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Bell work What are stars made of ? How do stars differ from one another? Do stars move? Write your answers in your science journal. Then, after you have completed this section, review your responses and change them if necessary. Stars Objectives for today! Describe how color indicates the temperature of a star Explain how a scientist can identify a star’s composition Describe how scientists classify stars. Describe different types of stars. Describe the quantities that are plotted in the H-R diagram. Explain how stars at different stages in their life cycle appear on the H-R diagram. Twinkle, Twinkle little star, how I wonder what you are? Most stars look like faint dots of light in the night sky. But stars are actually huge, hot, bright balls of gas that are trillions of kilometers away from Earth. Twinkle, Twinkle little star, how I wonder what you are? A star is made up of different elements in the form of gases. The inner layers of a star are very dense and hot. But the outer layers of a star, or a star's atmosphere, are made up of cool gases. Blue is hot and red is not? Although red and yellow may be thought of as "warm" colors blue may be thought of as a "cool" color, scientists consider red and yellow to be cool colors and blue to be a warm color. For example, the blue flame of the Bunsen burner is much hotter than the yellow flame of the candle. How bright are stars? The brightness of a light or star is called apparent magnitude. The closer a star is the brighter it may appear. Astronomers use a star's apparent magnitude and its distance from Earth to calculate its absolute magnitude. Absolute magnitude is the actual brightness of a star. If all stars were the same distance away, their absolute magnitudes would be the same as their apparent magnitudes. Variable star- a star whose brightness appears to change 1. Absolute vs. Apparent Magnitude 1. 2. 3. Variables which affect a star’s brightness: Star size Distance from Earth Star temperature Apparent magnitude: The amount of light received on Earth from a star. Absolute magnitude: How large and hot a star actually is in relation to other stars. Star Brightness Example: (Fig. 20.1) Sirius has a greater apparent brightness then Rigel, even though Rigel is a much hotter and brighter star. Why? Sirius is closer to Earth! Scientists classify stars by temperature and brightness. Objectives for today! Describe how color indicates the temperature of a star Explain how a scientist can identify a star’s composition Describe how scientists classify stars. Describe different types of stars. Describe the quantities that are plotted in the H-R diagram. Explain how stars at different stages in their life cycle appear on the H-R diagram. A. The Sun and You Our sun is a main sequence star according to the HR Diagram. The actual brightness is average for a star of its average size. 2. Layers of the Sun (Fig. 20.9) Dense inner core which is the site of hydrogen fusion. Radiation zone: Energy bounces back and forth before escaping. Convections zone: Cooler layer of gas that is constantly rising and sinking. Anatomy of Sun Photosphere: Bright source of much of the light we see. Chromosphere: Active layer which is home to many significant displays. Anatomy of Sun Corona: Outer layer which is a gradual boundary between sun and space. 3. Sunspots Sunspots: Cool dark areas on the sun’s surface. -First discovered by Galileo -Not permanent features—Will appear and disappear 4. Prominences and Flares Prominence: A huge arching column of gas. 4. Prominences and Flares Solar Flares: Violent eruptions near a sunspot which suddenly brighten and shoot outward at high speed. Distance to stars Because stars are so far away, astronomers use light-years to measure the distances from Earth to the stars. A light-year is the distance that light travels in one year. Light-year: (Equal to about 9.5 trillion kilometers) Light-Years It is easier to give the distance to the North Star as 431 light-years than as 4,080,000,000,000,000 km. Approximate distances: -Sun to edge of solar system = 5.5 light hours -Nearest star (Alpha Centauri) = 4.3 light years -Center to edge of Milky Way = 50,000 light years Objectives for today! Describe how color indicates the temperature of a star Explain how a scientist can identify a star’s composition Describe how scientists classify stars. Describe different types of stars. Describe the quantities that are plotted in the H-R diagram. Explain how stars at different stages in their life cycle appear on the H-R diagram. Life cycle of Stars There are two paths that stars can take, based on their mass. The 1st is for low and medium mass stars. The 2nd for high mass stars. st 1 All stars start as nebulas A cloud in space Made of gas and dust Can have stars inside Most of the ones we see are inside our Milky Way Galaxy Large, massive, bright nebulae Emission Nebula •The hot gas is emitting light Orion image at http://hubblesite.org/newscenter/archive/releases/2006/01/image/a/results/50/ Colder, darker nebulae Dark dust blocking the hot gas behind it NOAO/AURA/NSF Image from http://hubblesite.org/newscenter/archive/releases/nebula/dark/2001/12/image/c/results/50/ Protostars Gravity pulls huge nebulas of hydrogen gas and dust into a single spinning cloud. As the particles crash into each other, the matter heats up….about 15 million degrees. Fusion begins….. The mass begins to shine brightly. A star is born! Young stars form in nebulae from Small Magellanic Cloud Image at http://hubblesite.org/newscenter/archive/releases/2007/04/image/a/results/50/ Star-forming region in the Large Magellanic Cloud: http://hubblesite.org/newscenter/archive/releases/2008/31/image/a/results/50/ Interstellar “eggs” Movie at http://www.stsci.edu/EPA/PR/95/44/M16.mpg Medium-Sized Stars Hydrogen fuel is fused into helium at the star’s core until the hydrogen is used up. The core shrinks, heats up, & releases energy. The energy causes the star’s outer layers to expand, cool and become redder. The core heats up and releases energy. The star heats, expands, reddens and becomes a …… In a few Billion years… Red Giant Image at http://hubblesite.org/newscenter/archive/releases/1997/26/image/a/ Red Giant The helium core continues to heat. At ~ 200 million degrees the helium atoms fuse to form carbon atoms and the last of the hydrogen gases drift away. These drifting gases form a nebula ring around the star. The star is dying. It grows fainter and fainter. Gravity causes its matter to collapse inward. The star is squeezed into a…… White Dwarf They are superdense stars with carbon cores. Since all of the mass of a star the size of our sun has been squeezed into the size of the earth, all of the atoms are crowded together. By 5 billion years… White Dwarf Small, but very hot Image at http://hubblesite.org/newscenter/archive/releases/nebula/planetary/1998/39/results/50/ They are so packed together….. A chunk of the white dwarf the size of a sugar cube Would equal the mass of a CAR! When the white dwarf’s energy is gone, it becomes a dead star known as a black dwarf. This is a white dwarf with a nebula cloud around it. How long will a star live? Our sun, a medium sized star, will live about 10 billion years. Small stars will live about 100 billion years. Large stars will only live a few billion years.. Objectives for today! Describe how color indicates the temperature of a star Explain how a scientist can identify a star’s composition Describe how scientists classify stars. Describe different types of stars. Describe the quantities that are plotted in the H-R diagram. Explain how stars at different stages in their life cycle appear on the H-R diagram. Massive Stars are different On the “Main Sequence” but not for long Image from http://hubblesite.org/newscenter/archive/releases/nebula/emission/1997/33/results/50/ Massive Stars ……begin with about 6 times as much mass as our sun. Like medium sized stars: They burn hydrogen at first. They redden as their helium core forms. But, instead of becoming red giants …. they become supergiants. Betelgeuse—Red Supergiant Image from http://hubblesite.org/newscenter/archive/releases/star/massive%20star/1996/04/image/a/results/50/ As their cores heat up, they fuse from helium…. to carbon….. to oxygen and nitrogen…. to iron. Then the fusion stops. What happens next? When the fuel runs out, gravity collapses the core even more. The iron core absorbs energy instead of releasing it…. The star breaks apart in a terrific A Nova! The entire sky is lit up for weeks. The temperature can be more than 100 billion degrees. The iron atoms fuse into uranium. The uranium and gases explode into space. A gigantic cloud of gas and dust is left and may become a new nebula. If it is super big it is called a Supernova! New stars, like our earth, form from these remains. This supernova is surrounded by a nebula ring. Neutron Stars After the supernova explosion of a star 6-30 times the mass of the sun, a neutron star remains. How big is it? It’s only about 16Km in diameter, but it has so much mass packed into it, a chunk the size of a sugar cube has the mass of ~ 100 MILLION CARS!!! Stars 30 or more times the mass of our sun live even shorter lives and face a strange fate…… After the supernova explosion, the star’s core is so massive, it is swallowed up by its own gravity. The core’s gravity is so strong that even light can’t escape. The core has become a….. Black Hole The gravity from this black hole is pulling this matter into itself. Black Hole Image at http://hubblesite.org/newscenter/archive/releases/2002/30/image/a/results/50/ Black holes are like invisible cosmic vacuum cleaners. They swallow both matter and energy. How can we find them if we can’t see them? We can’t, for sure. Matter falling into a black hole releases large bursts of X-rays. We can detect these. Now, let’s do a quick review. Objectives for today! Describe how color indicates the temperature of a star Explain how a scientist can identify a star’s composition Describe how scientists classify stars. Describe different types of stars. Describe the quantities that are plotted in the H-R diagram. Explain how stars at different stages in their life cycle appear on the H-R diagram.