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					    Binary Stellar Evolution
•When stars form, common for two or
more to end up in orbit
   •Multiples more common than singles
   •Binaries are the most common multiples
•Most triples and higher are “hierarchical
binaries”
           Wide Binaries



•When binaries are far apart (several AU),
nothing unusual happens
  •More massive star lives, ages, dies
  •Less massive star lives, ages, dies
    Close Binaries
•When binaries are close together (AU or less),
they can interact extensively
  •During giant stages, gas can be transferred
  from one star to the other
  •This can affect evolution or appearance of
  star
             Roche Lobes
•In a binary system, the region of space
controlled by each star is called the Roche
lobe of that star
•Anything within the Roche lobe is likely to
be absorbed by the star
•The more massive star has a bigger Roche lobe
          Star A’s  No man’s land
      Roche Lobe              Star B’s
                             Roche Lobe
   Close Binary evolution: part 1
•During main sequence, nothing interesting
happens
•When the first star becomes a red giant, it
expands
  •If it expands enough, it can fill its Roche lobe
  •Any more expansion leads to mass transfer
          Accretion disks


                                Second star
•Incoming gas is rotating - from revolution
of two stars
   •Gravity pulling it towards object
   •Gravity vs. rotation = disk
•The system changes
   •Stars may merge or separate
   •Second star may become more massive star
Heavier star becomes a stellar corpse
•Compact object: any of the three
types of stellar corpses
    •White dwarf
    •Neutron star
    •Black hole
 •When second star becomes a
 giant, evolution gets interesting.
         Binary evolution
A binary star consists of a main sequence
star and a compact object. Which of these
is currently more massive?
A) The main sequence star
B) The compact object
C) It could be either one
Close Binary evolution: part 2
Some of the most bizarre and interesting
objects are binary combinations of giant
star/compact object
•Properties depend on type of compact
object
•White dwarf            •Neutron star
  •Nova                   •X-ray pulsar
  •Type I supernova     •Black hole
                          •X-ray binary
    C      B                      A

A is heavier than B is heavier than C. When
could this system first become one of the
peculiar binaries we are talking about?
A) When A becomes a giant star
B) When B becomes a giant star
C) When C becomes a giant star
Close Binary evolution: part 2
•Binary star: giant star/compact object
•Properties depend on type of compact
object


•White dwarf             •Neutron star
  •Nova                    •X-ray pulsar
  •Type I supernova      •Black hole
                           •X-ray binary
How to make a Nova          White
                            Dwarf




•White dwarf
  •Hydrogen gets added to
  carbon/oxygen layer
  •Builds up
  •Ignites and explodes
•Cycle repeats
 How to make a type I Supernova
•During each cycle the white dwarf gains mass
  •Shrinks slightly
•Reaches Chandrasekhar mass
  •Star begins to collapse
  •Heats up
  •Fusion begins
  •Whole star burns - explodes
•Star is completely destroyed
  •Burns mostly to iron
Close Binary evolution: part 2
Some of the most bizarre and interesting
objects are binary combinations of giant
star/compact object
•Properties depend on type of compact
object
•White dwarf            •Neutron star
  •Nova                   •X-ray pulsar
  •Type I supernova     •Black hole
                          •X-ray binary
  How to make an X-Ray Pulsar
 •Neutron star with hydrogen flowing in
 •Magnetic field of neutron star channels gas
 to magnetic poles




X-rays

•Large gravity – gas slams into neutron star
•Spot on neutron star gets very hot
How to make an X-Ray Pulsar
•The neutron star is rotating as well
•As viewed from here, hot spot appears and
disappears – it pulses
Close Binary evolution: part 2
Some of the most bizarre and interesting
objects are binary combinations of giant
star/compact object
•Properties depend on type of compact
object
•White dwarf            •Neutron star
  •Nova                   •X-ray pulsar
  •Type I supernova     •Black hole
                          •X-ray binary
  How to make an X-Ray Binary
                                       X-Rays


                                  Black Hole
What happens to the gas after it crosses
 •Accretion disk, as always
inside the event horizon?
 •Gas heats up even more, contributing
A) It  is going super fast - enormous gravity
 •Friction heats up the disk
more X-rays
 •X-rays from hot gas
B) It impacts on the surface of the black
 •Most efficient way to make energy
hole, producing additional energy
      •Even more efficient than fusion!
C) It disappears forever
           Doubly dead stars
•A binary system eventually ends as two
compact objects
   •Usually nothing else happens
•If very close (neutron stars or black holes)
more happens
   •Stars emit gravity waves – they move closer
   •Merge to make black hole

				
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posted:2/28/2010
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