What Are Stars? continued. What Are Stars? How are stars formed? Stars are powered by nuclear fusion reactions.

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1 What Are Stars? How are stars formed? Stars are formed from clouds of dust and gas, or nebulas, and go through different stages as they age. star: a large celestial body that is composed of gas and emits light light-year: the distance that light travels in one year; about 9.46 trillion kilometers What Are Stars? Stars are powered by nuclear fusion reactions. The core of a star is extremely hot, extremely dense, and under extreme pressure. Nuclear fusion takes place in the core of a star. Fusion combines the nuclei of hydrogen atoms into helium. When two particles fuse, energy is released. Visual Concept: Nuclear Fusion What Are Stars? Energy moves slowly through the layers of a star. Energy moves through the layers of a star by convection and radiation. During convection, hot gas moves upward, away from the star s center, and cooler gas sinks toward the center. During radiation, atoms absorb energy and transfer it to other atoms in random directions. Atoms near the star s surface radiate energy into space. Structure of the Sun Visual Concept: Structure of the Sun

2 Studying Stars How can we learn about stars if they are so far away? The telescope allowed astronomers to study stars in more detail for the first time. Some stars appear brighter than others. The brightness of a star depends on the star s temperature, size, and distance from Earth. The brightest star in the night sky, Sirius, appears so bright because it is relatively close to Earth. Studying Stars, We learn about stars by studying energy. Stars produce a full range of electromagnetic radiation, from high-energy X-rays to low-energy radio waves. Scientists use optical telescopes to study visible light and radio telescopes to study radio waves emitted from astronomical objects. Earth s atmosphere blocks some wavelengths, so telescopes in space can study a wider range of the spectrum. Studying Stars, Starlight Intensity Graph A star s color is related to its temperature. Hotter objects glow with light that has shorter wavelengths (closer to the blue end of the spectrum). Cooler objects glow with light that t has longer wavelengths (closer to the red end of the spectrum). Hot stars emit more energy at every wavelength than cooler stars do. Studying Stars, Visual Concept: Constellation Spectral lines reveal the composition of stars. The spectra of most stars have dark lines caused by gases in the outer layers that absorb light at that wavelength. Each element produces a unique pattern of spectral lines. Astronomers can match the dark lines in starlight to the known lines of elements found on Earth.

3 The Life Cycle of Stars What natural cycles do stars go through? In a way that is similar to other natural cycles, stars are born, go through various stages of development, and eventually die. The sun formed from a cloud of gas and dust. The sun formed about 5 billion years ago. The sun was born when the process of fusion began in the core. The Life Cycle of Stars, The sun has a balance of inward and outward forces. The fusion reactions in the core of the sun produce an outward force that balances the inward force due to gravity. Over time, the percentage of the sun s s core that is helium becomes larger. Scientists estimate that the sun can continue nuclear fusion for another 5 billion years. The Life Cycle of Stars, The sun will become a red giant before it dies. As fusion slows, the outer layers of the sun will expand. The sun will become a red giant. red giant: a large, reddish star late in its life cycle When the sun runs out of helium, the outer layers will expand and eventually leave the sun s orbit. The sun will become a white dwarf. white dwarf: a small, hot dim star that is the leftover center of an old star The Life Cycle of Stars, Supergiant stars explode in supernovas. Massive stars evolve faster, develop hotter cores, and create heavier elements through fusion. The formation of an iron core signals the beginning of a supergiant s death. Eventually the core collapses and then explodes in a supernova. supernova: a gigantic explosion in which a massive star collapses and throws its outer layers into space, plural supernovae The Life Cycle of Stars, Some supernovas form neutron stars and black holes. If the core that remains after a supernova has a mass of 1.4 to 3 solar masses, the remnant can become a neutron star. If the leftover core has a mass that is greater than three solar masses, it will collapse to form a black hole. black hole: an object so massive and dense that not even light can escape its gravity The Life Cycle of Stars, The H-R diagram shows how stars evolve. The vertical line on an H-R diagram indicates brightness in absolute magnitude. The horizontal line on the H-R diagram indicates temperature. Most stars appear in a diagonal line called the main sequence. As stars age and pass through different stages, their positions on the H-R diagram change. The sun is currently a main-sequence star.

4 H-R Diagram Visual Concept: Types of Stars Galaxies What is a galaxy and what is it made of? A galaxy is a collection of millions to billions of stars. galaxy: a collection of stars, dust, and gas bound together by gravity Galaxies Galaxies contain millions or billions of stars. Because stars age at different rates, a galaxy may contain many types of stars. Gravity holds galaxies together. Without gravity, everything in space might be a veil of gas spread out through space. Just as Earth revolves around the sun, our solar system revolves around the center of our galaxy. Galaxies, Gravity holds galaxies together in clusters. Galaxies are not spread evenly throughout space. The Milky Way galaxy and the Andromeda galaxy are two of the largest members of the Local Group, a cluster of more than 30 galaxies. Clusters of galaxies can form even larger groups, called superclusters. Visual Concept: Contents of Galaxies cluster: a group of stars or galaxies bound by gravity

5 Types of Galaxies Why are galaxies divided into three major types? The three types of galaxies have many stars, but differ in structure. We live in the Milky Way galaxy. Most of the objects visible in the night sky are part of the Milky Way galaxy. Scientists use astronomical data to piece together a picture of the Milky Way galaxy. Types of Galaxies, The Milky Way is a spiral galaxy. Our galaxy is a huge spiraling disk of stars, gas, and dust, with several distinct spiral arms. Our solar system is located within a spiral arm. The nucleus of the galaxy is dense and has many old stars. The gas and dust located between stars is called interstellar matter. interstellar matter: the gas and dust located between the stars in a galaxy Types of Galaxies, Elliptical galaxies have no spiral arms. Elliptical galaxies are spherical or egg shaped. They contain mostly older stars and have little interstellar matter. Because older stars are red, elliptical galaxies often have a reddish color. Types of Galaxies, All other galaxies are irregular galaxies. Irregular galaxies lack regular shapes and do not have a well-defined structure. Some irregular galaxies may be oddly shaped because the gravitational influence of nearby galaxies distorts their spiral arms. How Galaxies Change Over Time How do scientists know that galaxies change over time? Because the light from ancient galaxies takes so long to reach Earth, scientists do not know what far-away galaxies look like currently. By studying closer galaxies that might be similar to ancient ones, they can slowly piece together the puzzle of how galaxies evolve. How Galaxies Change Over Time, Quasars may be infant galaxies. In 1960, a faint object was matched with a strong radio signal. This object was called a quasar. Each quasar has a huge central black hole and a large disk of gas and dust around it. quasar: quasi-stellar radio sources; very luminous objects that produce energy at a high rate and that are thought to be the most distant objects in the universe

6 Visual Concept: Quasar How Galaxies Change Over Time, Galaxies change over time. Galaxies change as they use up their stores of gas and dust. Galaxies also change as a result of collisions. As galaxies approach each other, mutual gravitational attraction changes their shape. Collisions of gas and dust may cause new stars to begin forming. What Is the Universe? What makes up the universe? The universe consists of all space, matter, and energy that exists now, in the past, or in the future. universe: the sum of all space, matter, and energy that exist, that have existed in the past, and that will exist in the future What Is the Universe? Everything is part of the universe. We see the universe now as it was in the past. It takes time for light to travel in space. The farther away an object is, the older the light that we receive from that object. Most of the universe is empty space. Space is a vacuum with no air and no air pressure. What Happened at the Beginning? How did the universe begin? By studying ancient light and looking at other evidence, scientists have been able to theorize that the universe formed during a cataclysmic event known as the big bang. What Happened at the Beginning? The universe is expanding. Observations of spectral lines from other galaxies indicated that they were moving away from us. Red shift is an apparent shift toward longer wavelengths of light caused when a luminous object moves away from the observer. This red shift can be explained by the Doppler effect. Doppler effect: an observed change in the frequency of a wave when the source or observer is moving

7 Visual Concept: Red Shift What Happened at the Beginning? Expansion implies that the universe was once smaller. Long ago, the entire universe might have been contained in an extremely small space. All of the matter in the universe appears to expand rapidly outward, like a gigantic explosion. Visual Concept: Universal Expansion What Happened at the Beginning? Did the universe start with a big bang? Scientists have proposed several different theories to explain the expansion of the universe. The most complete and widely accepted theory is the big bang theory. big bang theory: the theory that all matter and energy in the universe was compressed into an extremely small volume that 13 to 15 billion years ago exploded and began expanding in all directions Visual Concept: Big Bang Theory What Happened at the Beginning? Cosmic background radiation supports the big bang theory. Cosmic background radiation is a steady but very dim signal from all over the sky in the form of radiation at microwave wavelengths. Many scientists believe that the microwaves are dim remnants of the radiation produced during the big bang.

8 What Happened at the Beginning? Radiation dominated the early universe. According to the big bang theory, expansion cooled the universe enough for matter such as protons, neutrons, and electrons to form. The big bang theory is constantly being tested. Like all theories, the big bang theory is constantly tested against each new discovery about the universe. The big bang theory may be refined, revised, or replaced as new information is discovered. Predicting the Future of the Universe How do scientists make predictions about the future of the universe? Scientists use their increasing knowledge of the universe to hypothesize what might happen to the universe in the future. Predicting the Future of the Universe, The future of the universe is uncertain. The universe is expanding, but the combined gravity of all the mass in the universe is also pulling the universe inward. The competition between these forces leaves three possibilities: 1. The universe will keep expanding forever. 2. The expansion of the universe will gradually slow down, and the universe will approach a limit in size. 3. The universe will stop expanding and start to fall back in on itself. Predicting the Future of the Universe, The future of the universe depends on mass. If there is not enough mass, the gravitational pull will be too small to stop the expansion. If the universe keeps expanding, it may expand at a steady rate or speed up and expand faster. If there is just the right amount of mass, the expansion will continually slow down, but will never stop completely. If there is too much mass, gravity will eventually overcome expansion and the universe will contract. Predicting the Future of the Universe, New technology helps scientists test theories. Powerful telescopes and other sensitive equipment help scientists study the universe. Scientists make observations to test theories and develop new explanations. Predicting the Future of the Universe, There is debate about dark matter. There is more matter in the universe than what is visible. Scientists call this dark matter. Dark matter may be planets, black holes, or brown dwarfs (starlike objects that lack enough mass to begin fusion).

9 Predicting the Future of the Universe, Visual Concept: Structure of the Universe Scientists use mathematics to build better models. Theories can be expressed in mathematical form. Mathematical models can be used to help test theories that are not easily observed. Albert Einstein developed the general theory of relativity, which he expressed in mathematical form. This mathematical model has been tested and supported by observation.