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Exam 3: 17 and 19
Lives of Stars
23
Astronomy
Undergraduate 2
11/13/2011

Additional Astronomy Flashcards

 


 

Cards

Term
Why can't iron be fused to release energy?
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.
Term
Cluster ages can be determined from
Definition
main sequence turnoff.
Term
Which stars does not have fusion occurring in its core and an expanding shell of hydrogen fusing around it?
Definition
a red giant
Term
What happens to the core of a high-mass star after it runs out of hydrogen?
Definition
It shrinks and heats up.
Term
True or False: Our Sun will end its life in a planetary nebula and become a white dwarf.
Definition
True
Term
What would happen if the Sun suddenly became a black hole without changing its mass?
Definition
Earth’s orbit would not change.
Term
Summarize some of the observational evidence supporting our ideas about how heavy elements form in massive stars.
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.
Term
What happens after a helium flash?
Definition
The core quickly heats up and expands.
Term
What happens when the gravity of a massive star is able to overcome neutron degeneracy pressure?
Definition
The core contracts and becomes a black hole.
Term
once hydrogen is exhausted in the core...
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
Term
what causes a helium flash?
Definition
very high temperatures and densities that are needed in order for helium fusion to begin
Term
planetary nebulae
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
Term
white dwarfs
Definition
the remnants of solar nebulae -- a small, dense carbon core
very faint because they are not fusing
Term
in the more massive stars, the rate of hydrogen fusion is higher, which leads to...
Definition
higher luminosities
Term
radiation pressure
Definition
the increased pressure support provided by the higher luminosities of more massive stars, as opposed to just thermal pressure
Term
the upper limit for stellar masses in an HR Diagram is set by...
Definition
the mass shedding of the outer layers of massive stars
Term
helium capture
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
Term
What evidence do we have that supports the theory of fusion of high-mass stars?
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.
Term
what happens once iron is in the core?
Definition
no more fusion takes place in the core, but fusion in the outer shell continues, so the core becomes more and more massive
Term
electron degeneracy pressure
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
Term
What happens once the iron core reaches a mass of 1.4 M(Sun)?
Definition
gravity becomes strong enough to overcome electron degeneracy pressure
the protons and neutrons fuse to form neutrinos
this happens instantaneously throughout the core
Term
What happens as a result of the core collapse?
Definition
the sudden flux of neutrinos tear apart the star, and its outer layers are thrown into space (supernova explosion)
Term
The Crab Nebula
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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