Chapter 13 The Bizarre Stellar Graveyard

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Chapter 13The Bizarre Stellar Graveyard
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What is a white dwarf?

• White dwarfs are the leftover cores of dead
  stars, made mostly of carbon.
• 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).

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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.
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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).
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White dwarfs cool off and grow dimmer with time
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Hubble space telescope photo of white dwarfs in a
globular cluster theyre very dim!
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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).
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White dwarfs shrink when you add mass to them
because their gravity gets stronger. Temperature
also increases.
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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

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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?)

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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
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What can happen to a white dwarf in a close
binary system?
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But firstHow are the lives of stars with close
companions different?
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Clicker Question

• The binary star Algol consists of a 4.3 MSun main
  sequence star and a 0.7 MSun subgiant star (a
  star just leaving the main sequence).
• Whats strange about this pairing?

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Algol is a binary a 4.3Msun MS star and a
0.7Msun star just leaving the MS. Whats odd
about that?

• Nothing
• 0.7Msun is not enough mass to form a true star
• A 4.3Msun star spends less time on the MS than a
  0.7Msun star
• Binary stars always have equal masses

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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
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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.
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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.
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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.
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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.
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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.
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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
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What would gas in an accretion disk do if there
was no friction in the disk?

• It would orbit forever
• It would eventually fall in
• It would be blown out of the disk

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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
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Nova explosion generates a burst of light lasting
a few weeks and expels much of the accreted gas
into space
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What happens to a white dwarf in a binary when it
accretes enough matter to reach the 1.4 MSun
limit?

• It explodes
• It collapses into a neutron star
• It gradually begins fusing carbon in its core
• Nothing special

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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
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One way to tell supernova types apart is through
their light curves (showing how luminosity
changes with time)
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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

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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)

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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

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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.

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Activity 25, Special Relativity, p. 91

• Are any of these the last 4 digits of your
  clickers device ID number?
• 01B4AA
• If so, REGISTER YOUR CLICKER!

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1. Youre standing in the aisle of a plane flying
at constant speed, and drop your headphones.

• They fall directly below your hand.
• They fall down, but land towards the back of the
  plane.
• They fall down, but land towards the front of the
  plane.

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1D. Velocity length divided by time. If
lightspeed is absolute

• Length and/or time must be absolute.
• Length must be relative.
• Time must be relative.
• Length and/or time must be relative.

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Part IV, page 93

• If youre interested in relativity, Part III is
  quite interesting, but we wont do it in class.

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6A Referring to Fig. 3, in which Graciela is
stationary, whose light pulses travel the greater
distance between ticks?

• The light pulses in Gracielas clock
• The light pulses in Dimitris clock
• Neither both their light pulses travel the same
  distance between ticks

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6C Graciela perceives that the interval between
ticks on Dmitris clock

• Is longer than the interval between ticks on her
  own clock
• Is shorter than the interval between ticks on her
  own clock
• Is the same as the interval between ticks on her
  own clock

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What we know Lightspeed c is a constant.
c(distance traveled by light)/(travel time). As
seen by Graciela, Dimitris light pulses travel a
greater distance. Therefore

• As seen by Graciela, the travel time (time
  between light pulses) is longer for Dimitris
  clock.
• As seen by Graciela, the travel time (time
  between light pulses) is shorter for Dimitris
  clock.

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MOVING CLOCKS RUN SLOW
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MOVING CLOCKS RUN SLOW

• If a clock is moving relative to you, it runs
  slower than your watch, which is not moving
  relative to you.

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MOVING CLOCKS RUN SLOW

• If a clock is moving relative to you, it runs
  slower than your watch, which is not moving
  relative to you.
• From the point of view of someone not moving
  relative to the clock, you and your watch are
  moving. So from that persons point of view,
  your watch is running slow relative to the clock.

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Dmitris perceives that the interval between ticks
on Gracielas clock

• Is longer than the interval between ticks on his
  own clock
• Is shorter than the interval between ticks on his
  own clock
• Is the same as the interval between ticks on his
  own clock

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6E Whose clock is keeping the right time?

• Gracielas
• Dimitris
• Both clocks
• Neither clock

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A CLOCK MOVING RELATIVE TO YOU RUNS SLOWER THAN A
CLOCK NOT MOVING RELATIVE TO YOU

• If a clock is moving relative to you, it runs
  slower than your watch, which is not moving
  relative to you.
• From the point of view of someone not moving
  relative to the clock, you and your watch are
  moving. So from that persons point of view,
  your watch is running slow relative to the clock.