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Stellar Evolution Chapter 30.2 Classifying Stars -H-R diagram- Used to plot surface temperature and absolute magnitude (brightness). -Main-sequence stars- stars that fall within a band running through the middle of diagram. (Fig. 1 page 781) – Cool, red, dim stars – lower right – Hot, blue, bright stars – upper left – Cool, red, bright stars – upper right – Hot, blue, dim stars – lower left Star Formation 1. Star’s begin as a nebula…cloud of gas & dust. -Gas consists mainly of Hydrogen and Helium. 2. Nebula forms into a Protostar…nebula begins to flatten into a disk of matter. Star Formation continued… 3. Nuclear fusion- A star begins to form when H + H → He; occurs in the core. -As gravity increases – the rate of fusion increases. -As long as fusion is occurring, the star becomes what we call a main sequence star. “Our Sun!” Star Formation continued… 4. Main Sequence…longest stage in a star’s life. -Energy continues to generate in the core (Fusion of H into He). -A star in this stage can stay there about 10 billion years. -When fusion stops, the star will move into another stage – no longer a main sequence star. Star Formation continued… 5. Giant and Supergiants- Almost all H atoms have fused into He atoms. -a star’s shell of gases grows cooler as it expands – begins to glow a reddish color. -they look bright because of their size. Star Formation continued…the Final Stages! 6. Energy is exhausted! -Planetary Nebulas form… a cloud of gas that forms around a sun-like star that is dying. -White dwarfs – As the planetary nebula disperses, gravity causes the remaining matter to collapse inward…what is left is hot & dense…it is called a white dwarf. -When a white dwarf no longer gives off light, it forms a black dwarf. Star Formation continued…the Final Stages! 7. Nova - from white dwarfs; large explosions which releases energy & stellar material into space. Supernova - from white dwarfs as well; (after super giant stage); a star that has such a tremendous explosion that it blows itself apart. -After a supernova the core may contract into a very small but dense ball of neutrons called a neutron star. -A neutron star may turn into a Pulsars - a very fast spinning star that emits pulses of radio & optical energy. -A supernova may lead to a Black Hole – a star so massive & dense that even light cannot escape its gravity.