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C05_232-237_RecipeforStar 7/12/05 5:36 PM Page 232 Recipe for a Star Several factors are important for star formation. All of these factors depend on the way materials interact. That is what you have studied in this unit, Matter Is Marvelous. First, gravity pulls a cloud of hydrogen gas together. Second, density increases within the cloud. The center of the cloud gets extremely hot. After a great deal of time, sometimes millions of years, the star finally “turns on.” What does that mean? What has to happen for a star to “turn on”? In this reading, you will follow a recipe that turns hot, dense matter into a star. Do you realize that virtually every element on Earth was once part of a star? Processes in stars formed the carbon in animals and plants, iron in your blood, nickel in your pocket, and oxygen you breathe. To understand how stars make these elements and how these elements got to Earth, you need to understand what makes a star. Imagine that there is a recipe for making stars. The recipe starts the same way for all stars. However, the end of the recipe varies because stars have a variety of masses. Look at the recipes for stars of different masses in figure 5.17. ) un ss (M S n’s ma ars the Su ss St s a n e u M m M S ti 12 d 12 Low M star< .25 an een 0 un< sity ity. 0.25M S (M star) is betw h den f grav r ans o igh enoug of sta e m y Mass h b with a ns. rogen ct hyd ve a cloud ion reactio time. e ll o C s a ng 1. right. uh r t fu for a lo s, this is b en yo e, sta n io s e 2. Wh emperatur u y ue f ur e em to and t ar to contin r. Close yo rs. Use th w st t sta r laye 3. Allo a red gian move oute e e k R a . dwarf. ore 4. M pse c . white rm a a ll a o m C r 5. nebula and fo ntly, and fo make re to cool nifica o c w ol sig 6. Allo core to co w 7. Allo dwarf. c la b k Figure 5.17 Recipe cards for stars. You can think of star formation in terms of recipes. The recipe for a star’s stages of formation depends on the original mass of gas in the nebular cloud. 232 Unit 1 Matter Is Marvelous H ig h - Ma s s S ta 12M rs Sun <M star <4 0M 1. Fo Sun llo 2 . F u w S te p s s 1 3. Fo e a new e –4 of the rm n ew la lement in Low-Mas 4. Try ye s th t 5. Wa o fuse iro rs from p e core o Star recip f th e revio n. S t tch o star. e. u a 6. S p u ew re t! Star w r will colla s core. ill ex 7. P r pse. mnan plo oduc e a n ts into sp de form ing a e u tro a n sta ce. supe r from rnova . th e c ollap sing core. Stars . Mass h g i ecipe rh Star r s Supe <M star s a a M Highrming 40M Sun of the llapsing, fo ar it. 6 – 1 t ne e co teps ow S ing ge ontinu 1. Foll core to c t let anyth w no 2. Allo hole. Do k c la b C05_232-237_RecipeforStar 7/12/05 5:36 PM Page 233 The general categories are low-mass stars, such as the Sun, with a mass of MSun, and high-mass stars a lot more massive than the Sun. As you will see, you can predict what elements a star can produce by noting the starting mass of the star. So making a star looks simple, doesn’t it? In fact, recipe cards are easy to follow if you know what each step means. For example, you might have tried to follow a recipe for making a cake, only to get to a step that did not make sense, such as “fold in the egg whites.” Coming across this step in a cake recipe, you might wonder how you “fold in” egg whites. As you read these recipes for stars, you might have trouble following them without getting some explanation of each step. Make note of these questions in your science notebook and read on to learn more. Recipe for a Low-Mass Star Step 1: Collect hydrogen and dust. A star starts out as a huge cloud of hydrogen gas. Gravity pulls the cloud together. The cloud begins to spin. This spinning cloud is called a protostar. Compared with actual stars, protostars are very bright, but their atmospheres are still relatively cool (3,000 Kelvin [K]). Step 2: Start fusion reactions. The protostar continues to contract. High energy strips nuclei of their electrons. Hydrogen nuclei collide constantly. As gravity continues to pull the nuclei inward, the cloud becomes denser. Hydrogen nuclei collide more frequently. The collapse causes the temperature and pressure in the protostar to rise. When the temperature gets high enough, hydrogen nuclei have enough energy to break through the electron repulsion of other hydrogen nuclei. Nuclear reactions then take place. Hydrogen nuclei begin to fuse. The name of this process is nuclear fusion. When nuclear fusion occurs, the star “turns on.” Hydrogen fusion results in the formation of a heavier element, helium. Vast amounts of nuclear energy are released, increasing the temperature of the star and giving off light. Step 3: Continue fusion. The energy of nuclear fusion causes the star to shine as the radiation moves outside of the core (see figure 5.18). The Sun is currently at this stage. Early stages of H-Fusion in a low-mass star Hydrogen fusion in star core (4H He + energy) Outer hydrogen layer Figure 5.18 Burning star. In the early stages of hydrogen fusion in a low-mass star, the core is surrounded by a thick outer layer of nonburning (nonfusing) hydrogen gas, which shines. Step 4: Make a red giant. Eventually, the amount of hydrogen decreases, which decreases the amount of hydrogen fusion. The star stops producing energy to shine. Without the outward pressure created by nuclear fusion, gravity causes the star to collapse. This initial collapse causes the core to get even more dense. Particles in the core collide more frequently and with greater energy. The core gets hotter. Chapter 5 Star Material 233 C05_232-237_RecipeforStar 7/12/05 5:36 PM Page 234 The higher temperature causes the outer layer to expand and cool. Now the star has a dense, hot core with an expanded outer layer. The star is called a red giant because it is brightest in the red portion of the spectrum. Because the star’s diameter is larger, it is much brighter than it was as a normal star. Red giants may vary in size depending on their mass. An example of a red giant is the star called Aldebaran, in the constellation Taurus. As described in the next step, this type of star can make carbon. Step 5: Collapse core. Make nebula. The core of a red giant is full of helium. This core continues to collapse. The core becomes hot and dense enough for helium fusion. When this happens, helium nuclei fuse to form carbon nuclei. The periodic table shows that carbon is heavier than helium or hydrogen. As a result, gravity pulls carbon to the star’s core. Without their electrons, helium and hydrogen gradually concentrate in surrounding outer layers because red giant core they are less dense than carbon. The core of the star is nonfusing carbon, surrounded by a shell of mostly helium, then another shell of mostly hydrogen (see figure 5.19). The helium fusion in the core continues until helium is all used up in making carbon. Most of the red giants’s mass remains concentrated in the dense carbon core. These red giants are not hot enough to start carbon fusion. This means that carbon does not change to heavier elements. Red giants, therefore, are the main source of carbon for life in the universe. In fact, most of the carbon in your body was produced in red giants that shone long before Earth was formed. Time passes. The red giant’s layers expand. These outer layers keep expanding. Eventually, these layers escape the surface of the star. They are literally blown into space. The matter from these layers becomes a nebula. A nebula is a cloud of gas and dust that floats around a former star. Light from the star allows astronomers to see the nebula. Figure 5.20 shows one such nebula. carbon core He–fusing shell H–fusing shell nonfusing H shell Figure 5.19 Red giant. This figure shows a four-layered star. It is large and luminous, making it a red giant. The core consists of carbon surrounded by helium and then hydrogen shells undergoing fusion. These shells are then surrounded by hydrogen gas in a thick outer layer that is not undergoing fusion. 234 Unit 1 Matter Is Marvelous Figure 5.20 Helix Nebula. The Helix Nebula is a cloud of dust and gas left over from the red giant stage. It is one of the closest and largest planetary nebula. The nebula is about 450 lightyears (ly) from the Sun in the direction of the constellation Aquarius. C05_232-237_RecipeforStar 7/12/05 5:36 PM Page 235 Step 6: Allow core to cool to a white dwarf. You might predict that the carbon atoms will fuse. This does not happen. There is not enough energy in a low-mass star to fuse carbon. The low energy makes it impossible for carbon to fuse. Much of the star’s mass is blown away in the nebula; the remaining core becomes a white dwarf. It is called a white dwarf because it is still very hot (white hot), but also small (dwarf) and not very luminous. White dwarfs are dim and hot stars. Step 7: Let core get cold and become a black dwarf. Eventually, white dwarfs cool completely. They become cold black dwarfs. The life of a low-mass star is over! Recipe for a High-Mass Star To review, the recipe for a low-mass star begins with a protostar that becomes a lowmass star, usually less than eight times the mass of the Sun. Then in a series of steps, the star changes into a red giant, a nebula, a white dwarf, and finally a black dwarf. But not all stars begin with a low mass. Some stars begin fusion with much more hydrogen in the initial cloud. How do you make a highmass star? To find out, follow these easy recipe directions. Step 1: Follow Steps 1–4 of low-mass star recipe. To make a high-mass star, follow the recipe for a low-mass star through Step 4. Then switch to the recipe card for high-mass stars and begin with Step 2, which follows. Step 2: Use nuclear fusion to make carbon in core. The core of a high-mass star collapses rapidly through the force of gravity. It becomes dense and hot enough to make three helium nuclei fuse. This results in one carbon nucleus. Carbon then can fuse with itself and lighter nuclei to make heavier elements. This is where things really get exciting! For example, how can a star manufacture elements with an atomic mass greater than carbon? The star has to fuse carbon with hydrogen to form nitrogen. You probably know that nitrogen is a vital element for life. If carbon fuses with helium, it forms another vital element for life, oxygen. The core region of the star now consists of a mixture of carbon, nitrogen, oxygen, helium, and hydrogen. All of those elements are undergoing nuclear fusion and beginning to separate into distinctive layers. Step 3: Form new layers as more fusion reactions begin. The number of fusion reactions increase, making more and more nitrogen, carbon, and oxygen. These reactions produce heavier and heavier elements from the periodic table. Gravity draws these elements toward the core of the star. Lighter elements move outward to form a layer. The heaviest elements in the core then undergo more nuclear fusion, making heavier and heavier elements. Heavier elements displace lighter elements, forcing them outward from the core. The star has a layered appearance, something like the concentric layers of an onion. Fusion reactions continue in the core of a high-mass star. Each new layer results from the fusion deep in the core. The star continues to produce layers corresponding to heavier elements such as oxygen, neon, magnesium, silicon, and sulfur. As you will see, this sequence of reactions slows down when the core of the star consists of greater and greater amounts of iron. Iron is not able to begin fusion because the star Chapter 5 Star Material 235 C05_232-237_RecipeforStar 7/12/05 5:36 PM Page 236 Multilayered Fusing Star nonfusing hydrogen hydrogen fusion helium fusion carbon fusion oxygen fusion neon fusion magnesium fusion silicon fusion iron core Image of Betelgeuse Betelgeuse core Size of Star Size of Earth's Orbit Size of Jupiter's Orbit Orion Constellation Figure 5.21 Multilayered fusing star. The diagram shows the core of a high-mass star in the supergiant stage. The center of the core consists of iron (a nonfusion core). Layers surrounding the core are undergoing nuclear fusion reactions and consist of shells of Si, Mg, Ne, O, C, He, and H. The nine-layered core would be surrounded by a thick, nonfusing hydrogen gas shell. These layers mix some at boundaries between layers. does not produce enough energy to make iron nuclei fuse. The iron then forms a nonfusing core. Astronomers say that this part of a star’s life is the supergiant stage (see figure 5.21). An example of a red supergiant is Betelgeuse. Figure 5.21 shows Betelgeuse at Orion’s left shoulder. What is the pattern you see in the layers of elements when you locate the elements in the periodic table? Step 4: Try to fuse iron nuclei. The star will collapse. Fusion continues until elements with atomic numbers 1–26 form. On the periodic table, those are all of the elements up to iron. When the star reaches this stage, things get even more exciting because, until now, fusion reactions produced heat. In contrast, the fusing nuclei of iron absorb heat. 236 Unit 1 Matter Is Marvelous The iron core of the star absorbs energy from the layer above it. The outward force to expand the star decreases. Gravity pulls the material inward from outer layers. The star collapses on itself extremely rapidly, and the core temperature and pressure increase tremendously. Soon the temperature becomes so hot that nuclei fall apart into protons and neutrons. The core becomes a soup of protons, neutrons, and electrons. Step 5: Explodes to a massive-star supernova. (Stand back and wear your safety goggles!) Still, the collapse continues. Eventually, it squashes protons and electrons together, causing them to become neutrons. The star is now a core made of neutrons surrounded by collapsing outer layers. The star “bounces” against its own neutron core, and all of the layers explode into space, leaving the core behind. C05_232-237_RecipeforStar 7/12/05 5:36 PM Page 237 This explosion is known as a massive-star supernova. The explosion is so powerful that it can briefly outshine an entire galaxy. It is the most violent and spectacular explosion in the universe. While the core explosion occurs in about one second, the massive-star supernova may continue to shine for weeks or months. Massive-star supernovae are incredibly important because a flurry of fusion reactions before and during the explosions creates all elements in the natural environment with atomic masses greater than iron. Look at the periodic table. How many elements have you heard of with atomic masses greater than iron? These less familiar elements are rare because they form only during a massive-star supernova explosion. Step 6: Scatter remains of massive-star supernova event. Take everything from the supernova explosion and scatter it in space. This will form a blurry-looking nebula around the exploded core of the star. This matter may eventually become the building material for the other bodies in space, such as planets like Earth, and your very own body! Step 7: Collapse core to a neutron star. All that remains of the core after a massivestar supernova event is a very hot, small, and dense core of neutrons. This is called a neutron star. A neutron star is so dense that 1 tsp of it would weigh 100 million tons on Earth! Extra recipe steps for extra high-mass stars: Make a black hole. If the remaining mass at the core is great enough, the core of a neutron star can collapse even further. After blowing away much of the star during the massive-star supernova, the remaining mass of the neutron star collapses to a black hole. These bizarre objects are areas in space that pull in everything around them—even light! Because they do not emit light, black holes are invisible. If black holes are invisible, what makes astronomers think they exist? Astronomers know black holes exist because they can watch the behavior of stars and other material orbiting around black holes. People also have observed stars, matter, and even light being pulled into regions of space that could be black holes. In fact, it is a good thing that the solar system is 28,000 ly from the center of the Milky Way galaxy—strong evidence indicates that the center of our Milky Way contains a black hole! Chapter 5 Star Material 237