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Star Light, Star Bright... By Laura G. Smith Name: ____________________________________ Date: _______________________ Period:_________ How many stars can you see on a clear night? About 5,000 stars are visible from Earth without the use of a telescope, but you can only see a fraction of them from any particular viewing point at any particular time. With the help of a powerful telescope, it's possible to see more than 3 billion of the hundreds of billions of stars in our Milky Way Galaxy. 1 Stars are huge masses of hot, glowing gases, made up of about 75% hydrogen, 22% helium, and traces of oxygen, neon, carbon, nitrogen, and other elements. Hydrogen provides the fuel for stars to release heat, light, and other forms of energy through a process called nuclear fusion. When the center of a star reaches a temperature of 2,000,000 degrees Fahrenheit, hydrogen gas is changed to helium gas. The extra hydrogen that is not used during nuclear fusion is instead changed into energy. This energy is transferred to the surface of the star and then passed into space in the form of light, heat, and radio waves. This release of energy is what causes stars to "shine." 2 The combination of elements and amounts of each one found in a star is described as its chemical composition. As the chemical composition of a star changes over time, its appearance is also altered. For example, a star that is in the late stages of its existence may have exhausted its supply of hydrogen fuel, causing it to burn helium and heavier elements. This causes the star to expand greatly, transforming it into what is called a red giant. As the red giant loses material on its surface, gravity causes the star to begin to collapse. As the star's pressure and temperature increase, the star may explode into a supernova (a star that suddenly becomes extremely bright). The star may then eventually become a white dwarf, which is the last visible stage of a star. If the star continues to cool down and contract, it may become a collapsed star called a black hole. 3 Astronomers learn about a star's mass, temperature, size, and distance from the earth by studying the energy it gives off. The brightness or magnitude of a star indicates how much mass the star has. A star's color indicates the temperature of the star's surface. Stars have a spectrum of colors ranging from blue to red, with the hottest stars giving off more blue than red (reaching surface temperatures of up to 50,000 degrees Fahrenheit). Stars with lower surface temperatures (about 5,000 degrees Fahrenheit) give off more red than blue. 4 Stars are divided by size into four main groups: Dwarfs, as you might guess, are the smallest stars. Maanen's Star, the smallest dwarf, has a diameter of 5,200 miles (about 2800 miles less than the diameter of the earth.) The next-largest group of stars, simply named Medium-Sized Stars, includes the sun. The diameters of these stars range from one-tenth of that of the sun to about 10 times greater than that of the sun. Stars classified as Giants have diameters ranging from 10 to 100 times as great as the sun's diameter. Supergiants, the largest stars, earn their name because of huge diameters measuring up to a thousand times that of the sun. If one of these stars were placed in the center of our solar system, it would easily cover Earth, Mars, Jupiter, and Saturn. 5 Great distances separate stars from the earth and from each other. The sun, our nearest star, (about 93 million miles from the earth) appears different from the stars visible in the night sky because it is about 250,000 times closer to the earth than the next closest star. The earth's second-closest star looks like a tiny pinpoint next to the sun. 6 Although stars appear to stay in the same place in the flat-looking patterns of the constellations, they are actually moving at high speeds that can be measured over long periods of time by small changes in the stars' positions. This movement of stars is known as proper motion and is separate from the apparent motion of stars across the sky. That apparent motion is caused by the earth's rotation. 7 Star Light, Star Bright... 1. Heat and light is released from the surface of stars through a process called A. Star shine B. Chemical composition C. Nuclear fusion D. Heat transfer 3. Each star maintains the same chemical composition as all the other stars. A True B. False 5. Stars that have a surface temperature of 50,000 degrees Fahrenheit are mostly ________ in color. A. Red B. Yellow C. Blue D. White 7. The movement of stars that is measured over long periods of time is referred to as Apparent motion Proper motion Stellar motion Slow motion 2. A "red giant" is a star that A. No longer reflects light B. Is in its last visible stage as a star C. Has exhausted its supply of hydrogen fuel D. Has suddenly become very bright 4. As a star's pressure and temperature increase, it may explode into a ____________________ causing a sudden burst of brightness. A. Supernova B. Superstar C. Star burst D. Yellow dwarf 6. Astronomers learn about a star's mass, temperature, size, and distance from the earth by studying the Movement of the star Age of the star Energy the star emits Star's relationship to the sun 8. Explain what causes stars to "shine."