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KEY CONCEPT Stars change over their life cycles. Sunshine State STANDARDS SC.E.1.3.3: The student understands that our Sun is one of many stars in our galaxy. SC.E.1.3.4: The student knows that stars appear to be made of similar chemical elements, although they differ in age, size, temperature, and distance. BEFORE, you learned NOW, you will learn • The Sun is our local star • The other stars are outside our solar system • There are huge distances between objects in the universe • How stars are classified • How stars form and change EXPLORE Characteristics of Stars How does distance affect brightness? PROCEDURE 1 VOCABULARY light-year p. 786 parallax p. 787 nebula p. 789 main sequence p. 790 neutron star p. 790 black hole p. 790 In a darkened room, shine a flashlight onto a dark surface from 30 cm away while your partner shines a flashlight onto the surface from the same distance. Observe the two spots of light. MATERIALS • 2 flashlights • meter stick • dark surface 2 Move one of the flashlights back 15 cm and then another 15 cm. Compare the two spots of light each time you move the flashlight. WHAT DO YOU THINK? • How did distance affect the brightness of the light on the dark surface? • How does the distance of a star from Earth affect our view of it? MAIN IDEA WEB A main idea web would be a good choice for taking notes about the characteristics of stars. We classify stars by their characteristics. Like our Sun, all stars are huge balls of glowing gas that produce or have produced energy by fusion. However, stars differ in size, brightness, and temperature. Some stars are smaller, fainter, and cooler than the Sun. Others are much bigger, brighter, and hotter. Stars look like small points of light because they are very far away. At most, only a few thousand can be seen without a telescope. To describe the distances between stars, astronomers often use a unit called the light-year. A light-year is the distance light travels in one year, which is about 9.5 trillion kilometers (6 trillion mi). Outside the solar system, the star closest to Earth is about 4 light-years away. 786 Unit 6: Space Science Brightness and Distance If you look at stars, you will probably notice that some appear to be brighter than others. The amount of light a star gives off and its distance from Earth determine how bright it appears to an observer. A star that gives off a huge amount of light can appear faint if it is far away. On the other hand, a star that gives off much less light can appear bright if it is closer to Earth. Therefore, to determine the true brightness of a star, astronomers must measure its distance from Earth. One way astronomers measure distance is by using parallax, which is the apparent shift in the position of an object when viewed from different locations. Look at an object with your right eye closed. Now quickly open it and close your left eye. The object will seem to move slightly because you are viewing it from a different angle. The same kind of shift occurs when astronomers view stars from different locations. To measure the parallax of a star, astronomers plot the star’s position in the sky from opposite sides of Earth’s orbit around the Sun. They then use the apparent shift in position and the diameter of Earth’s orbit to calculate the star’s distance. Check Your Reading What factors affect how bright a star appears from Earth? Parallax SKILL FOCUS How does the distance of an object affect parallax? Measuring PROCEDURE 1 MATERIALS Stand 1 m away from a classmate. Have the classmate hold up a meter stick at eye level. 2 With your left eye closed, hold a capped pen up close to your face. Look at • meter stick • capped pen TIME 10 minutes the pen with your right eye, and line it up with the zero mark on the meter stick. Then open your left eye and quickly close your right eye. Observe how many centimeters the pen seems to move. Record your observation. 3 Repeat step 2 with the pen held at arm’s length and then with the pen held at half your arm’s length. Record your observation each time. WHAT DO YOU THINK? • How many centimeters did the pen appear to move each time you observed it? • How is parallax affected when you change the distance of the pen from you? CHALLENGE How could you use this method to estimate distances that you cannot measure directly? Chapter 22: Stars, Galaxies, and the Universe 787 Size It is hard to get a sense of how large stars are from viewing them in the sky. Even the Sun, which is much closer than any other star, is far larger than its appearance suggests. The diameter of the Sun is about 100 times greater than that of Earth. A jet plane flying 800 kilometers per hour (500 mi/h) would travel around Earth’s equator in about two days. If you could travel around the Sun’s equator at the same speed, the trip would take more than seven months. A star the size of the Sun Diameter = 1.4 million kilometers (900,000 mi) Some stars are much larger than the Sun. Giant and supergiant stars range from ten to hundreds of times larger. A supergiant called Betelgeuse (BEET-uhl-JOOZ) is more than 600 times greater in diameter than the Sun. If Betelgeuse replaced the Sun, it would fill space in our solar system well beyond Earth’s orbit. Because giant and supergiant stars have such huge surface areas to give off light, they are very bright. Betelgeuse is one of the brightest stars in the sky, even though it is 522 light-years away. There are also stars much smaller than the Sun. Stars called white dwarfs are about 100 times smaller in diameter than the Sun, or roughly the size of Earth. White dwarfs cannot be seen without a telescope. Color and Temperature If you observe stars closely, you may notice that they vary slightly in color. Most stars look white. However, a few appear slightly blue or red. The differences in color are due to differences in temperature. White dwarf 1/100 the Sun’s diameter Giant star 10–100 times the Sun’s diameter Supergiant star 100–1000 times the Sun’s diameter You can see how temperature affects color by heating up metal. For example, if you turn on a toaster, the metal coils inside will start to glow a dull red. As they get hotter, the coils will turn a brighter orange. The illustration on page 789 shows changes in the color of a metal bar as it heats up. Like the color of heated metal, the color of a star indicates its temperature. Astronomers group stars into classes by color and surface temperature. The chart on page 789 lists the color and temperature range of each class of star. The coolest stars are red. The hottest stars are blue-white. Our Sun—a yellow, G-class star—has a surface temperature of about 6000°C. Stars of every class give off light that is made up of a range of colors. Astronomers can spread a star’s light into a spectrum to learn about the star’s composition. The colors and lines in a spectrum reveal which gases are present in the star’s outer layers. Check Your Reading 788 Unit 6: Space Science How does a star’s temperature affect its appearance? Color and Temperature Objects that radiate light change color as they heat up. When heated to about 1500°C, a steel bar gives off white light. Classification of Stars Class Color Surface Temperature (°C) O blue-white above 25,000 B blue-white 10,000–25,000 A white 7500–10,000 F yellow-white 6000–7500 G yellow 5000–6000 K orange 3500–5000 M red below 3500 Stars are classified according to their colors and temperatures. The Sun is a G-class star. At about 1200°C the metal gives off yellow light. A steel bar glows red when heated to about 600°C. Stars have life cycles. Although stars last for very long periods, they are not permanent. Like living organisms, stars go through cycles of birth, maturity, and death. The life cycle of a star varies, depending on the mass of the star. Higher-mass stars develop more quickly than lower-mass stars. Toward the end of their life cycles, higher-mass stars also behave differently from lower-mass stars. Colors have been added to this photograph of the Omega Nebula in order to bring out details. Stars form inside a cloud of gas and dust called a nebula (NEHB-yuh-luh). Gravity pulls gas and dust closer together in some regions of a nebula. As the matter contracts, it forms a hot, dense sphere. The sphere becomes a star if its center grows hot and dense enough for fusion to occur. When a star dies, its matter does not disappear. Some of it may form a nebula or move into an existing one. There, the matter may eventually become part of new stars. Check Your Reading How is gravity involved in the formation of stars? Chapter 22: Stars, Galaxies, and the Universe 789 Stages in the Life Cycles of Stars The diagram on page 791 shows the stages that stars go through in their life cycles. Notice that the length of a cycle and the way a star changes depend on the mass of the star at its formation. RESOURCE CENTER CLASSZONE.COM Learn more about life cycles of stars. FLORIDA Content Review reminder Notice that in the lives of stars, as useful energy of the star system decreases, disorder in the system increases. The stage in which stars produce energy through the fusion of hydrogen into helium is called the main sequence. Because they use their fuel slowly, lower-mass stars can remain in the main-sequence stage for billions of years. The Sun has been a mainsequence star for 4.6 billion years and will remain one for about another 5 billion years. When a lower-mass star runs out of hydrogen, it expands into a giant star, in which helium fuses into carbon. Over time a giant star sheds its outer layers and becomes a white dwarf. A white dwarf is simply the dead core of a giant star. Although no fusion occurs in white dwarfs, they remain hot for billions of years. Lower-Mass Stars Stars more than eight times as massive as our Sun spend much less time in the main-sequence stage because they use their fuel rapidly. After millions of years, a higher-mass star expands to become a supergiant star. In the core of a supergiant, fusion produces heavier and heavier elements. When an iron core forms, fusion stops and gravity causes the core to collapse. Then part of the core bounces outward, and the star erupts in an explosion called a supernova. Higher-Mass Stars For a brief period, a supernova can give off as much light as a galaxy. The outer layers of the exploded star shoot out into space, carrying with them heavy elements that formed inside the star. Eventually this matter may become part of new stars and planets. Neutron Stars and Black Holes The collapsed core of a supergiant star may form an extremely dense body called a neutron star. Neutron stars measure only about 20 kilometers (12 mi) in diameter, but their masses are one to three times that of the Sun. Neutron stars emit little visible light. However, they strongly emit other forms of radiation, such as x-rays. Some neutron stars emit beams of radio waves as they spin. These stars are called pulsars because they seem to pulse as the beams rotate. A pulsar emits beams of radio waves as it spins rapidly. The pulsar seems to pulse as the beams rotate toward and away from Earth. 790 Unit 6: Space Science Sometimes a supernova leaves behind a core with a mass more than three times that of the Sun. In such a case, the core does not end up as a neutron star. Instead, it collapses even further, forming an invisible object called a black hole. The gravity of a black hole is so strong that no form of radiation can escape from it. Check Your Reading How do lower-mass stars differ from higher-mass stars after the main-sequence stage? Life Cycles of Stars A star forms inside a cloud of gas and dust called a nebula. The life cycle of a star depends on its mass. Lower-Mass Stars A lower-mass star can fuse hydrogen into helium for billions of years. This stage is called the main sequence. Higher-Mass Stars A higher-mass star remains in the main-sequence stage for millions of years. After the main-sequence stage, the star expands into a supergiant. After the main-sequence stage, the star expands into a giant star. When a giant star sheds its outer layers, it leaves behind a dead core called a white dwarf. When fusion can no longer occur in the supergiant, it undergoes an explosion called a supernova. A high-mass star leaves behind a densely packed core called a neutron star. A star with an extremely high mass leaves behind an invisible black hole. Astronomers can sometimes detect matter and energy around a black hole. How do the stars shown in this illustration differ in the main-sequence stage of their life cycles? Chapter 22: Stars, Galaxies, and the Universe 791 Star Systems Unlike our Sun, most stars do not exist alone. Instead, they are grouped with one or more companion stars. The stars are held together by the force of gravity between them. A binary star system consists of two stars that orbit each other. A multiple star system consists of more than two stars. In many star systems, the stars are too close together to be seen individually. However, astronomers have developed ways of detecting such systems. For example, in a binary star system, one of the stars may orbit in front of the other when viewed from Earth. The star that orbits in front will briefly block some of the other star’s light, providing a clue that more than one star is present. The illustration at right shows a binary star system that can be detected this way. Sometimes astronomers can also figure out whether a star is really a star system by studying its spectrum. Binary Star System Some binary star systems appear to dim briefly when one star orbits in front of the other and blocks some of its light. When neither star is in front of the other, the star system appears to give off more light. Star systems are an important source of information about star masses. Astronomers cannot measure the mass of a star directly. However, they can figure out a star’s mass by observing the effect of the star’s gravity on a companion star. Check Your Reading Why are star systems important to astronomers? KEY CONCEPTS CRITICAL THINKING 1. Why must astronomers figure out a star’s distance to calculate its actual brightness? 4. Analyze Some of the brightest stars are red supergiants. How can stars with cooler red surfaces be so bright? 2. How are color and temperature related in stars? 3. How does a star’s mass affect its life cycle? 792 Unit 6: Space Science 5. Infer Will the Sun eventually become a black hole? Why or why not? CHALLENGE 6. Infer At what stage in the life cycle of the Sun will it be impossible for life to exist on Earth? Explain.