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ASTR 114 Exam 3 - ASTR 114 Exam 3 17. Stellar Evolution 1...

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ASTR 114Exam 317. Stellar Evolution 1 Star Formation and Main SequenceMain-sequence stars are fusing hydrogen into helium in their cores like the Sun.Luminous main-sequence stars are hot (blue).Less luminous ones are cooler (yellow or red).Mass measurements of main-sequence stars show that the hot, blue stars are much moremassive than the cool, red ones. (L≈M4)The mass of a normal, hydrogen-burning star determines its luminosity and spectral type.Stars form in dark clouds of dusty gas in interstellar space.The gas between the stars is called interstellar medium.Most of the matter in star-forming clouds is in the form of molecules (H2, CO, etc.)These molecular clouds have a low temperature and density.A typical molecular cloud must contain at least a few hundred solar masses for gravity toovercome pressure.Emission lines from molecules in a cloud can prevent pressure buildup by converting thermalenergy into infrared and radio photons that escape the cloud.Fragmentation- begins with a turbulent cloud containing 50 solar masses of gas; randommotions of different sections of the cloud cause it to become lumpy; each lump of the cloud inwhich gravity can overcome pressure can go on to become a starAs contraction packs the molecules and dust particles of a cloud fragment closer together, itbecomes harder for infrared and radio photons to escape.Thermal energy then begins to build up inside, increasing the internal pressure.Contraction slows down, and the center of the cloud fragment becomes a protostar.Matter from the cloud continues to fall onto the protostar until either the protostar of aneighboring star blows the surrounding gas away.Collisions between particles in the cloud cause it to flatten into a disk.Rotation also causes jets of matter to shoot out along the rotation axis.Jets are observed coming from the centers of disks around protostars.The formation of stars like the Sun:1.Interstellar cloud starts to contract, probably triggered by shock or pressure wave fromnearby star. As it contracts, the cloud fragments into smaller pieces.2.Individual cloud fragments being to collapse. Once the density is hot enough, there is nofurther fragmentation.3.The interior of the fragment has begun heating and is about 10,000 K.
4.The core of the cloud is now a protostar and makes its first appearance on the H-Rdiagram.A protostar looks starlike after the surrounding gas is blown away, but its thermal energy comesfrom gravitational contraction, not fusion.Contraction must continue until the core becomes hot enough for nuclear fusion.Contraction stops when the energy released by core fusion balances energy radiated from thesurface—the star is now a main-sequence star.A life track illustrates a star’s surface temperature and luminosity at different moments in time.Luminosity and temperature grow as matter collects into a protostar.Surface temperature remains near 3000 K while convection is main energy transportmechanism.

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Term
Spring
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Tags
Galaxies, Star Formation, Shell, White dwarf

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