The Life and Death of Stars.pdf - Astronomy 103 The Lives...

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Astronomy 103The Lives and Deaths of Stars, and the Origin ofthe Elements
A star like our sun might live for 10 billion years.Some starslives for 4 million years. So how do we know how stars liveand die if humans have only been studying stars for the last2000 years?
For stars like the sun, their evolu+on can be broken into sixstages (in Roman numerals)I.Collapse of a gas cloud to form a starII.Main SequenceIII.Red GiantIV.Helium FlashV.Second Red Giant (Asympto+c Giant Branch [AGB])VI.Planetary Nebula and forma+on of White DwarfIt is helpful to keep in mind that stars are a ba;le betweengravity and nuclear energy.And gravity always wins in theend.
II.Main sequence starThe main sequence isboring, where H fuses inthe core to make He.But where a star sits onthe main sequencedepends on its mass.
The energy of a main sequence star isproduced by fusion of H to He in the star’scoreII. Main sequence
Hydrosta±c balanceRecall that stars are a balancebetween pressure pushing stu'out and gravity pulling stu' in.Because, star are constantlylosing energy, they wouldcontract if it were not for a heatsource (nuclear fusion) in thecenter.
At some point, the stars willrun out of fuel. But it wills+ll shine and so gravitytakes over: the core againbegins to contract.As it contracts, the coreheats up (remember thenega+ve heat capacity– ifyou cool it, it gets ho;er).
At some point thestar gets hot enoughto burn hydrogen inthe outer layeraround the He core.This is calledHydrogen shellburningThis next stage is theRed Giant stage.II. Red Giant
II. Red GiantBecause the core is denser and ho;er than before, the Hfusion is much faster in the shell and the energy output ismuch greater than when the star was on the mainsequence.
The huge amount of energy pouring out from the centralregion of the star in?ates the outer star like a balloon:
Why is a red giant red?
Why is a red giant red?

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Term
Fall
Professor
TWEEDALE
Tags
Supernova, White dwarf

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