2. The UniverseSection 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?

3. The UniverseSection 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 https://www.youtube.com/wa tch?v=PM9CQDlQI0A

4. The UniverseSection 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.

5. The UniverseSection 1 Visual Concept: Nuclear Fusion

6. The UniverseSection 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.

7. The UniverseSection 1 Structure of the Sun

8. The UniverseSection 1 Visual Concept: Structure of the Sun

9. The UniverseSection 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.

10. The UniverseSection 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.

11. The UniverseSection 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.

12. The UniverseSection 1 Starlight Intensity Graph

13. The UniverseSection 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.

14. The UniverseSection 1 Visual Concept: Constellation

15. The UniverseSection 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.

16. The UniverseSection 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.

17. The UniverseSection 1 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, continued

18. The UniverseSection 1 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, continued

19. The UniverseSection 1 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, continued

20. The UniverseSection 1 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 Life Cycle of Stars, continued

21. The UniverseSection 1 H-R Diagram

22. The UniverseSection 1 Visual Concept: Types of Stars https://www.youtube.com/wa tch?v=FCH2s-4b1mM