Term
| what is interstellar medium (ISM)? |
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Definition
| gas & dust found in pace btw stars p 512 |
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Term
| the very first stars were born from clouds made only of H & He; what elements have they transformed since then and how much? |
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Definition
| since that time, stars have transformed small fraction of H & He into heavier elements p 512 |
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Term
| what mainly comprises ISM? |
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Definition
| 70% H; 28% He; 2% other heavy elements p 513 |
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Term
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Definition
| in particularly dense & cold interstellar clouds known as molecular clouds p 513 |
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Term
| why are molecular clouds called as such? |
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Definition
| because they are cold enough & dense enough to allow atoms to combine together into molecules p 513 |
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Term
| what is typical temp of molecular cloud? |
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Definition
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Term
| what is the average density of molecular cloud? |
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Definition
| 300 molecules per cubic cm, which is high by interstellar standards, but almost a millions times density of sea level air on Earth p 513 |
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Term
| why is molecular H abundant in these clouds, but why can't we easily detect it? |
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Definition
| b/c H & He are the most abundant elements and He atoms do not combine with other atoms into molecules; hard to detect because clouds usu too cold to produce emission spectral lines p 513 |
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Term
| about how much of a molecular clouds mass is heavier elements? |
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Definition
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Term
| which type of photons passes through ISM dust better? one with a short wavelength or a long one? |
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Definition
| long, redder photons, e.g. why Sun looks redder at times when viewed through smoke or smog p 515 |
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Term
| why does star formation not occur everywhere? |
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Definition
| in most places in our galaxy, gravity is not strong enough to overcome internal pressure of interstellar gas (grav. equilibrium); molecular clouds are the exception p 516 |
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Term
| what is thermal pressure? |
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Definition
| temperature-dependent pressure in ordinary gas clouds (like blowing balloon with air or heating the air, which increases density thus increasing particles movement speedp 516 |
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Term
| why can the thermal pressure in most interstellar clouds resist gravity? |
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Definition
| b/c their low gas densities keep gravity quite weak p 516 |
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Term
| why is gravity strong enough to overcome pressure in a molecular cloud? |
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Definition
| gas is denser, so more molecules are packed into each cubic cm, which increases gravity. However, thermal pressure isn't much greater because of low temps p 516 |
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Term
| why doesn't gravitational contraction cause pressure to build up in molecular clouds and halt star formation? |
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Definition
| b/c they quickly rid themselves of thermal energy that builds up b/c collisions among gas molecules transform the thermal energy into photons that escape thus allowing gravity to overcome thermal pressure p 516 |
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Term
| why do stars form in clusters? |
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Definition
| b/c gravity is stronger in high-mass cloud p 517 |
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Term
| what are 2 factors that likely play important roles in why star-forming clouds with 1000s times mass of Sun resist gravity long enough to grow to large masses before they begin to form stars? |
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Definition
| 1) gravity must be strong enough to overcome turbulence of gas clumps 2) magnetic fields help the cloud resist gravity |
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Term
| how do magnetic fields help the cloud resist gravity? |
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Definition
| light from stars is us random, but light passing through ISM often has its electric & magnetic fields aligned in certain directions (polarization p 145) Magnetic field threading thru cloud prevents charge particles from moving perpendicular to the lines. Friction from particles moving along the lines is great enough to inhibit all movement in a cloud. Can halt or slow grav,. collapse. p 517 |
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Term
| why doesn't a molecular cloud form one giant star? |
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Definition
| gravity follows inverse square law, meaning gravity increases as cloud's size shrinks, giving it an advantage over thermal pressure. because molecular clouds are lumpy, small dense clumps w/in cloud start shrinking on their own, splitting into fragments that become star system p 518 |
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Term
| how dense and cool does a cloud need to be to form a star, even if it's not massive? |
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Definition
| density of few tens of 1000s of molecules per cubic cm; temp of 10 K p 518 |
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Term
| which elements were in the clouds where the very first generation of stars must have been born? |
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Definition
| only hydrogen and helium p 519 |
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Term
| why don't we find many first-generation stars today? |
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Definition
| the high-temp of 1st-generation clouds made it difficult for gravity to overcome pressure meaning they would have formed in hi mass clouds forming hi mass stars, which have short lifetimes p 519 |
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Term
| what slows the contraction of a star-forming cloud? |
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Definition
| the increased density makes it hard for photons to escape, which raises the temp, which slows the contraction. But begins to change again when the density increases further, changing the collisions back into thermal energy p 520 |
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Term
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Definition
| sometimes referred to as a pre-main-sequence star, it is a clump of gas that will become a new star p 520 |
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Term
| why is a protostar not yet a true star? |
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Definition
| because its cores not yet hot enough for nuclear fusion p 520 |
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Term
| what is a protostellar disk? |
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Definition
| when a cloud's rotation increases to conserve its total momentum, gas is prevented from raining directly down on a a protostar, and instead settles into a disk p 521 |
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Term
| which law do gas particles in a protostellar disk obey? |
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Definition
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Term
| what does Kelper's 3rd law cause the gas particles in a protostellar disk to do and what does this eventually do to their orbits? |
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Definition
| The outer particles move slower than the inner particles. The difference in orbital speeds creates a rubbing and generates friction and heat p 521 |
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Term
| what does the friction and heat cause the gas particles to do and what is this process called? |
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Definition
| causes the gas particles's orbits to slow down and fall back down on the disk thereby increasing its mass; process by which material falls onto another body is accretion p 521 |
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Term
| what is a likely cause of protostellar jets? |
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Definition
| magnetic fields link the angular momentum pf the protostar;s disk to the outflowing gas in the jet. Magnetic field lines passing thru the disk get twisted into rope-like configuration, which may help channel jets of charged particles along the rotation axis p 521-2 Also see Fig 16.15 |
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Term
| how do stars become binary? |
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Definition
| Gravity pulls 2 neighboring protostars together, but they don't crash because they each need to conserve a retain amount of angular momentum. The larger the momentum, the larger the orbit p 522 |
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Term
| how does nuclear fusion begin in a newborn star? |
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Definition
| protostar accretes enough mass so its interior becomes very hot. Ultimately, the interior becomes high enough for nuclear fusion p 522 |
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Term
| what is the typical central temp of a protostar? |
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Definition
| typical central temp of a protostar is 1 million K. It needs to contract further to ignite fusion p 523 |
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Term
| When does a protons become a true star? |
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Definition
| when its core temp exceeds 10 million K, making it hot enough for hydrogen fusion to operate efficiently. Fusion halts the gravitational contraction and marks what we consider the birth of a star p 523 |
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Term
| to what temp does a protostar's surface get where it remains steady for the rest of the contraction process? |
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Definition
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Term
| why aren't there stars outside the range of 0.08 and 150 solar mass? |
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Definition
| the battle between gravity & degeneracy pressure p 524 |
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Term
| the temp of a low-mass star (<0.08 solar mass) never reaches the 10 million K threshold. What kind of pressure halts its grav. contraction? |
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Definition
| degeneracy pressure (like chairs in an auditorium; only so many chairs for people [quantum mechanics]) p 524 |
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Term
| How does degeneracy pressure differs from thermal pressure? |
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Definition
| degeneracy pressure depends only on density and not on temperature p 525 |
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Term
| why doesn't the gradual cooling of a brown dwarf's interior weaken its degeneracy pressure? |
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Definition
| because degeneracy pressure does NOT rise and fall with temperature p 525 |
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Term
| what is the maximum mass of a solar star? |
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Definition
| best estimation is 150 solar mass, but not as well defined as minimum mass. Stars in Large Magellanic Cloud have been claimed to be as much as 300 solar masses, but not binary, so we can't infer their masses p 526 |
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Term
| what type of pressure is the reason stars have a maximum mass? |
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Definition
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Term
| what is radiation pressure? |
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Definition
| photons of light exert slight pressure on matter. On Earth, we don't notice it b/c any light source on Earth is small compared to its gravity. However, in massive stars, nuclear fusion generates so much energy that radiation bouncing around inside the stars stronger than thermal pressure p 526 |
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Term
| For every star with a mass between 10 and 150 solar masses, how many stars are there between 10 & 2 solar masses? Btw 2 and 0.5? Btw 0.5 and 0.8? |
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Definition
| 10 stars between 10 and 2 solar masses; 50 between 2 and 0.5; and 200 btw 0.5 and 0.08 p 527 and Fig 16.21 on p 526 |
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Term
| Are most stars more or less massive than our Sun? |
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Definition
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