Term
| Why can't iron be fused to release energy? |
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Definition
| Iron cannot be fused to release energy because for elements heavier than iron, the mass per nuclear particle increases, so fusing two iron nuclei requires more energy than it produces. |
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| Cluster ages can be determined from |
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Definition
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Term
| Which stars does not have fusion occurring in its core and an expanding shell of hydrogen fusing around it? |
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Definition
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Term
| What happens to the core of a high-mass star after it runs out of hydrogen? |
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Definition
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Term
| True or False: Our Sun will end its life in a planetary nebula and become a white dwarf. |
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Definition
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Term
| What would happen if the Sun suddenly became a black hole without changing its mass? |
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Definition
| Earth’s orbit would not change. |
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Term
| Summarize some of the observational evidence supporting our ideas about how heavy elements form in massive stars. |
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Definition
| One piece of evidence that supports our theories about how heavy elements form in high-mass stars is the chemical composition of older stars. Our theory predicts that the older stars should have fewer heavy elements in their compositions. Observations indicate that this is so. Another piece of evidence supporting our theories is the relative abundances of the various elements. For example, since the helium-capture reactions are an important series of reactions in high-mass stars, we expect to see more elements with even numbers of protons than odd numbers of protons. This predicted pattern agrees with the observations quite well. |
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Term
| What happens after a helium flash? |
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Definition
| The core quickly heats up and expands. |
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Term
| What happens when the gravity of a massive star is able to overcome neutron degeneracy pressure? |
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Definition
| The core contracts and becomes a black hole. |
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Term
| once hydrogen is exhausted in the core... |
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Definition
the thermal pressure is no longer strong enough to prevent gravitational collapse
The core is now composed entirely of helium, but the region just outside is still hydrogen and so fusion begins there
As this fusion proceeds, the mass of helium in the core keeps increasing
With no fusion to halt the core contraction, this increasing mass causes the core to contract and heat up.
As the core heats up, the fusion rate in the H-layer increases, causing the luminosity to go up and the star to swell
Expansion causes the surface to cool, making the star appear red, even though its core is white hot
---> RED GIANT |
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Term
| what causes a helium flash? |
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Definition
| very high temperatures and densities that are needed in order for helium fusion to begin |
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Term
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Definition
stars whose outer layers are continually expanding and cooling
eventually temperature and density will decrease to the point where fusion stops
The hot carbon core will continue to emit huge amounts of thermal radiation and the Sun's stellar wind will push its outer layers into space |
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Definition
the remnants of solar nebulae -- a small, dense carbon core
very faint because they are not fusing |
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Term
| in the more massive stars, the rate of hydrogen fusion is higher, which leads to... |
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Definition
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Term
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Definition
| the increased pressure support provided by the higher luminosities of more massive stars, as opposed to just thermal pressure |
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Term
| the upper limit for stellar masses in an HR Diagram is set by... |
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Definition
| the mass shedding of the outer layers of massive stars |
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Term
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Definition
the fusion of heavier elements in the carbon core
the carbon fuses with helium to create heavier elements
depending on the mass of the core, this fusion may continue all the way up the periodic table to iron |
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Term
| What evidence do we have that supports the theory of fusion of high-mass stars? |
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Definition
If fusion does proceed by helium capture, we should observe a higher fraction of elements (starting from carbon) with even numbers of protons than odd numbers, which we do
If the universe began with only hydrogen and helium, the oldest stars should have a low fraction of heavy elements in their spectra; conversely, young stars should have a higher fraction of heavy elements.
This is observed to be the case. |
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Term
| what happens once iron is in the core? |
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Definition
| no more fusion takes place in the core, but fusion in the outer shell continues, so the core becomes more and more massive |
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Term
| electron degeneracy pressure |
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Definition
pressure that can exist at any temperature, meaning that even if the core is not hot enough to begin fusion, the density may reach a point at which the atoms cannot be packed any tighter.
It is this pressure in the iron core which can hold a star up against gravitational collapse, even if there is no fusion going on |
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Term
| What happens once the iron core reaches a mass of 1.4 M(Sun)? |
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Definition
gravity becomes strong enough to overcome electron degeneracy pressure
the protons and neutrons fuse to form neutrinos
this happens instantaneously throughout the core |
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Term
| What happens as a result of the core collapse? |
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Definition
| the sudden flux of neutrinos tear apart the star, and its outer layers are thrown into space (supernova explosion) |
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Term
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Definition
a core collapse supernova, resulting from an iron core overcoming neutron degeneracy pressure and fusing into a neutron star
it is still held together by degeneracy pressure because the neutrons are so tightly packed together, making it the densest object in the universe |
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