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The Universe Section 1 Key Ideas 〉 How are stars formed? 〉 How can we learn about stars if they are so far away? 〉 What natural cycles do stars go through? The Universe What Are Stars? 〉 How are stars formed? Section 1 https://www.youtube.com/wa tch?v=PM9CQDlQI0A 〉 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 The Universe Section 1 What Are Stars? continued • 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. The Universe Section 1 Visual Concept: Nuclear Fusion The Universe Section 1 What Are Stars? continued • 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. The Universe Structure of the Sun Section 1 The Universe Section 1 Visual Concept: Structure of the Sun The Universe Section 1 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. The Universe Section 1 Studying Stars, continued • 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. The Universe Section 1 Studying Stars, continued • 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 has longer wavelengths (closer to the red end of the spectrum). – Hot stars emit more energy at every wavelength than cooler stars do. The Universe Starlight Intensity Graph Section 1 The Universe Section 1 Studying Stars, continued • 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. The Universe Section 1 Visual Concept: Constellation The Universe Section 1 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 Universe Section 1 The Life Cycle of Stars, continued • 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 core that is helium becomes larger. – Scientists estimate that the sun can continue nuclear fusion for another 5 billion years. The Universe Section 1 The Life Cycle of Stars, continued • 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 Universe Section 1 The Life Cycle of Stars, continued • 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 Universe Section 1 The Life Cycle of Stars, continued • 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 Universe Section 1 The Life Cycle of Stars, continued • 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. The Universe H-R Diagram Section 1 The Universe Section 1 Visual Concept: Types of Stars https://www.youtube.com/wa tch?v=FCH2s-4b1mM