Lecture 8-Chapter 16-Star Birth

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Astronomy & Astrophysics
Stefan Mochnacki

Chapter 16 Star Birth Where Stars Form The Interstellar Medium N The gas and dust that fill the spaces between stars within a galaxy is the interstellar medium. N The gas between the stars is composed mostly of hydrogen and helium. N We use spectroscopy to measure the new elements that stars have added to the interstellar medium. DIAGRAM 16.2 absorption line measurement N The interstellar medium in our region of the Milky Way consists of 70% hydrogen, 28% helium, and 2% heavier elements. Star-Forming Clouds N Molecular Clouds are where stars are born in, because these clouds are cold enough and dense enough to allow atoms to combine together into molecules (10- 30 K). N Molecular hydrogen is the most abundant molecule, due to large amounts of hydrogen and helium and that helium atoms do not combine into molecules. N By radio emission lines, more than 120 molecules have been identified, which are carbon monoxide (CO), water, ammonia, ethyl alcohol, etc. Interstellar Dust N About half the atoms of elements heavier than helium are found in tiny, solid grains of interstellar dust. N %K:841,24O0.:O,7.O4:82,88L8.4389L9:90-L3907890OO,7:89J7,L38 since the atoms heavier than helium make up about 2% of the mass of the interstellar medium. DIAGRAM 16.5 visible light image vs. infrared image Why Stars Form N Gravity causes the cloud to contract, continuing until the central object becomes hot enough to sustain fusion in its core, at which point it has become a star. N However, star formation does not occur everywhere because gravity is not strong enough to overcome the internal pressure of interstellar gas, but in molecular clouds, gravity can win the battle against pressure and start the formation of stars. Gravity vs. Pressure N Gravity can create stars only if it can overcome the outward push of the pressure within a gas cloud, which depends on the density and the temperature of the cloud. N Thermal pressure can resist gravity in most interstellar gas clouds, because their gas densities are comparatively low, keeping gravity quite weak. N However, in molecular clouds, where gas densities are tens to thousands of times greater than average, gravity is strong enough to overcome pressure. Preventing a Pressure Buildup N Regions of a molecular cloud in which the gravitational attraction is stronger than the thermal pressure are forced to contract. www.notesolution.com
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