How are the lives of stars with close companions different?

1
How are the lives of stars with close companions
different?
2
How could this strange pairing have come
about? Stars in Algol are close enough that
matter can flow from the subgiant (which just
left the main sequence) onto the main-sequence
star
3
Left-hand star is now a subgiant (just leaving
the MS), but was originally more massive, say 4.5
solar masses, than its companion (which started
with, say, 0.5 solar masses). These
original-mass stars are shown at top, on MS.
4
Left-hand star is now a subgiant (just leaving
the MS), but was originally more massive, say 4.5
solar masses, than its companion (which started
with, say, 0.5 solar masses). As the left-hand
star reached the end of its MS life and expanded,
it began to transfer mass to its companion.
5
Left-hand star is now a subgiant (just leaving
the MS), but was originally more massive, say 4.5
solar masses, than its companion (which started
with, say, 0.5 solar masses). As it reached the
end of its MS life and expanded, it began to lose
mass to its companion. Now the companion star is
more massive (it went from 0.5 to 4.3 solar
masses), while the mass-losing star (now a
subgiant) went from 4.5 to 0.7 solar masses.
6
Eventually the mass-losing subgiant star (the
star on the left) will become a white dwarf.
What happens after that? Role reversal! When
the star on the right becomes a giant, the white
dwarf gains matter from it.
7
Chapter 13The Bizarre Stellar Graveyard
8
What is a white dwarf?

• White dwarfs are the leftover cores of dead
  stars, usually made mostly of carbon (some are
  made mostly of helium others of oxygen or other
  elements heavier than carbon, up to and including
  iron).
• Their name comes from the fact they are 'born'
  glowing white-hot with high temperatures
  (remember that the core of a normal star has a
  higher temperature than the surface of the star).

9
In this Hubble space telescope photo we see
Sirius A, the visually brightest star in the sky,
and the white dwarf Sirius B as a tiny dot at the
lower left. (The spikes of light are artifacts
of the camera.)
10
In X-rays (photo at left), Sirius B, the white
dwarf, is brighter than its binary companion
Sirius A, the visually brightest star in the sky.
11
Electron degeneracy pressure supports white
dwarfs against gravity, and doesn't depend on
temperature. So a white dwarf has the same
temperature inside as on its surface (unlike
normal stars or planets).
12
White dwarfs cool off and grow dimmer with time
13
Hubble space telescope photo of white dwarfs in a
globular cluster theyre very dim!
14
White dwarfs cool off and grow dimmer with
time. So not all white dwarfs are white they
have colours from blue-white (young) to
orange-red (old).
15
(No Transcript)
16
White dwarfs shrink when you add mass to them
because their gravity gets stronger. Temperature
also increases.
17
Shrinkage of White Dwarfs

• White dwarfs shrink when they get heavier!
• Quantum mechanics says that electrons in the same
  place cannot be in the same state
• Adding mass to a white dwarf increases its
  gravity, forcing electrons into a smaller space

18
Shrinkage of White Dwarfs

• Quantum mechanics says that electrons in the same
  place cannot be in the same state
• Adding mass to a white dwarf increases its
  gravity, forcing electrons into a smaller space
• In order to avoid being in the same state in the
  same place some of the electrons need to move
  faster. That increases the temperature, but not
  the pressure - degeneracy pressure doesn't depend
  on temperature
• Is there a limit to how much you can shrink a
  white dwarf? (That is, how much mass a WD can
  have?)

19
The White Dwarf Mass Limit Einsteins theory
of relativity says that nothing can move faster
than light. The speed of limit is the same
relative to all observers. When electron speeds
in a white dwarf approach the speed of light,
electron degeneracy pressure can no longer
support the white dwarf. Chandrasekhar found (at
age 20!) that this happens when a white dwarfs
mass reaches 1.4 MSun
20
What can happen to a white dwarf in a close
binary system?
21
White dwarfs gravity pulls matter off of giant
companion, but angular momentum prevents the
matter from falling straight in instead, it
forms an accretion disk around the white dwarf.
22
Friction in disk makes it hot, causing it to
glow Friction also removes angular momentum from
inner regions of disk, allowing them to sink onto
white dwarf
23
Hydrogen that accretes onto a white dwarf builds
up in a shell on the surface When base of
shell gets hot enough, hydrogen fusion suddenly
begins and causes a nova
24
Nova explosion generates a burst of light lasting
a few weeks and expels much of the accreted gas
into space
25
Two Types of Supernova
Massive star supernova (Type II) Iron core of
massive star reaches white dwarf limit and
collapses into a neutron star rest of star
'bounces' off neutron star and explodes White
dwarf supernova (Type Ia) As white dwarf in
close binary system reaches white dwarf limit,
carbon fusion begins suddenly, throughout the
white dwarf (uniform temperature) complete
explosion of white dwarf into space
26
One way to tell supernova types apart is through
their light curves (showing how luminosity
changes with time)
27
Nova or White Dwarf Supernova?

• Supernovae are MUCH, MUCH more luminous (about 10
  million times)
• Nova H to He fusion in a surface layer, white
  dwarf left intact
• White dwarf Supernova complete explosion of
  white dwarf, nothing left behind

28
Supernova Type Massive Star or White Dwarf?

• Light curves differ (brightness changes over time
  are different)
• Spectra differ (exploding white dwarfs dont have
  hydrogen absorption lines --- they're made of
  carbon and some oxygen, but essentially no
  hydrogen)

29
What have we learned?

• How are the lives of stars with close
  companions different?
• When one star in a close binary system begins to
  swell in size at the end of its hydrogen-burning
  life, it can begin to transfer mass to its
  companion. This mass exchange can then change the
  remaining life histories of both stars.
• Sun

30
What have we learned?

• What is a white dwarf?
• A white dwarf is the core left over from a
  low-mass star, supported against the crush of
  gravity by electron degeneracy pressure.
• What can happen to a white dwarf in a close
  binary system?
• It can acquire hydrogen from its companion
  through an accretion disk. As hydrogen builds up
  on the white dwarfs surface, it may ignite with
  nuclear fusion to make a nova, or compress the
  white dwarf until carbon fusion creates a
  supernova.