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Figure 1- Our Sun as a Red Giant star, swollen near its doomed planet, Earth A Celestia Educational Activity The Life and Death of Stars version 6 rev 4, September, 2004 Celestia Educational Activity 6 – The Life and Death of Stars pg 2 of 29 Table of Contents The Life and Death of Stars (click on any title below to go there) Background Information .............................................................................................................................3 Introduction ...................................................................................................................................................4 In the Heart of the Milky Way ....................................................................................................................5 1. The Birth of a Star - The Nebula Stage ..............................................................................................7 2. The Protostar Stage ............................................................................................................................10 3. The Main Sequence or Yellow Supergiant Stage ...........................................................................12 3a. Brown Dwarfs – failed stars .............................................................................................................15 4. The Red Giant and Supergiant Stage ..............................................................................................15 5. The Death of Earth (and the inner planets) .....................................................................................18 6. A new Beginning on Pluto? ................................................................................................................20 7. The Death of a Red Giant Star – the Planetary Nebula stage .....................................................20 8. The White Dwarf and Black Dwarf Stages.......................................................................................22 9. The Death of Large Stars – Supernova Stage ................................................................................23 10. The Death of Large Stars - Neutron Star and Pulsar Stage .......................................................25 11. The Death of Large Stars - The Black Hole ..................................................................................27 12. Conclusion ..........................................................................................................................................28 13. Credits: ................................................................................................................................................29 Celestia Educational Activity 6 – The Life and Death of Stars pg 3 of 29 Background Information This is a Celestia educational activity. It will take you on a special journey through space and time, to examine the birth, life and death of stars. Along the way, you will encounter some of the most amazing objects in the universe, including Black Holes, Red Giants and Pulsars. You will also discover the fate of Earth. Celestia activities are designed for middle and high school students in grades 7 to 12, but can be used by anyone who wishes to explore our universe. This activity employs the use of a visually stunning space simulation program named Celestia. Celestia runs on most computers but does require your computer to have the following minimum capabilities: 1. A CPU speed of at least 800 MHz 2. An operating system running Windows 98, ME, 2000, NT, XP, or MAC OSX 3. RAM memory of at least 128 MB (256 – 512 MB preferred) 4. A video graphics card with at least 32 megabytes of RAM (64 – 128 MB recommended) 5. OpenGL graphics rendering capability If you are not sure if your computer has these things, we suggest you download Celestia and try it out. During this activity, you will be asked to use certain buttons and menu controls within Celestia. If you are unfamiliar with the program, don’t worry. We will guide you step-by-step through your journey. Celestia is provided to you absolutely free of charge for use with this activity. If you are within a school, your computer may already have Celestia loaded on it. Ask your teacher. If it is not there or if you are working from home, you must download Celestia. To do so, visit our website at http://www.shatters.net/celestia/download.html and locate the version of Celestia that best matches your computer’s operating system. Once downloaded, left-click on it twice (double-click) and it will install itself on your computer (Windows systems) or install the program as your MAC OX, Linux or Unix system instructs you. After it installs, return to this document to begin our Celestia journey together. If you are using this activity in school, your teacher will tell you how to arrange your computer screen so that you can both read this open document and use Celestia. If you are doing this activity from home, we suggest you read it on screen while also viewing and using Celestia, by offsetting the two “windows” so that the top of both windows is always visible. A shortcut way of switching views is simply to press the [Alt] and [Tab] keys on the keyboard at the same time (Windows systems). Press them again to switch views back. A student worksheet is provided as a separate document (Activity 6 worksheet). You may want to keep that worksheet handy for filling in information as you discover it. Your teacher may ask you to fill in this document for a grade. Celestia was created and is generously provided to you free of charge by Chris Laurel and the Celestia Open Source group and is available to everyone on the internet at www.shatters.net/celestia. We encourage you to visit the site. If you wish to explore this great program in more depth, we urge you to download the Celestia User’s Guide, Part 1 from that site. It will teach you how to use every control and feature of the program. Celestia Educational Activity 6 – The Life and Death of Stars pg 4 of 29 Introduction When Celestia opens, press the [V] key to turn off all text visible on the screen. Then, dim the room lights if possible. Block out all external distractions and just contemplate the view in front of you for at least a minute. Press the [6 →] key or [4 ←] key to slowly rotate your view around completely. Take your time. Not many people on Earth get to observe the incredible beauty of space from your vantage point. If you’ve had an opportunity to take the journey titled, “Celestia Educational Activity 1 and 2 – the Universe”, you have discovered that the depths of space are almost infinite in size and stretch to limits almost beyond our capability to imagine. Populating that dark cold space are the stars, newly estimated in number at over 140 sextillion (140,000,000,000,000,000,000,000). If you added up all the grains of sand on all the beaches and at the bottom of all the oceans on Earth, you would not have enough grains to represent the stars we believe exist in the Universe. Incredible but true! Celestia is displaying only a tiny portion of the stars in our nearby neighborhood, the amount your eyes might pick up looking into space without a telescope. However, hidden in that darkness are billions more stars, too dim or far away to see with just our eyes. To see more of them, press and hold down the { ] } key on your keyboard. WOW! The stars you see before you are part of a much, much bigger group. Stars exist in space as enormous collections of stars called Galaxies. A Galaxy contains many billions of stars, held together by the force of gravity. ALL stars occur only in galaxies. Between the stars is mostly empty space but there are Figure 2 – a representation of a galaxy not too far from our own Milky Way galaxy. Each tiny white dot is a star. The colored material is nebula gas and dust. The bright yellow glowing blob in the center is a huge concentration of stars, billions of them Celestia Educational Activity 6 – The Life and Death of Stars pg 5 of 29 places where huge thin clouds of gas and dust collect. The gas is mainly Hydrogen (H), with some traces of Helium (He) and Lithium (Li). The dust represents small pieces of all of the other 90 elements that make up the Periodic Table of the natural elements. These include Carbon (C), Oxygen (O), Nitrogen (N), Calcium (Ca), Iron (Fe) and yes, even Gold (Au). Since all stars exist only in galaxies, that means our own Sun and the stars you see in your viewscreen are part of a galaxy of stars. Our galaxy’s name is the Milky Way. We are one of over 200 billion stars in our Milky Way. Location of our Sun Figure 3 – An artist’s sketch of our own Milky Way. Our Sun is approximately positioned where the arrow points. We can see the Milky Way with our own eyes by simply looking up at night. Every star in the sky is part of it. On a dark night, far from city lights, the densest region of the Milky Way is a hazy band of whitish matter that looks a bit like a faint cloud in space. In your viewscreen, it is crossing the center of the screen and is where the highest concentration of stars exists. That white stuff is actually stars … billions of stars … clustered so close together that their light merges into a broad milky haze. Mixed in with the stars is also gas and dust … countless trillions and trillions of tons of it. How all those stars, gas and dust got there, and what will become of them over time, is the journey of discovery you are about to take. Through the magic of Celestia, we are going to take you to the birthing grounds of stars and explain how they form. We will then lead you through their life story, and end with their deaths. Yes, stars are born and eventually, they die. Yet as you will learn, their deaths can lead to new re-births, of new stars and possibly of planets suitable for the development of life. In fact, you exist on Earth because some stars died in this part of space long ago. The atoms that make up your body actually came from stars that blew up and died billions of years ago. Note: For ease of use, we suggest you arrange this document and Celestia together on the same screen in a “tiled” fashion, so that you can see the borders of both of them. To switch from the document to Celestia and back again, just click once on the border. Alternatively, you can press the [Alt & Tab] keys at the same time. In Windows® systems, this will switch from document to program and back again. In the Heart of the Milky Way 1. Let’s turn off some of the stars. Press the [V] key to turn back on your screen’s text display. Then, repeatedly press the { [ } key so that your text display says the “Magnitude limit” is between 9.7 and 9.9. The number of stars you see will return to a typical view as seen in space with our eyes only. 2. You are in a very special and very fast spaceship. Its name is the Celestia 2. It is one of just two such ships that have been built for use in the Celestia universe. You may have already ridden on the Celestia 1 in a prior Celestia adventure. Celestia Educational Activity 6 – The Life and Death of Stars pg 6 of 29 3. Would you like to see the Celestia 2? You can, by launching an external camera. To do so, click here once only (it is important to click only once) to activate the camera. The camera will take position off the starboard (right) side of your beautiful ship. She looks fast, doesn’t she? To get a better look, point onto the Celestia viewscreen with your mouse, press/click the right mouse button and while holding it down, drag the mouse. Your view will turn around the ship. This right-click-drag action will be used a lot, so remember it. Examine your ship from all sides. 4. Then, turn off the camera and return to the inside of your ship by clicking here once. You will be looking through your front view window, with the Sun reflecting off your ship’s metallic nose. The ship’s internal lights are off so you can see better. Your ship’s powerful engines are also off at the moment. It is quiet now and your ship is silent. Outside the ship, it is in fact, totally quiet. In space, there is no air or atmosphere. Sound requires something to pass through. If there is no air, sounds cannot be heard. It is also incredibly cold outside your ship’s hull. The temperature of deep space is more than -400° F (-240°C) below zero. 5. Lets’ review your ship’s HUD text displays. In the upper right corner, you will see today’s Date and Time display. The date is shown as (year, month, day). The time is in the format (hours, minutes, seconds Universal Time or local time) on a 24-hour clock. For example, 2003 06 18 01:33:33 UTC means you are viewing the sky as it looks in the year 2003 on June (the 6th month) 18 (the 18th day) at about 1:33 (and 33 seconds) AM, Universal Time (UTC). Universal time is commonly used by Astronomers and the military. Alternatively, your time may be set to your local time zone. 6. Under the date and time display are the words, “Time stopped.” That means that everything on the screen is stopped at the moment. Of course, time stops for no one, but in Celestia, it is sometimes helpful to be able to pause time to give you an opportunity to read these instructions. To resume Real time, return to Celestia and locate and press the [\] key now. 7. In space, stars and planets are very large and take a lot of time to move (rotate or travel). Since we do not have a lot of time, we may wish to speed time up on occasion. To do so, return to Celestia and on the keyboard, type the letter [L]. The display in the upper right corner will change to read, “10x faster. Type [L] again and it becomes 100x faster. Hit [L] again and the time clock speeds up again. To slow time down, type the letter [K]. To return quickly to Real time, press the [\] key again. 8. In the upper left of your ship’s display will be the word, “Celestia 2”. This advises you that your ship’s navigation computer is locked onto your own ship. In a few moments, we will begin our journey and command the Celestia 2 to lock onto another object. Once it does so, that object’s name will appear in the upper left corner instead. 9. In the lower left corner is your ship’s speed. You are stopped now so your speed is 0.000 m/s (meters per second). Later, we will be accelerating to very high speeds and this display will change. 10. What are stars? Where do they come from? How do they form? How long do they exist? Do they end? If so, what happens to them? These are very good questions. For most of our history, we simply didn’t know these answers. Of course, many people took a guess, but in most cases, it turned out to be wrong. 11. Beginning in the 16th Century, things began to change. Thinkers and inventors in Europe like Copernicus, Brahe, Galileo, Kepler, Cassini and Newton began to develop telescopes, tools and ideas that for the first time, gave us a new window into the blackness of space. They began a long process of discovery that continues today as the modern science of Astronomy. Literally hundreds of Celestia Educational Activity 6 – The Life and Death of Stars pg 7 of 29 thousands of men and women worldwide now study space in detail and entire government agencies such as the National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA) are devoted to space science. These men and women have invented powerful telescopes, and have launched hundreds of satellites into space that enable us to see far into the cosmos, detecting minute details of galaxies, stars and planets. Through these tools, we have made great progress in unraveling the mystery of where stars come from, how they form, and what kind of life cycles they undergo. Let’s begin our story by firing up our swift spacecraft and traveling to a nursery … where new stars are being born! Are you ready? 1. The Birth of a Star - The Nebula Stage 12. To position your ship for the trip, click here once. The Milky Way and Sun will be visible in your ship’s window. Your ship is far from Earth, drifting near the outer planets of our Solar System. Now, press the [C] key once. This will instruct your ship to swing and point its nose (the bow) directly toward a faint colored cloud in space named “M42”. Notice that in the same info display, the distance from you to that cloud is listed as 1,177.7 ly. The “ly” stands for “Light Year”. It is not a time, but a distance. It is the distance a beam of light will travel through space in one year. 13. Light travels faster than any other thing in the entire universe. Its speed is an incredible 186,000 miles/sec (300,000 km/sec) and is abbreviated “c”. Nothing else travels that fast … no spaceship or asteroid or moving star … nothing even comes close. That speed is so fast that you could travel completely around the Earth 7 times every second. In a year moving at that speed, light will travel 5.9 trillion miles (9.5 trillion km). In scientific notation terms, a light year is 5.9 x 1012 miles, or 9.5 x 1012 km. Notice that M42 is over 1,177 light years away. That means that if you could travel in a ship at the incredible speed of light toward that distant cloud you see before you, it would still take you 1,177 YEARS to get there (provided we ignore a bizarre principle of Physics known as the Time Dilation Effect of Special Relativity). The span of space between you and M42 is ENORMOUS. 14. Fortunately, the Celestia 2 is a “special” ship. It can travel faster than the speed of light. We won’t have to wait 1,177 years to make our first stop in the story of a star’s life cycle. 15. M42 is a cloud of gas and dust in space called a “Nebula”. It really is in the night sky where you see it, and it does look like this, but your eyes are too small to see it clearly without a telescope. However, on a clear night, you may be able to make out its dim outline, if your know where to look. To help orient you, locate and press the [/] key. This will turn on the outlines of the constellations. Constellations are patterns of stars in the sky that some folks think look like something familiar. M42 is called the “Orion Nebula” because it is located within the famous constellation of Orion, the Hunter. Orion is visible on most dark nights in the Northern Hemisphere during the autumn and winter months. Look for the 3 bright stars that cross the hunter’s belt. Notice where M42, the Orion Nebula is in relation to the constellation of Orion. It is below the belt of stars. When done, press the [/] key again to turn off constellation outlines. 16. To begin our trip, press the [F] key once. This will command your ship’s computer to lock onto M42 and “follow” it. A display in the lower right corner of your screen will now say, “Follow M42”. 17. Now, let’s fire up Celestia 2’s powerful engines. To do so, press the [A] key on the keyboard. You will begin moving forward slowly through space. Your speed will be listed in the lower left corner of the screen. You will begin moving at a very slow speed, measured in meters per second (m/s), but as Celestia Educational Activity 6 – The Life and Death of Stars pg 8 of 29 you continue tapping the [A] key, your speed will increase. Soon, you will be moving at thousands of meters/sec (km/s), then at fractions of the speed of light (abbreviated c) per second. If you go too fast, press the [Z] key. This slows you down. Remember these commands; A to speed up, Z to slow down. To stop your ship completely, press the [S] key. 18. Let’s accelerate to a speed far in excess of the speed of light (c). To reach it quickly, press the [ F 7 ] function key at the very top of the keyboard. This is a shortcut command to the Celestia 2’s engines to accelerate to the impossible speed of one light year per second (1.000 ly/s). This is over 30,000,000 (30 million) times faster than the speed of light. Yet even at this speed, it will still take us 1,177 seconds (about 19 minutes) to reach M42. Let’s shorten that journey even more. Tap the [A] key several times more to accelerate from 1 ly/s to between 8 and 9 ly/s. Now, your flight to M42 will take only a few minutes. Sit back and enjoy it. Stars will begin passing by on both sides. Watch your Distance display in the upper left corner. When you get between 40 and 30 ly of M42, press the [S] key on the keyboard. This turns off your engines and also brings your ship to a quick stop. 19. What you are looking at is the birthplace of stars, a nursery that is helping to create and nurture stars. It was created here a long time ago, when gases and atoms of elements drifting in space between the stars began to gather and collect by the force of gravity. Gravity can be thought of as a powerful attractive force that all objects and matter have. It comes from all atoms that exist in the universe. Even the atoms in your body are giving off a gravitational force that is pulling on Figure 4 - M42, the things around you. It makes no Orion Nebula difference what the object is ... a computer, TV, person, rock, tree or star. Everything attracts everything else by gravity. Gravity also works over very long distances. Although it gets weaker as distances between objects increase, it never really disappears entirely. Your body for example, is giving off a gravitational force that is pulling on the Earth and moon, on the Sun itself and on every star you can see in the sky. It is even pulling on M42 before you. Likewise, the Earth and moon, Sun and stars and M42 are all pulling on you with their own gravitational forces. Because the Earth is so large and so close to you, its gravitational force is the most powerful around you and pulls you down to the ground. You stay put here on Earth. 20. What if we took you off the Earth, however, and put you far away in space, far from other stars. That is where the Orion nebula is. Out here, exist mostly gases and small particles of many other elements in nature. About 90% of all the visible mass in the universe is the simplest element, Hydrogen. All star-forming nebula are made mostly of it. Second in concentration is Helium (a bit less than 10%). The small percentage left consists of traces of “dust” made of such elements as nitrogen, oxygen, carbon, lithium, iron and even gold and silver. In fact, all 90 natural elements that exist in nature can be found in space in differing trace amounts. They all drift around in thin clumps of gas and dust. 21. Over time (a LOT of time), that gas and dust will be attracted to each other through the force of gravity. Over millions of years, gravity will pull a wisp of gas here and a grain of dust there together until it collects as a big cloud of material, a fog gathered into a large clump in space we call a Nebula. M42, the Orion Nebula in front of you is one such cloud. Actually, it is two clouds. Notice that on its right side, a second purplish nebula has formed. It is called NGC 1977, and is similar to M42. Celestia Educational Activity 6 – The Life and Death of Stars pg 9 of 29 22. How is M42 and NGC 1977 a birthplace for stars? The answer is simple. Stars are made of the material you see in front of you. M42 contains the “food” that feeds a new star. Its concentrated clump of mostly hydrogen gas plus some elemental dust contains the starting material … the nutrients that nurture the growth and development of a new star. 23. Notice a bright part of M42 near the top right corner? That is a new “star” developing, slowly gathering matter from the Nebula around it through the force of gravity. It is not yet a true star but it is growing every day, getting hotter and bigger. Someday, it will become a full-fledged star. Lets’ move toward it. Our control needs to be tight here. We don’t want to fly into the Nebula. 24. Before we approach it, you will need to learn how to steer your ship. Figure 5 – a new star forming On the number keyboard on the right side of your keyboard, locate in M42 the [8↑], [2↓], [4←] and [→ 6] keys. Do you see them? There should be a small light on above those numbers, labeled Num Lock. If the light is not on, press the key that says [Num Lock] so that the light comes on. 25. Then, return to Celestia and tap the [8↑] key. Your view of M42 will move (pitch) downward. The [2↓] key pitches your view upward. Tap the [4←] key. Your view will move to the left. Tap the [→ 6] key and your view will move to the right. The [7] key and [9] keys rotate your view. Practice with these keys for a minute, until you are comfortable with what they do. You will use them often. When you are ready, click here. 26. Press and hold down the [A] key and accelerate to a speed of about 0.2 to 0.3 ly/s. Slowly, the Orion Nebula will enlarge. Steer toward the bright region in figure 5 above, using your steering keys. When you are within about 7 light years (7 LY) on the Distance display, press [S] to stop your ship. Now, swing your view around using the six steering keys and enjoy a view no human has ever actually seen in real life; a closeup of M42 from only 7 light years out. What a beautiful nursery! All stars, including our Sun, started out in a place like this; a giant Nebula cloud of Hydrogen gas and elemental dust which gathered by gravity in the cold of space. 27. Celestia has several beautiful star nursery nebulae to show you. Your ship is programmed to take you to a second one. To visit one of the most active, click here. You will be taken to the Rosette Nebula, a massive nebula that is giving birth to hundreds of stars. To take a short tour, press the [ F 7 ] key. Your hyperdrive engines will come on, and you will begin moving toward the nebula at the incredible speed of 1 ly/sec. Use your steering keys and fly through the stars in the Nebula for a minute or two. Enjoy the beauty of these young stars, some just born, some teenagers. Remember the size of this nebula. These stars in front of you are trillions of miles apart from each other, and the Rosette Nebula is hundreds of light years wide. Press the [S] key to stop your ship when done exploring. 28. Our next stop is one of the most beautiful birthing places in the Milky Way. It is called the Eagle Nebula. To go there, click here. What a striking place! Press the [F7] key and fly toward it for a bit. Turn and explore for a minute or so. Press [S] when done. The nebula gases and dust in front of you represent so many colors because they are particularly rich in different elements. In fact, all 90 elements from the Periodic Table are dense here. The stars that are forming in the Eagle will have dense solar systems of planets, developing from all that nebula material. How planets form is part of the next chapter in our story of the life of a star. 29. If part of a class lesson, go to the worksheet you’ve been provided and complete the questions prior to and including Nebula. Go to Worksheet. Celestia Educational Activity 6 – The Life and Death of Stars pg 10 of 29 2. The Protostar Stage 30. What is going to happen to the many bright glowing regions of gas that you see in front of you? Over many thousands of years, the force of gravity will pull different clumps of them tighter and tighter together, pulling the gases and heavy elements toward their centers. Repeated in different regions throughout the nebula, atoms will bang and collide in space and the core of these regions will heat up hotter and hotter from the friction of atoms rubbing, banging and crushing together. Eventually, these hot clumps will exceed a million degrees and still they will get hotter, and denser. One by one, the gases will glow with heat much brighter than you see them now. 31. Nebula clouds, and regions of dense clumping within nebula clouds, will also begin to spin. Due to the way atoms come together in space by gravity, most very young stars begin to spin as they collect nebula gases and dust. Let’s journey to two such young stars, still in the process of forming. To enjoy our view, press the [V] key until no text appears on the screen. 32. To position you for the first journey, click here. The Celestia 2 will depart the Eagle Nebula and fly you at hyperspeed to a section of the Rosette Nebula that has begun to pull together by gravity into a new star, and has begun rotating. This is the beginning of step 2 in a star’s life cycle, called the “Protostar” stage. 33. To fly into the heart of the rotating nebula/protostar, press the [F6] key. Your engines will come on at an impossible speed of 1 au/s. Steer with your steering keys as outlined earlier. Dip toward the nebula and fly/skim along its top. Pass the central protostar, fly across to the other side of the nebula and turn around with your [4←] and [→ 6] keys. Notice that this new star is part of the Rosette nebula which you visited a few minutes ago. That is how almost all stars form. Gravity collects a small clump of gas and dust of a much bigger nebula into a protostar. When done exploring, press [S] to stop your ship. 34. Over time, this baby protostar will develop further. Gravity will pull it tighter and it will begin to glow very hot. Let’s visit a star that has reached a later stage in Protostar development. It is also part of the Rosette nebula. To go there, click here. Your ship’s NAV computer will steer your ship to a most amazing object. It is almost formed. Gravity has pulled it into a tight glowing ball. Surrounding it is part of the nebula that it formed from, captured by its gravity and spinning also. Figure 6 – a Protostar with nebula, rotating and growing 35. You may be wondering about those two amazing “jets” which are coming out the top and bottom of the protostar. They are part of the nebula gas, forced to “squirt” out, guided by the electrical and magnetic forces that are being generated inside the rotating star. Over time, the jets will disappear, but for thousands of years, they will be there, spraying energetic high speed atoms of gas away from the young star. Most young protostars have them. 36. The reddish colored “disk” of gas and dust that is spinning around the protostar has a special role to play in the life cycle of this star. Do you notice that some areas of the nebula are thicker and denser Celestia Educational Activity 6 – The Life and Death of Stars pg 11 of 29 than others? That disk is full of both gas and dust containing heavy elements such as iron and silicon, calcium, aluminum and carbon. Slowly, gravity is forcing the thicker, denser regions of that spinning disk to clump also. Little whirlpools of very dense material are forming where these clumps are thickest … and from that material … you guessed it … planets and moons are growing. Spread out before you is not only a new star in the making, but an entire solar system, not yet fully developed but soon to be so. How long will the wait be? Due to its enormous size, this new solar system may take millions of years to fully develop, but someday in the future, a complete system of planets and moons surrounding this hot young star will be here. 37. In fact, there are already two or three new planets present. To visit one, click here. Your ship will move to a position off the hot protostar. Look closely in front of the star, at the rotating nebula. Can you see a small dot moving, imbedded in the nebula? Wait until it passes by the front of the star, then press the [F] key. Your ship will lock onto it and follow it. Figure 7 – a hot new planet 38. Now, press the [G] key and your rocket engines will come on and take you automatically to this new protoplanet. It has no name yet. It is simply called “B”. It is hot and still semi-molten. Lava is flowing freely across its surface as space dust and rocks continue to rain down on it, smashing into it and causing it to grow bigger and denser. Currently, it is smaller than Earth but year by year, “B” is enlarging and maturing. Press the [L] key once to slow down time a bit and witness this baby planet rotating for a minute. Right-click-drag around it a bit. WOW! 39. Billions of years ago, our solar system formed this way, and our Earth looked much like this young planet. Celestia Educational Activity 9 is the story of the Primitive Earth and Moon system, at a time long ago when both were young, hot and violent. Look for it on our website. Celestia Educational Activity 9 The Primitive Earth and Moon Examines how we think the Moon/Earth system formed. It is under development. You can find it at http://www.fsgregs.org/celestia, or at the Celestia add-on repository at http://www.celestiamotherlode.net 40. Let’s get really close to this hot new protostar. To do so, click here. The Celestia 2 will take you close to the central hot core, but not too close. Enjoy the view. This protostar is hot and glowing. It’s internal core temperature is well over 4 million degrees C and climbing. Its surface temperature is close to 6,000° C. Yet, its heat comes primarily from friction and gravitational compression of its atoms. It has not yet grown up to become a full star. That is the next stage in our journey. 41. Go to the worksheet and complete the questions on Protostars. Go to Worksheet. Celestia Educational Activity 6 – The Life and Death of Stars pg 12 of 29 3. The Main Sequence or Yellow Supergiant Stage 42. Deep within the protostar, something is happening. As the gravity of the star pulls more and more nebula material onto it, pressing down and crushing it denser and denser, its central core is being compressed to incredible pressures. This compression forces its atoms to rub and crush together. The result is an increase in temperature. The core will get hotter and hotter. It will exceed 4 million degrees C and still it will climb. Year after year, century after century, it will get hotter. 43. The material in new stars being formed today is primarily hydrogen gas (about 75%), helium gas (about 24%) and 1% of everything else on the periodic table. Hydrogen is a simple substance with an atomic mass of 1. In atomic terms, hydrogen is just a single proton surrounded by one buzzing electron. 44. When the temperature in the core of a new star reaches in excess of 10 million° F (5.5 million° C), the protons of hydrogen actually smash into each other with such force and energy that they begin to FUSE. The process is called Hydrogen Fusion. Four Hydrogen protons fuse together and form a nucleus of Helium (made of 2 protons and 2 neutrons). As you can note, two protons will change to neutrons in the fusion process. That results in a release of LARGE amounts of energy into the core, which migrates up to the surface of the star and escapes into space as huge bubbles of heat and gas. It is the same process that occurs in a hydrogen bomb. 45. To see that process in action, click here once and wait for the animation to open. Left-click on the play button (the one in the middle). You will see atoms colliding faster and faster until they begin to fuse, while the core of the star begins to glow white-hot with radiant energy, giving off vast amounts of heat and radiation into space. 46. In reality, stars that get hot enough in their cores to begin fusing hydrogen to helium actually begin blowing up, exploding like a bomb. However, gravity, which has pulled all that material together in the first place, prevents these hydrogen fusion explosions from destroying the star. Instead, the star swells from the explosions to a particular size and stabilizes there. It is exploding outward and is being pulled back by gravity inward at the same time. A balance occurs … called fusion equilibrium. Figure 8 - a star undergoing nuclear fusion explodes outward and is pulled inward by gravity. It stabilizes in size and is in “equilibrium.” Celestia Educational Activity 6 – The Life and Death of Stars 47. pg 13 of 29 When a star begins fusing hydrogen into helium in its core, it is now a fully grown, fully developed star. No more material falls onto it because the explosions in its core heat it up to such a high temperature (over 20 million° F, or 11 million° C) that hot matter from the star begins flying off into space at high speed, like a wind blowing away from the star. As seen in the prior movie, this “Solar Wind” of hot gases moves away from the star at over 1 million mi/km per hour and blows away the remaining gas and dust that once surrounded the protostar. Figure 9 – The solar wind from a new main sequence star blows the nebula away. 48. The new star, now actively fusing hydrogen into helium in its core, goes by several names. If it is a star similar in size to our Sun, it is called a “Main Sequence” star. The color of such stars range from blue-white to orange. They have enough hydrogen to burn for hundreds of millions to billions of years. For example, our Sun is a small Main Sequence star. It is not burning hydrogen into helium as fast as some stars, so it is a bit cooler and less energetic. It will fuse hydrogen to helium for close to 10 billion years before it runs out of hydrogen fuel in its core. It is about 4.6 billion years old now, so it has over 5 billion years to go before hydrogen fusion stops inside its core. Most stars in the night sky are Main Sequence stars and live for similar periods. 49. The color of a star depends upon its surface temperature. Hot stars are blue-white in color. Average temperature stars are white to yellowish, and cooler stars are orange to pink in color. The hotter the star, the faster it goes through its hydrogen fuel and the shorter is its lifespan. 50. Our Sun is an average to small sized star. In fact, in some Astronomy texts, it is identified as a “yellow dwarf”. Many stars in the universe are average in size, a bit bigger than our Sun. However, there is no reason why a star has to be average sized and in fact, more than 80% of the stars you can see in the night sky are bigger in diameter than our Sun. If the nebula cloud from which it grows is denser and wider than the one that formed our Sun, gravity will organize it into a larger clump. A larger protostar will form and from it, a larger hot star will ignite. 51. Stars in their hydrogen fusion stage can in fact, grow to very large proportions and burn through their fuel quickly. For example, stars 5 to 500 times larger than our Sun populate the night sky. Stars that are several times larger than our Sun fuse hydrogen at a much faster rate because their mass is greater, their core temperatures are hotter and they are exploding more violently. These stars are not only much hotter, but they expand to a bigger size during hydrogen fusion and are called “Yellow or White Giants”. They go through their “main sequence phase” in millions rather than billions of years. Really BIG stars can get so hot during fusion that they turn blue-white in color, swell to massive proportions and are called Blue Supergiants. These stars go through their fuel furiously. They have only enough hydrogen in the core to last only 10 million years or less, even though they are much more massive. They are very short-lived stars. Celestia Educational Activity 6 – The Life and Death of Stars pg 14 of 29 52. In summary, a Main Sequence star is one that has grown up and has begun fusing hydrogen into helium in its central core, causing the star to explode with energy and heat. Gravity keeps it from blowing itself to bits. Our Sun is a Main Sequence star. Most typical main sequence stars are white to orange in color and will convert Hydrogen into Helium for hundreds of millions to billions of years. 53. If the star is larger than our Sun, it may develop into a Yellow, White or Blue Giant or even into a Supergiant star that is much bigger and is burning/fusing its hydrogen so fast that it will last for only 100 million years or less. 54. The Celestia 2 is ready to take you to some of these stars. Let’s start with our own Sun. Return to Celestia, press the [K] key and slow time back down to Real time. Next, press the [H] key (think of Home), then press the [C] key and [F] key. Your ship will turn and lock onto the Sun, which is just a tiny star over 5,500 light years from your current position. Press the [G] key and you will be taken at hyperspeed to our Sun. Zoom in closer with the [Home] key and speed up time to 10000x faster with the [L] key. To restore your HUD text display, press the [V] key once. 55. The surface of our Sun is about 10,000° F (5500° C). Its core temperature, however, is over 20 million ° F. It has been fusing hydrogen into helium for billions of years. As mentioned, it is actually exploding, but gravity is holding its size steady. It is in equilibrium. It has enough hydrogen in its core to continue hydrogen fusion for about 5 billion more years. 56. Although your ship is reasonably close to the Sun now, you are safe only because the Celestia 2 has a special hull to protect you from the enormous radiation and energy pouring out of the Sun. In fact, it is enough to warm the Earth over 93,000,000 miles (1 Astronomical Unit) away. That energy is due to the ferocious fusion of Hydrogen occurring inside the core. As you watch, large amounts of hot gas from the surface of the Sun are also being blasted off the star and thrown into space as the Solar Wind. Some of it is even hitting your ship now. 57. Let’s attempt a very dangerous maneuver. Let’s try to skim over the outer atmosphere of the Sun. You can only do so for a few moments without overloading your ship’s cooling systems. Press the [A] key and accelerate toward the Sun at about 0.2 c. As the Sun approaches, use your arrow keys and change course so that you just skim over the top of the Sun, passing through its yellowish atmosphere but not plunging into its surface. Can you do it? WHEW! When done, press [S] to stop your ship. 58. Let’s visit some stars that are larger than our Sun. Press the [B] key to turn on star labels and rightclick-drag until you locate the star named “Sirius”. Left-click on it to select it, followed by the [G] key. Your ship will take you to a bright, large white star that is the brightest star in our sky, next to the Sun itself. Zoom in close with the [Home] key. Press the [V] key again to turn on your “complete” information display. Slow down time to 1000x faster with the [K] key. In the upper left corner, you will notice that this star has a radius of 1.88 times that of the Sun (1.88Rsun). In other words, it is almost twice as large as our Sun. Its brightness is given by the word, “Luminosity”. Notice that it is over 24 times brighter than our Sun. It is a larger White Main Sequence star and is burning its hydrogen fuel faster than our Sun, causing it to also be hotter (whiter) in color. It is so bright because it is very close to our Solar System (only 8.6 ly away). 59. Next, press the [Enter] key and type “Canopus”. Press [Enter] again and [G] to go there. Zoom in close. Canopus is a very big star 71 times larger than our Sun and 14,000 times more luminous (bright). It is burning its Hydrogen fuel far faster than our Sun. It is currently a Yellow –White Giant star, based upon its temperature. It will enter old age and the next stage of its life cycle long before our Sun. When finished viewing, press the [B] key to turn off star labels. 60. To visit a truly monster star that is simply burning itself up, press [Enter], type “Rigel”, press [Enter] again and [G] to go there. Zoom in and speed up time to 10000x faster. This Blue Supergiant star is Celestia Educational Activity 6 – The Life and Death of Stars pg 15 of 29 simply incredible. It is 17 times more massive, 60 times bigger in diameter and is over 40,000 times brighter than our Sun. Its surface temperature is almost double our Sun’s surface temperature and its internal core temperature is over 50 million °F. If a planet existed anywhere near Rigel, it would be baked, fried and sterilized by the ferocious radiation. There would be no possibility of life on it. Someday in the not too distant future, Rigel will explode with a gigantic force that will be felt here in our Solar System, 42 ly away. 61. Answer the questions on Main Sequence stars. Go to Worksheet. 3a. Brown Dwarfs – failed stars 62. As you’ve just learned, nebula that are large enough will give birth to a protostar that will eventually ignite in the fires of nuclear fusion, and burn for millions to billions of years. But what about small nebula? After all, just because there is some gas in space does not mean a star will form. What if the nebula is simply too thin and doesn’t have the gravitational mass to collapse a star into a density that will start a nuclear fusion chain reaction? Are there such nebula? What happens to the protostars that form in them? 63. The answer is interesting. The protostar gets hot and begins to glow with friction, but never reaches fusion temperatures. It never ignites. In a way, it is an aborted star. Gravitational friction will keep it hot and it will spin, but its glow will be just a dull red, dim and practically invisible among all the other stars. Over millions of years, it will slowly cool and die down until it becomes a dense “Brown Dwarf”. That is the name Astronomers give to stars that never make it … and fail to begin nuclear fusion. They really are brownish red. 64. Let’s go to one. Click here and your ship will take you to a spot in space only 2,500,000 km from a Brown Dwarf star. The dwarf has no name, but it orbits a star called 61 Figure 10 – a Brown dwarf Cyg B. Zoom in close and right-click-drag around it. You are in front on a failed star that began as a companion to 61 Cyg B and 61 Cyg A (nearby), but which failed to capture enough nebula material to form more than a small protostar … not enough to ignite into fusion. It glows now only dimly as gravitational compression keeps it modestly hot from friction alone. 65. Answer a few questions on Brown Dwarfs. Go to Worksheet. 4. The Red Giant and Supergiant Stage 66. All main sequence and giant fusion stars live long lives by fusing hydrogen in their cores into helium. However, all things come to an end. At some point, they run out of hydrogen fuel near the core. There is still plenty of hydrogen gas in a thick ring around the outside regions of the star, but it is far too cool to fuse into helium. The fires of nuclear fusion finally die down in the core region and the star, which has been exploding for millions or billions of years, calms down. The explosions that have been trying to blow the star up fade away. Now, gravity, which is always present, seizes the Celestia Educational Activity 6 – The Life and Death of Stars pg 16 of 29 star’s mass and begins pulling it again toward the center. The diameter of the star shrinks rapidly and the core is crushed again by enormous gravitational pressures. Day by day, week by week, the mass of the star tightens and presses down on the core. Week by week, the temperature rises from the pressures. It exceeds 50 million degrees and still continues to get hotter. Month by month, the now old, burned out Main Sequence or Yellow Giant star gets smaller and internally hotter. 67. Finally, as it collapses, the temperatures in the core region begin to exceed the incredible temperature of 100 million degrees. At that point, two things happen: 68. A) The leftover hydrogen that is still outside the core but close to it gets hot enough to resume fusing furiously into helium in a large “ring” of burning hydrogen fusion surrounding the core. The ring expands outward and “pushes” the star to begin swelling in size/diameter. 69. B) The helium nuclei that make up the core of the star become so hot that they also begin FUSING HELIUM into Carbon, Oxygen and Neon. Helium fusion is more energetic than Hydrogen fusion and releases more energy. 70. The star stops collapsing. Instead, it now blows up again, expanding and swelling away from its core in enormous explosions, but this time, there are two sources of explosion, the hydrogen fusion explosions going on in a ring around the core, and the helium fusion explosions occurring in the core. The star rapidly swells outward to 100 - 200 times its original size and more. It becomes a truly GIANT star. However, gravity is still powerful enough a force to keep the star from blowing itself to bits. It settles into a new size and a new equilibrium, much, much larger than before, but still holding together. 71. In fact, the surface of the star, the only part we see, is so far away from the heat generating fusion zone and core (millions of miles/km), that it cools off significantly and becomes red in color, rather than hot white or yellow. The visible star becomes a RED GIANT or SUPERGIANT. 72. Our Sun is facing this fate. Calculations reveal that our Sun will swell to a Red Giant about 5 billion years from now. Figure 11 – Our Sun below, will become a Red Giant someday Hydrogen fusion region Our Sun now Helium fusion Celestia Educational Activity 6 – The Life and Death of Stars pg 17 of 29 73. What happens if the star is already large; that is, if it starts out as a Yellow, White or Blue Giant? The answer is that when it runs out of hydrogen, it undergoes the same process as a smaller Main Sequence star. It contracts, heats up and again explodes with both hydrogen and helium fusion occurring. However, it is so large a star and is so hot, that it becomes a GIGANTIC Red Supergiant. Some of these massive stars can reach a diameter over 1,000 times that of our Sun. 74. Would you like to visit a few of these giants? We can fly the Celestia 2 to do so, but we have to be careful. The radiation is stupendous near these Red Giant and Supergiant stars. We can’t linger long. 75. Return to Celestia, press [H] to select Home, and [G] to go there. We will return to our Solar System. Next, press [Enter] and type the word, “Betelgeuse”. Press [Enter] again, then [C] to center it in your viewscreen. A modestly bright reddish star will be centered. Press the [/] to temporarily turn on constellation borders, and you should discover that Betelgeuse is the top, Orion the hunter reddish star of the constellation “”. Press the [/] to temporarily turn off borders. 76. Betelgeuse is a massive Red Supergiant star over 400 times the diameter of our Sun. It is about 500 light years from Earth and is rapidly and ominously burning itself up. Press [G] to go there and zoom in with the [Home] key. The view of the amazing star in front of you is Figure 12 – the ominous surface of Betelgeuse based upon what we can make out of its surface from our best Earth telescopes. It has a multicolored surface, with regions that are hot and others that are cooler, making them look black in comparison. Don’t be fooled, however. This star is a monster and someday, it is going to undergo a cataclysmic explosion known as a Supernova (see later discussion) When it does, it will become the brightest object in the sky next to the Sun for several weeks, possibly even outshining the moon. It may even shower Earth with dangerous radiation that could endanger life on our planet. What is so strange is that it is so far away (500 light years), that it takes light 500 years to get here from there. It may already have exploded centuries ago, but the radiation and light is still in route; to arrive tomorrow … or in 300 years. 77. Next, press [Enter] and type the word, “Antares”. Press [Enter] again, then [G] to go there. Zoom in close. Antares is also a Red Supergiant star. It is about 15 times the mass of our Sun, is 11,000 times brighter than our Sun now and over 400 times wider in diameter. Speed up time with the [L] key to 100000x faster until Antares is turning slowly in space. Are you ready for this? Antares is so large that 91 million (91,000,000) of our Suns could fit inside it. It also has a companion, another large blue-white star that orbits it. Antares is not the biggest Red Supergiant we have ever found, however. That distinction goes to Mu Cephei, a truly colossal star over 5,000 light years from Earth. Fortunately, our Celestia computer database includes the star and our ship can take you to it. 78. Let’s visit it. Click here. Incredible, isn’t it? Its radius is unbelievably almost 1,100 times wider than our Sun. To understand that size, if Mu Cephei was a sphere the diameter of a 20-story office building, our Sun would be the size of a golf ball. Good Grief!!! We really do not want to be around it when it finally ends its life in a colossal explosion. The explosion for a short time, will be one of the brightest lights in the entire universe. Celestia Educational Activity 6 – The Life and Death of Stars pg 18 of 29 5. The Death of Earth (and the inner planets) 79. What of the planets that might be around a star that has swelled to a Red Giant or Supergiant? The answer is not good. As the star swells, it will get big enough to actually swallow any planets that are within 100 to 200 or so solar radii of it. The planet is consumed inside the fires of the star. Planets that are further away may be spared, but they will get much hotter as the surface of the Red Giant reaches toward them, sometimes hot enough for rocks on them to begin melting and gas atmospheres to vaporize into space. The planets far from the star, however, can enjoy a time of mild warmth, as the Red Giant finally warms their environment. 80. When the time comes, our Sun will get big enough to swallow and consume Mercury and Venus. Astronomers are divided on whether its size will swell to engulf Earth. Some believe it will, others are not sure. In either case, about 5 billion years from now, our beautiful planet will come to an end, either swallowed and vaporized into atoms inside our Sun, or located so close to it that it melts and becomes just a boiling blob of molten rocks. Mars will get very hot but will survive. Jupiter and Saturn will get hot, and Uranus, Neptune and Pluto will be bathed in dangerous radiation. If humans are still around when our Sun swells to a Red Giant billions of years from now, we will have to abandon Earth and move beyond Neptune to our outermost planet, Pluto, if we ever hope to survive as a species of life. 81. Through the magic of Celestia, we are going to take you to that time and place. You will get to witness the death of our beautiful home as we know it. Are you ready? 82. Let’s travel far into the future aboard the Celestia 2, to a time billions of years from now. Our Sun has run out of hydrogen fuel in its core. Gravity has caused it to shrink and heat up internally, as the crushing pressures shoot up its core temperature to over 100 million degrees. Suddenly, Hydrogen fusion begins again around a gas shell surrounding the core, and the internal core begins undergoing Helium fusion also. Within days to weeks, our Sun explodes outward, rapidly swelling to a diameter beyond the planets of Mercury and Venus. These planets are swallowed and evaporate inside the Red Giant. Still the surface of the Sun swells and approaches … Earth. Gravity grabs the exploding star and begins to slow its swelling, but … 83. Return to Celestia and press the [V] key until the information text disappears, then click here. 84. The wounded dying planet in front of you is … Earth, our home. Gone are its blue oceans and Figure 13 –The Earth, melting as our Red giant Sun pale blue atmosphere, its forests and animals, approaches it its birds and people. The planet is melting ... it is already semi-molten. Surface temperatures exceed 2000° F and rocks are melting back into the lava from which they originally formed. Watch for a minute or two. If humans still exist as a species billions of years from now, we will have had to abandon the Earth centuries earlier. … And the source of all that heat? It is just to the left of Earth. To see what our Sun will become, tap your [4←] key on the numeric keyboard a few times. 85. Incredible, but true. Our Sun will someday swell to this size and look just like this. Earth will be doomed. Left-click on the Sun to select it and press the [F] key to follow it. Zoom out with the Celestia Educational Activity 6 – The Life and Death of Stars pg 19 of 29 [End] key if you wish. Then, Right-click-drag around it. Unfortunately, the Sun has not yet reached its final equilibrium size. As each day passes, it continues to swell, getting closer and closer to Earth. 86. If the Sun does reach the orbit of Earth our planet will melt, then turn to gases and vaporize within the fiery sphere of our Red Giant Sun. 87. To visit that time and place, click here. You are actually now inside the Red Giant’s fiery atmosphere. The Earth has melted and vaporized. This is its final resting place. Its atoms and molecules, including those of all the plants, animals and people who ever lived on Earth, will become part of our Sun again. Perhaps it is somehow fitting that Earth formed from the gases and dust of that original nebula gathered by our Sun when it first developed, and it will die by having those same atoms merge again with the Sun. Since your atoms are a part of Earth, your atoms also will someday merge with those of your Sun. 88. What of the other planets? Once our Sun reaches it final Red Giant equilibrium size, it will stay that size for millions of years. Could humanity move to Mars and set up a new home there … or perhaps to a moon of Jupiter? 89. To find out, click here. You will be taken to a position in space locked onto … Mars. It is that tiny red dot to the left of the swollen and bloated Red Giant Sun. Do you see it? Press the “[G] key and your ship will take you there. As you can see, Mars is not a home for humanity. It is so close to the Red Giant that it too is melting. Its surface temperature is very hot. Its atmosphere has also blown away and its ice caps have long ago melted and vaporized into space. 90. Press and hold down the [Shift + left ← ] keys together and see what the doomed and cooked Mars of our future will look like. 91. Jupiter also and all of its moons will be too close to our Red Giant star to live comfortably. Jupiter of course, is a gas giant planet and has no solid surface. However, it has over 60 moons and all of them are rocky, with large pockets of ice. Ganymede and Europa for example, are large moons that have extensive water. They are large enough to hold onto a thin atmosphere that could support life. Unfortunately, our Red Giant star is expected to make even the Jovian (Jupiter) system too hot. To see why, click here. The Red Giant on the right will far overshadow tiny Jupiter, just visible near the left side of your view window. Press the [G] key to go to Jupiter. When you get there, press and hold down the [Shift + left ← ] keys together. Your ship will rotate around Jupiter and you will see it as it might look billions of years from now. Its clouds will be different and it will have a distinct reddish glow from the light of the star. Its atmosphere will be puffed out from the heat of the Red Giant. 92. To sense what Jupiter is like, position yourself behind the planet with the Red Giant in front of you (see figure 14). Press the [\] key to resume Real time, and the [V] key to restore your HUD text. Now, press the [A] key and accelerate toward the planet at 4000 – 5000 km/s. If you go too fast, slow down with the [Z] key. Steer your ship and fly above Jupiter (don’t crash). When you pass the planet and it is behind you, press the [Shift and *] keys together to turn on your rear viewscreen. You can watch Jupiter recede from you. When done, press the [Shift+*] keys again to restore forward view. 93. Press the [S] key to stop your ship. Figure 14 – our Sun from Jupiter Celestia Educational Activity 6 – The Life and Death of Stars pg 20 of 29 94. Jupiter and all of its moons will be bathed in heat and electromagnetic radiation too severe and harsh for any life to survive. Even the rocky, ice covered moons of Saturn and Uranus will receive too much radiation to support life safely over the long term. Based on our latest estimates, humanity will have to travel to the farthest moons of Neptune or to our outermost planet, Pluto, to survive the fierce energy pouring out of this Red Giant. 95. Far in the future, if humans are still around, we will have prepared for the end of Earth, and may have moved our civilizations to Pluto and its moon, Charon. Let’s go there now, and see what awaits us. 6. A new Beginning on Pluto? 96. To go to Pluto, click here. Your ship will fly you at hyperspeed to New Pluto space. This tiny world has become the new home of humanity. Zoom in. To see all of the cities, speed up time with the [L] key to view the planet rotating before you, its cities turning on their lights as Pluto turns into the dark side. To see its new and advanced civilization, right-click-drag around the planet. In the distance, the Red Giant glows with heat, fainter and cooler way out here, but still warm enough to keep this world alive and give us a home. 97. Swing the planet around and look for Pluto’s brown moon Charon in the distance. It too has extensive cities and colonies, visible as it too rotates beyond your ship. 98. When done, resume Real time with the [\] key. 99. Answer all questions about the Red giant stage of a star life cycle. Go to Worksheet. 7. The Death of a Red Giant Star – the Planetary Nebula stage 100. Billions of years from now, our Red Giant Sun will burn helium in its core and fuse it into Carbon, Oxygen, Neon and a few other elements. Eventually, however, it will run out of helium fuel in the central core. When that happens, fusion will again stop and again, gravity will grab the star’s internal mass and compress it, crushing it toward the center of the star. The Red Giant’s core will shrink in size. It will happen fast, in a matter of weeks to months. Again, temperatures will skyrocket in the core, this time to impossible temperatures approaching hundreds of millions of degrees. 101. Once again, as temperatures get to such a fierce level, the remaining helium surrounding the now stupendously hot core will be compressed by gravity and again begin fusing in a shell of helium fusion. Beyond that, the remaining hydrogen in the star will begin fusing in a much larger, outer shell of fusion. This will swell the star outward again. It will happen quickly. The star will explode again from the truly enormous energies released by such fusion. This time, however, gravity will not be able to hold the star in one piece. Entire layers of the Red Giant sun will be blown away and be thrown into space, expanding as a ring of debris called a Planetary Nebula (the name has nothing to do with planets). The remains of the star will temporarily stop fusing, gravity will grab what’s left of it and compress it and again, intense fusion will occur in shells around the core, causing another giant Celestia Educational Activity 6 – The Life and Death of Stars pg 21 of 29 explosion and another layer of the Sun will be thrown off. Piece by piece, layer by layer, our Sun will puff up and blow itself to bits, coughing out parts of itself like smoke rings. 102. The layers flying off the Sun form a glowing cloud of debris in space called a Planetary Nebula. 103. When our Red Giant Sun does finally explode layer by layer, it will signal the end for most of the remaining planets in our Solar System. The shock waves and heat from the explosions will incinerate Mars, Jupiter and Saturn, and all of their moons. The shock waves from the expanding debris will deform and possibly blow apart the atmospheres of Uranus and Neptune, an enormous “solar wind” that knocks the gases of the outer giant planets into space. Although Pluto may survive the initial blasts, no human colonies living on it will be able to do so and will be destroyed. In the end, the Sun will lose so much mass that what is left of our solar system including Pluto will either be blown out of orbit and become adrift in space, or be forced to orbit what is left of our Sun at vast distances far beyond their current positions, alone in the frigid cold and darkness of space. 104. Such explosions signal the beginning-of-the-end of the life cycle of a main sequence star similar to our Sun in size, and its Solar System. Over 99% of the stars you see in the night sky will have such a fate. The universe is obviously full of the recent remains of such explosions. Lets’ visit four of them now in the Celestia 2. 105. Press the [Enter] key and type “Eight Burst Nebula”. Note: Your ship’s computer will try to guess where you want to go and display a list below your typing. If you see the name, press the [Tab] key to cycle through the choices. When you stop on the one you want, just press the [Enter] key again and [G] to go there. You will be whisked to a place where a Red Giant star exploded thousands of years ago. Its remains are present now only as an expanding “shell” of gas and atomic debris that is spreading into surrounding space at a speed of Figure 15 – thousands of miles/km every hour. It is made of many the Helix of the lighter elements in the periodic table, from Nebula hydrogen to iron. Year by year, this Planetary Nebula gets wider and dimmer. Zoom in with the [Home] key if you wish and just enjoy its tragic beauty for a moment. 106. Stop two is to the Helix Nebula. Press the [Enter] key and type “Helix Nebula”. Press [Enter] again and [G] to go there. What a dramatic series of explosions, tossing off several layers that are still glowing with energy as they spread out. 107. Our third stop will take you to an older Planetary nebula. This one has been expanding a long time in a nice round spherical shape. You can see through it, in fact. It is called the Abell 39 nebula. To go there, click here. Wild, isn’t it. Right-click-drag around it. It really is here in space and looks just like this. 108. Lastly, click here to go to the famous Ring Nebula. Notice the extent of explosive layers in this nebula. Imagine the fate of any planets near the central Red Giant when it finally ran out of fuel and blew up in a final set of explosions. 109. Where will all of those newly created atoms go as they spread into space? The answer is … quite amazing. They will merge into space, mixing with other thin nebula gases that are already there. Sometime far in the future, they may in fact, become part of a new nebula cloud that is pulled by gravity into a new protostar. In short, the atoms that these exploded stars forge in the furnace of its exploding core may one day recycle and become a new star … or a new planet. Celestia Educational Activity 6 – The Life and Death of Stars pg 22 of 29 110. In fact, you live on such a planet. We are made of such atoms. You have large amounts of carbon and oxygen, nitrogen and calcium in your bodies. The atoms that make you up came from the Earth and the Earth in turn is made of atoms that were created in the heart of an exploding planetary nebula and supernova (more about that shortly) long ago … a star that threw its debris far into space, to become incorporated into the nebula that our Sun and solar system eventually formed from. In a way, we and all life on Earth are related. We are all “children” of the universe. 111. Complete the questions on the Planetary Nebula stage in the worksheet. Go to Worksheet. 8. The White Dwarf and Black Dwarf Stages 112. Look in the center of the ring nebula and you will see a small white star. In fact, this is all that remains of the central core of the massive Red Giant star that blew up here. As it blew up, pieces of its mass were thrown into space to form that nebula, but some of its densest core remained, held closely together by gravity. Over time, gravity compressed that dense material into a super-hot ball of elements called a “White Dwarf”. A white dwarf could still be considered a “star” because it glows very bright for its tiny size, and is ferociously hot (hundreds of millions of degrees) from the heat of that fiery explosion. However, it is actually better considered a huge hot ball of debris. It is no longer fusing elements; it is not burning anything anymore. It is just a super-dense pile of rubble, tightly packed into a glowing hot ball perhaps the size of the Earth (only 1/10,000th the size of the Red Giant from which it remains). 113. All stars that explode into planetary nebula will leave a white-hot dwarf remnant, a core of debris. When our Sun explodes in the future, it too will leave a white dwarf. 114. Let’s visit two white dwarf stars. You’ll discover something very unusual about them. To go to the first, click here. It is the White Dwarf imbedded in the Abell 39 nebula that you visited a bit ago. Time has not been speed up but is passing as Real time. What do you notice about a White Dwarf’s rotation rate? Go to Worksheet. 115. White Dwarfs are also very very hot! White Dwarfs may not be fusing elements, but they are the very dense remains of an explosion that reached a temperature of 200 – 400 million° F. Even in the cold of space, their dense mass takes billions of years to cool off. They also spin … fast. Gravity forces them to rotate rapidly on their axis, due to a law of Physics known as the Law of Conservation of Angular Momentum. As they spin and cool slowly, their brightness and color will begin to fade. Over billions of years, they will change from bright white to yellow, to orange, then to a Red Dwarf. Eventually, they will fade to brown in color and finally … to black. They will cool down to a frozen Figure 16 – a temperature exceeding -400° F below zero White Dwarf star and become just a cold, dense black “lump” of stellar debris in space. It is not Celestia Educational Activity 6 – The Life and Death of Stars pg 23 of 29 clear how many Black Dwarfs actually populate the universe yet. The time for a White Dwarf to cool to black is estimated in 10’s to 100’s of billions of years, and the universe may not yet be old enough for many White Dwarfs to have cooled down to black. 116. The Celestia 2 is programmed to take you to an older White Dwarf, adrift in the Milky Way just 15 light years from Earth. To do so, click here. The white dwarf in front of you is named Van Maanen’s star or Wolf 28 (named after the men who found it independently). It has slowed down a bit from TYC-2-900-0 due to its age and has cooled off somewhat. What is amazing, however, is the fact that it will still spin like this silently in space here for billions of years more (it makes no noise), slowly fading in color and temperature … slowly cooling off. 117. What will the end of our Sun’s life cycle look like? Let’s visit a theoretical “Black Dwarf”. Click here. In the distance, almost invisible, is a model of a fading black dwarf. It is centered in your screen. Can you see it? If not, right-click-drag and see if you can find it. Press the [G] key once to go to it and tap the [L] key once. If we could visit a Black Dwarf, this is all that would remain of a once mighty star. It is fading now. Its spin has slowed to a crawl. Just a dim glow remains of the fires that once powered this star. The nebula that it threw off is long ago gone, spread out into wisps of gas and dust in space. In a few billion more years, this dim glowing blob of matter will be as black as the blackness of space. Here, it will drift throughout the Milky Way, possibly for as long as our galaxy lasts. If it once had planets, most would be long destroyed or lost to space. It is possible, however, that far from this dwarf, there may still be a few lone asteroids or frozen comets or even a small planet remnant circling it, orbiting it in a cold lonely dance that will continue forever. This is the end of a star the size of our Sun. This will be the end of our Sun also. Return to Celestia and contemplate it for a few moments. 118. Go to the worksheet and complete all questions on White and Black Dwarfs. Go to Worksheet. 9. The Death of Large Stars – Supernova Stage 119. What of the giant stars, the massive Red Supergiants like Betelgeuse or Mu Cep? Will they also end their lives in a series of explosions, producing a series of expanding shells of gas (planetary nebula)? The answer is no! Their fate is much more violent … and unbelievable! 120. When a Red Supergiant runs out of helium fuel, its nuclear fires in its core calm down and like its smaller cousins the Red Giants, gravity pulls it inward and it collapses. Again, its core, which is now primarily carbon and oxygen, heats up as gravity collapses the star, shrinking it in size to a diameter not much larger than Earth. The core’s temperature and pressures now skyrocket to levels almost impossible to conceive … temperatures far above a billion degrees. There, an enormous amount of atomic mass, far more than a normal Red Giant, is ready to blow. At that point ... Figure 17 – a Red Supergiant goes Supernova Celestia Educational Activity 6 – The Life and Death of Stars pg 24 of 29 121. The entire Red Supergiant’s core begins fusing into heavier and heavier elements, virtually all at the same time. In a few seconds, nuclei are crushed into each other with such force that ALL of the 90 elements in the Periodic Table are created. This furious fusion inside a celestial body millions of millions in diameter causes the star to explode with a force that staggers the imagination. The explosion is called a Supernova! 122. Gravity has no chance of containing such an explosion. The star is literally blown to bits in a cataclysmic explosion of epic proportions. It disintegrates and rushes outward at enormous speed of several million miles/km per hour. 123. A major part of the star’s mass is thrown into space. All of the newly formed atoms and nuclei that were created by fusion near the core in a few moments, become debris shot at high speed into space. Over time, that debris will mix with hydrogen, helium and other atoms from other star explosions in space and become part of a new nebula. Millions, perhaps billions of years later, a new star may form from that nebula material … a star with planets and a solar system. 124. As you read this, realize this! The hydrogen in your body dates back to the beginning of the universe over 13.7 billion years ago. The carbon that makes up your muscles, skin and bones came from an exploded planetary nebula some time billions of years ago. The heavier elements in your body, such as magnesium, potassium, iron, silicon, etc. were formed inside the core of a massive Red Supergiant star that exploded in a Supernova explosion over 6 billion years ago, somewhere in our area of space. Our Sun and all of its planets and moons (including Earth), formed from part of that nebula. You are literally made of the stuff of a violent universe. 125. To see an animation of what a Supernova might look like, click here and wait for the movie to open. Press [Alt + Enter] to go to full screen. Press again and close the movie screen when done. 126. The Celestia 2 is ready to take you to a Supernova explosion in space. Press the [Enter] key, type “M1”, press [Enter] again and [G] to go there. You will be flown at hyperspeed to the location of one of the largest explosions ever to occur in the neighborhood of our part of the Milky Way. You are looking at the Crab Nebula. Zoom in to a distance of about 10 ly. 127. In the year 1054 CE, some folks in China noticed a bright star in the sky that suddenly appeared. It was visible even in the daytime for several weeks. Within a few months, its brightness had faded to a dim glow, visible only at night. They recorded the sighting and position in some ancient writings dating from that time. 128. Today, modern telescopes have found that location and have discovered ... the Crab Nebula. It is the site of a Supernova explosion less than 1000 years ago. It is a long way from our Sun (over 6,000 light years), but that is good. Had it been much closer, the Earth would have received not only visible light from the explosion, but some of the deadly radiation that poured out of the Red Supergiant that blew up here. Had it been 100 times closer to Earth when it blew, it could have caused a mass extinction of much of life on Earth, and none of us would be here today. 129. The Crab Nebula is one of hundreds of such locations that have been chronicled in our night sky. The stellar debris you see as colored nebula represents all 90 elements (they glow different colors). It is still expanding, spreading into space at high speed. It will continue to do so for thousands of years. Slowly, that debris will mix with the gases of deep space and will stop glowing. Hundreds of thousands of years from now, the Crab will fade from view. Those atoms, however, will remain in space, available to form new stars someday, should gravity re-organize this region of space into a new stellar nursery. 130. Answer the questions on supernova. Go to Worksheet. Celestia Educational Activity 6 – The Life and Death of Stars pg 25 of 29 10. The Death of Large Stars Neutron Star and Pulsar Stage 131. From our earlier discussion, please recall that when a Red Giant blows up, some remnant of the core remains as a super-hot, dense White Dwarf. Well, something similar occurs when a larger Red Supergiant star blows up in a Supernova explosion. This time, however, things get really strange. 132. A supernova explosion is so enormous that when it blows, the energy is sent in all directions at once. Not only does the explosion expand outward, but it also squashes inward. The central core of the massive Red Supergiant is compressed by such titanic forces so hard, that the matter trapped there is squashed into a sphere no bigger than about 10 miles across. 133. That sphere is called a Neutron Star. It has that name because the compression and enormous forces change the nuclear structure. The electrons and protons that make up a large part of the mass of all stars are crushed together so severely that they merge and become neutrons. The entire central core of the exploding Supernova becomes one, incredibly dense, squashed, super-hot mass of neutrons. 134. The properties of a Neutron Star are so strange that they are hard to believe. First, they are the most dense objects in the visible universe. The neutrons are packed so tightly together that there is simply no empty space ... neutrons are practically touching each other. There are so many of them that the mass of an entire star is packed into a volume only a few miles across. This results in gravitational forces being emitted so powerful that if you could somehow collect 1 teaspoon of a neutron star, it would weigh close to 100 million tons. Imagine it … a teaspoon weighing 100 million TONS. If you put such a weight on the ground of Earth, it would instantly disappear into the ground, boring its own hole all the way to the center of the Earth. Were you to get too close to a neutron star, its gravity would pull you and your ship down to it to splat onto its glowing solid surface like a drop of hot oil on a hot skillet. Every atom of your body and ship would be plastered against the surface of the star and ripped apart to be converted to neutrons. In short, you would become a very thin, super-hot, neutron film smeared onto the star (Ugh!). 135. A second amazing fact is a neutron star’s spin. As it is crushed into a tiny ball during the supernova explosion, the mass of a neutron star begins to spin as it shrinks and packs more tightly together. It is the same principle that causes white dwarfs and nebula and stars to spin … the principle of Conservation of Angular Momentum. In this case, a neutron star’s spin becomes incredibly fast. A newly formed neutron star will spin on its axis faster than an ice skater spinning in her best pirouette. The spin can exceed a whopping 50 times a second. As it spins, a neutron star is also hot … incredibly hot. It is truly a hot, blue-white whirling demon! 136. To witness a neutron star up close and personal, click here. The Celestia 2 will take you safely to the outskirts of one. Press the [A] key and accelerate to between 5 – 6 km/s. As the star approaches, tap the steering keys and fly closely by it on the left or right. After you pass it, press the [S] key to stop your ship. 137. Although no nuclear fusion is occurring, energy and heat are pouring off the star. As such, Neutron stars are just like White Dwarfs. They are super hot, they spin, and they take billions of years to cool off. Eventually, they too will end their existence as a Black Dwarf. Celestia Educational Activity 6 – The Life and Death of Stars pg 26 of 29 138. Sometimes, however, nature has an even weirder surprise. Whenever a Supernova explodes, titanic magnetic forces are also being generated around the star. In some newly formed neutron stars that are still surrounded by the debris from a recent supernova explosion, gravity will pull that supernova debris down toward the neutron star, accelerating it to enormous speeds. It will then impact onto the surface of the neutron star. The result is what can best be described as a “squirt”. Matter circles the neutron star as it Figure 18 – the Pulsar falls in and some of it is squirted back away from the star at high speed, channeled into two narrow beams of matter called “jets” by the steering effects of the magnetic fields. Sometimes the jets point away from the Earth and we don’t see them. Sometimes, however, they point directly at Earth. As the star spins, what we will see is twin concentrated jets of matter going by rapidly. The star will appear to pulse or blink. If it does, the neutron star is called a Pulsing Neutron Star, or Pulsar for short. 139. The Celestia 2 contains a pulsing neutron star in its database. In fact, it is inside the Crab Nebula. The neutron star that formed inside the Crab is in fact, a Pulsar. Its jets happen to be pointing toward Earth. Let’s go there again. To do so, click here. Then press the [G] key. When you get closer, zoom in with the [Home] key. 140. The Crab Pulsar is spinning 32 times per second. To stop its spin a moment, press the [Spacebar] at the bottom of the keyboard. This pauses time. Press it again to resume Real time. The jets of beamed energy sweeping across your ship are in fact, horrendously deadly. They are concentrated beams of high-energy photons that would kill you instantly if your ship did not have good protection. The gravity from the neutron star is also pulling you toward it. Only your ship’s automatic controls are keeping you from splatting onto the surface. 141. This pulsar is really there in space at this spot and will spin like this for millions of years. Slowly, over time, the spin will slow down. The rest of the supernova gases that are feeding this hot core will slowly spread out across space and the neutron star will run out of material to fall onto it. The jets will die away. Its color will fade from white hot, to red, then to maroon, then eventually to black. Billions of years from now, this neutron star will become a super-dense cold and dead Black Dwarf, drifting through the galaxy like all of its brethren. 142. Complete the questions on Pulsars. Go to Worksheet. Celestia Educational Activity 6 – The Life and Death of Stars pg 27 of 29 11. The Death of Large Stars The Black Hole 143. The universe is surely strange, but it has left its strangest thing for last. 144. On a rare occasion, a REALLY BIG Red Supergiant blows up in a supernova explosion (a star like Mu Cep or Antares). When that happens, the core of the star will be crushed inward and a neutron star will form. However, … gasp! … scientists have discovered that if there is enough material available to be crushed, the diameter of the neutron star will not stop at 10 miles or so. It will be crushed to a size so small that the object becomes “impossibly” dense. It is crushed far beyond the neutron star stage. All of its neutrons in fact, become crushed by the explosion so much that they literally squash “out of existence”. They stop being made of matter. They become … well …. nothing! They actually disappear from the universe. They create a “hole” in space itself. The hole is very tiny. In some cases, it can be smaller than the nucleus of an atom. It is called a singularity. This place … this hole in space … is called a “Black Hole”. It really is black. Gravity is so strong here that nothing, not even a beam of light, can escape. Nothing that falls into or is sucked into a Black Hole is ever heard from again. Even light falling into the hole disappears. 145. The gravity of a Black Hole is so strong that even space itself is sucked into it for some distance around it (Huh????). The tiny singularity in the center creates a bigger zone of complete blackness around it that may be 10 miles or more in diameter. The edges of that zone are called the “event horizon”. Anything … matter, light and space itself … within the event horizon is sucked out of existence. Nothing can be seen coming from it. 146. What would a Black Hole look like? Let’s go find out. Click here. You will be taken far from our solar system to a place where very long ago, a massive supernova explosion occurred. The nebula cloud of debris has long ago spread out into space or been sucked into ? Nothing is visible except stars, the Milky Way galaxy and … a Black Hole. Can you find it? It is small and silhouetted against the haze of the Milky Way. Search carefully. To go to it, press the [G] key. 147. The Black Hole in front of you is spinning at close to 100 times per second. Although it is a “hole” in space, it is a spinning hole. It is the weirdest thing to try to imagine. Its gravity is so powerful that if you were in front of it in real space-time, your ship, your body and every atom in you would be instantly ripped apart and be sucked down into that black tear in space. The eerie glow around it is actually wisps of stray interstellar gas being sucked into the hole and falling inward, glowing as it accelerates and disappears from the universe forever, almost as if the gas was “screaming” as it fell in. 148. Because the star it formed from once orbited the Milky Way as one of billions of such stars, so does the Black Hole. It moves silently and eerily through space, orbiting the Milky Way at a similar rate as its now destroyed parent star, silent and deadly, a one-way trap to oblivion. 149. As it moves, Black Holes can encounter other objects. On occasion, things do get too close to a Black Hole. Entire stars, dense nebula gases, planets … anything that strays too close will be ripped apart by the gravity and sucked into the hole, disappearing forever. When that happens, the gases and atoms falling into the hole accelerate to very high speeds as they are sucked in. The atoms collide with each other on the way in and frequently will glow with energy as they are ripped apart. That glow is visible to us folks on Earth as a glowing “disk”, spiraling around the jet-black event horizon of the hole in the center. It is called an “Accretion Disk”. 150. The Celestia 2 is programmed to take you to a rotating Black Hole with an accretion disk of matter falling into it. To go there, click here. Celestia Educational Activity 6 – The Life and Death of Stars 151. That colored disk is nebula and star material being sucked into the black hole. As it does so, it is rotating around the hole. Let’s challenge your ship and you in piloting skills. Aim for the rim of the accretion disk and press the [A] key to start your engines. Accelerate to about 2.5 km/s and fly toward the black hole, skimming just along the top of the accretion disk (see figure 19). At the last minute, pull up, or even the Celestia 2 will not be able to save you from instant destruction. pg 28 of 29 Figure 19 – being sucked down a Black Hole 152. Black Holes appear to exist forever. Once “space” itself is punctured with this incredible object’s presence, we do not believe a repair occurs. The “hole” stays there, drifting through space. Right now, a Black Hole could be drifting through our part of the Milky Way galaxy. Someday, one could even drift into the neighborhood of our Sun. If that occurred, it might be possible that our solar system (and us along with it), could be sucked into the passing hole. That would be the end … of us. 153. In summary, the life of a massive star ends when it blows up in a supernova explosion and its remaining core forms a Neutron Star, which may or may not become a Pulsar. In either case, the neutron star or pulsar will eventually become a cold Black dwarf. A supernova core, however, can instead quickly compress past the neutron star stage directly into a rotating Black Hole. 12. Conclusion 154. We hope you have enjoyed this celestial journey into the life cycle of stars aboard the Celestia 2. If you have not done so, we urge you to explore our other Celestia activities and enjoy the program on your own. Our new educational website has great facts and pictures about this amazing universe we call home. 155. If you have been taking this voyage of discovery as part of a school lesson, and have been writing your answers on a printed worksheet, it is time to turn your work into your teacher. Make sure your name is on the top of the worksheet. To close this document, simply click the small “X” in the top right corner, or follow your normal procedures for closing documents, if you are running another operating system. You may close Celestia or continue exploring the universe on your own. We urge you to journey through other Celestia Educational Activities at http://www.fsgregs.org/celestia or at the Celestia add-on repository at http://www.celestiamotherlode.net Celestia Educational Activity 6 – The Life and Death of Stars pg 29 of 29 13. Credits: This document was written by Frank M. Gregorio, Manassas, VA Version 6, rev 4. revised September, 2004 Copyright © 2003, 2004 - Frank M. Gregorio Celestia – courtesy of : Chris Laurel Clint Weisbrod Fridger Schrempp Bob Ippolito Christophe Teyssier Copyright © 2000, 2004 - Chris Laurel Many thanks to the gifted artists and graphics designers who contributed add-on textures to this Activity. Star Textures - Copyright © and courtesy – Kris Rassilon Red Giant texture - Copyright © and courtesy – Kris Rassilon Dying Earth Textures - Copyright © and courtesy – Don Edwards Dying Mars Textures - Copyright © and courtesy – Kris Rassilon Dying Jupiter Textures - Copyright © and courtesy – Kris Rassilon New Pluto and Charon textures - Copyright © and courtesy – Don Edwards Protostar textures - Copyright © and courtesy – David Nessler Protostar with jet textures - Copyright © and courtesy – Kris Rassilon Brown Dwarf - Copyright © and courtesy – Frank Gregorio Betelgeuse Surface texture - Copyright © and courtesy – S. Bowers Neutron star - Copyright © and courtesy – Frank Gregorio White Dwarf - Copyright © and courtesy – Black Hole Accretion Disk- Copyright © and courtesy - Jorge Omar Leyra Black Hole 2 texture - Copyright © and courtesy – Frank Gregorio Black Dwarf - Copyright © and courtesy – Frank Gregorio Orion Nebula - Copyright © and courtesy – Mind-toAsk Eagle nebula - Copyright © and courtesy – Mind-to-Ask Crab Nebula - Copyright © and courtesy – Killeen Rosette Nebula and planets - Copyright © and courtesy – Kris Rassilon Pulsar Model - Copyright © and courtesy – Kris Rassilon Ring Nebula - Copyright © and courtesy - Vandenrijt Jean-François Eight Burst nebula - Copyright © and courtesy – Killeen Abell 39 nebula - Copyright © and courtesy – Astroboy Helix Nebula - Copyright © and courtesy – Astroboy Supernova animation - Copyright © and courtesy – NASA Hubble Space Telescope Institute Supernova drawing © 2003, Patrick Towen Pulsar Drawing © 2002, David King Nebula Solar Wind drawing - © 2002, David Hardy Permission is granted to copy and distribute this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover Texts, and with no Back-Cover Texts. A copy of the license is included in "GNU Free Documentation License".