Stars and Galaxies Evolution of Stars What do you think? Read the two statements below and decide whether you agree or disagree with them. Place an A in the Before column if you agree with the statement or a D if you disagree. After you ve read this lesson, reread the statements to see if you have changed your mind. Before Statement After Key Concepts How do stars form? How does a star s mass affect its evolution? How is star matter recycled in space? 5. The more matter a star contains, the longer it is able to shine. 6. Gravity plays an important role in the formation of stars. Life Cycle of a Star Stars have life cycles that can be compared to the life cycles of living things. They are born, and after millions or billions of years, they die. Stars die in different ways, depending on their masses. But all stars from white dwarfs to supergiants form in the same way. Nebulae and Protostars Stars form deep inside clouds of gas and dust. A cloud of gas and dust is a nebula (plural, nebulae). Star-forming nebulae are cold, dense, and dark. Gravity causes the densest parts to collapse, forming regions called protostars. Protostars continue to contract. As they contract, they pull in surrounding gas. Eventually, their cores are hot and dense enough for nuclear fusion to begin. As they contract, protostars produce enormous amounts of thermal energy. Birth of a Star Over many thousands of years, the energy produced by protostars heats the gas and dust around the protostars. Eventually, the gas and dust blow away, and the protostars become visible as stars. Some of this material might later become planets or other objects that orbit the star. During the star-formation process, nebulae glow brightly. Ask Questions As you read, write your questions about stars on a sheet of paper. Answer your questions as you read the lesson a second time. Discuss any questions that you can t answer with your teacher. Key Concept Check 1. Summarize How do stars form? Reading Essentials Stars and Galaxies 217
Make a vertical five-tab book to organize your notes on the life cycle of a star. Protostar Main Sequence Red Giant Red Supergiant Supernova Visual Check 2. Name what forms in only the most massive stars. Main-Sequence Stars Recall the main sequence of the Hertzsprung-Russell diagram. Stars spend most of their lives on the main sequence. A star becomes a main-sequence star as soon as it begins to fuse hydrogen into helium. It remains on the main sequence for as long as it continues to fuse hydrogen into helium. Lower-mass stars such as the Sun stay on the main sequence for billions of years. High-mass stars stay on the main sequence for only a few million years. Even though massive stars have more hydrogen than lower-mass stars, they process it at a much faster rate. When a star s hydrogen supply is nearly gone, the star leaves the main sequence. It begins the next stage of its life cycle, as shown in the figure below. Not all stars go through all phases shown in the figure below. Lower-mass stars, such as the Sun, do not have enough mass to become supergiants. A Massive Star s Life Cycle Massive star Helium fuses and forms carbon. Hydrogen fuses and forms helium. Carbon Neon Oxygen Silicon Iron Red giant Hydrogen Helium When the star s hydrogen supply is gone, gravity causes the core to contract and heat up. Thermal energy in the star's center causes the star s outer layers to expand and cool. The star becomes a red giant. Eventually, the interior becomes hot enough to resume nuclear fusion. The outer layers contract, and the star begins to fuse helium nuclei and form carbon. Red supergiant Carbon fuses and forms other elements. Hydrogen fuses and forms helium. Helium fuses and forms carbon. Larger red giant When helium in the core runs low, the core again collapses under gravity and the outer layers expand. The star becomes a red giant for a second time, but this time it is even larger. It again contracts when it begins to fuse carbon and form other elements. The process repeats again and again. The star becomes a red supergiant as different elements are formed during fusion. Iron nuclei form in the most massive stars. 218 Stars and Galaxies Reading Essentials
End of a Star All stars form in the same way. But stars die in different ways, depending on their masses. Massive stars collapse and explode. Lower-mass stars die more slowly. White Dwarfs Lower-mass stars, such as the Sun, do not have enough mass to fuse elements beyond helium. They do not get hot enough. After helium in their cores is gone, the stars cast off their gases, exposing their cores. The core becomes a white dwarf, a hot, dense, slowly cooling sphere of carbon. Reading Check 3. Point Out What determines the way a star will die? The Sun as a Red Giant What will happen to Earth and the solar system when the Sun runs out of fuel? When the Sun runs out of hydrogen, in about 5 billion years, it will become a red giant. Once helium fusion begins, the Sun will contract. When the helium is gone, the Sun will expand again, probably absorbing Mercury, Venus, and Earth, and pushing Mars and Jupiter outward. The Sun as a White Dwarf Eventually, the Sun will become a white dwarf, as shown in the figure below. Imagine the mass of the Sun squeezed a million times until it is the size of Earth. That s the size of a white dwarf. Scientists hypothesize that all stars with masses less than 8 10 times that of the Sun will eventually become white dwarfs. With a white dwarf at the center, the solar system will be a cold, dark place. White dwarf Jupiter The Sun as a White Dwarf Asteroid belt Mars Reading Check 4. Summarize What will happen to Earth when the Sun runs out of fuel? Visual Check 5. Locate Circle the planet closest to the white dwarf. Reading Essentials Stars and Galaxies 219
Reading Check 6. Explain Why does a massive star lose its internal energy source when iron forms in its core? REVIEW VOCABULARY neutron a neutral particle in the nucleus of an atom Key Concept Check 7. Explain How does a star s mass determine if it will become a white dwarf, a neutron star, or a black hole? Supernovae Stars with more than 10 times the mass of the Sun do not become white dwarfs. Instead, they explode. A supernova (plural, supernovae) is an enormous explosion that destroys a star. In the most massive stars, a supernova occurs when iron forms in the star s core. Iron is stable and does not fuse. After a star forms iron, it loses its internal energy source. Without its energy source, the core collapses quickly under the force of gravity. The collapse of the core releases so much energy that the star explodes. When it explodes, a star can become 1 billion times brighter and form elements even heavier than iron. Neutron Stars Have you ever eaten cotton candy? A bag of cotton candy is spun from just a few spoonfuls of sugar. Cotton candy is mostly air. Similarly, atoms are mostly empty space. During a supernova, the collapse is so violent that it eliminates the normal spaces inside atoms, and a neutron star forms. A neutron star is a dense core of neutrons that remains after a supernova. Neutron stars are only about 20 km wide. Their cores are so dense that a teaspoonful would weigh more than 1 billion tons. Black Holes For the most massive stars, atomic forces holding neutrons together are not strong enough to overcome so much mass in such a small volume. Gravity is too strong, and the matter crushes into a black hole. A black hole is an object whose gravity is so great that no light can escape. A black hole does not suck matter in like a vacuum cleaner. But a black hole s gravity is very strong because all of its mass is concentrated in a single point. Because astronomers cannot see a black hole, they only can infer its existence. For example, if they detect a star circling around something but they cannot see what that something is, they suspect it is a black hole. Recycling Matter At the end of a star s life cycle, much of its gas escapes into space. This gas is recycled. It becomes the building blocks of future generations of stars and planets. 220 Stars and Galaxies Reading Essentials
Planetary Nebulae You read that lower-mass stars, such as the Sun, become white dwarfs. When a star becomes a white dwarf, it casts off hydrogen and helium gases in its outer layers. The expanding, cast-off matter of a white dwarf is a planetary nebula. Most of the star s carbon remains locked in the white dwarf. But the gases in the planetary nebula can be used to form new stars. Planetary nebulae have nothing to do with planets. They are called planetary because early astronomers thought they were regions where planets were forming. Supernova Remnants During a supernova, a massive star comes apart. This sends a shock wave into space. The expanding cloud of dust and gas is called a supernova remnant. Like a snowplow pushing snow, a supernova remnant pushes on the gas and dust it encounters. In a supernova, a star releases the elements that formed inside it during nuclear fusion. Almost all of the elements in the universe other than hydrogen and helium were created by nuclear reactions inside the cores of massive stars and released in supernovae. This includes the oxygen in air, the silicon in rocks, and the carbon in you. Gravity causes recycled gases and other matter to clump together in nebulae and form new stars and planets. As you will read in the next lesson, gravity also causes stars to clump together into even larger structures called galaxies. Reading Check 8. Relate How are a white dwarf and a planetary nebula related? Key Concept Check 9. Describe How do stars recycle matter? Reading Essentials Stars and Galaxies 221
Mini Glossary black hole: an object whose gravity is so great that no light can escape nebula: a cloud of gas and dust neutron st ar: a dense core of neutrons left from a supernova supernova: an enormous explosion that destroys a star white dwarf: a hot, dense, slowly cooling sphere of carbon 1. Review the terms and their definitions in the Mini Glossary. Write a sentence that describes how a supernova and a neutron star are related. 2. Complete the life cycle of a massive star by writing the following in the correct sequence in the circles of the diagram: larger red giant, protostar, red giant, red supergiant, supernova remnants. main-sequence star massive star nebulae What do you think Reread the statements at the beginning of the lesson. Fill in the After column with an A if you agree with the statement or a D if you disagree. Did you change your mind? supernova ConnectED Log on to ConnectED.mcgraw-hill.com and access your textbook to find this lesson s resources. END OF LESSON 222 Stars and Galaxies Reading Essentials