Transcript What is a white dwarf?

Clicker question:
How are the lives of stars with
close companions different?
Algol is a binary: a 4.3Msun MS star and a 0.7Msun star
just leaving the MS. What’s odd about that?
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1. Nothing
2. 0.7Msun is not enough
mass to form a true
star
3. A 4.3Msun star spends
less time on the MS
than a 0.7Msun star
4. Binary stars always
have equal masses
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
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.
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.
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.
Eventually the masslosing 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.
Chapter 13
The Bizarre Stellar Graveyard
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).
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.)
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.
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).
White dwarfs cool
off and grow
dimmer with time
Hubble space telescope photo of white dwarfs
in a globular cluster… they’re very dim!
White dwarfs cool
off and grow
dimmer with time.
So not all white
dwarfs are white:
they have colours
from blue-white
(young) to orangered (old).
A solar-mass white dwarf is about the same size as Earth
White dwarfs shrink when you add mass to them because
their gravity gets stronger. Temperature also increases.
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
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?)
The White Dwarf Mass Limit
Einstein’s 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.
S. Chandrasekhar
Chandrasekhar found (at age 20!)
that this happens when a white
dwarf’s mass reaches 1.4 MSun
What can happen to a white
dwarf in a close binary system?
White dwarf’s 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.
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
What would gas in an accretion disk do
if there was no friction in the disk?
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1. It would orbit
forever
2. It would eventually
fall in
3. 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
Nova explosion generates a burst of light lasting a few weeks
and expels much of the accreted gas into space
What happens to a white dwarf in a binary when it
accretes enough matter to reach the 1.4 MSun limit?
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1. It explodes
2. It collapses into a
neutron star
3. It gradually
begins fusing
carbon in its core
4. 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
One way to tell supernova types apart is through their
light curves (showing how luminosity changes with time)
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
Supernova Type:
Massive Star or White Dwarf?
• Light curves differ (brightness changes over
time are different)
• Spectra differ (exploding white dwarfs don’t
have hydrogen absorption lines --- they're
made of carbon and some oxygen, but
essentially no hydrogen)
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
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 dwarf’s
surface, it may ignite with
nuclear fusion to make a
nova, or compress the white
dwarf until carbon fusion
creates a supernova.
Activity 25, Special Relativity, p. 83
Parts I, II and IV (skip part III for now)
1. You’re standing in the aisle of a plane flying
at constant speed, and drop your headphones.
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1. They fall directly
below your hand.
2. They fall down, but
land towards the
back of the plane.
3. They fall down, but
land towards the
front of the plane.
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2A. According to Einstein, what
will Pertti measure for the speed of
the pulse sent by Riitta?
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1. Lightspeed c minus
rocket speed V
2. Lightspeed c
3. Lightspeed c plus
rocket speed V
:10
2B. What will Pertti (on a rocket
skateboard) measure for the speed of light
from a streetlight in front of her?
1. Lightspeed c minus
skateboard speed v
2. Lightspeed c
3. Lightspeed c plus
skateboard speed v
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2C. What will Pertti (on a rocket
skateboard) measure for the speed of a light
pulse sent by Riitta in her rocket ship?
1. Lightspeed c minus
skateboard speed v
minus rocket speed V
2. Lightspeed c
3. Lightspeed c plus
skateboard speed v
plus rocket speed V
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2D. Velocity = length divided by time.
If lightspeed is absolute…
1. Length and/or time
must be absolute.
2. Length must be
relative.
3. Time must be
relative.
4. Length and/or time
must be relative.
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Part IV, page 85-86
6A: Referring to Fig. 3, in which Graciela is
stationary, whose light pulses travel the greater
distance between ticks?
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1. The light pulses in
Graciela’s clock
2. The light pulses in
Dimitris’ clock
3. 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 …
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1. Is longer than the
interval between ticks
on her own clock
2. Is shorter than the
interval between ticks
on her own clock
3. Is the same as the
interval between ticks
on her own clock
:10
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:
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1. As seen by Graciela, the
travel time (time between
light pulses) is longer for
Dimitris’ clock.
2. As seen by Graciela, the
travel time (time between
light pulses) is shorter for
Dimitris’ clock.
:10
MOVING CLOCKS RUN SLOW
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.
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 person’s point of view,
your watch is running slow relative to the clock.
Dmitris perceives that the interval between
ticks on Graciela’s clock …
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1. Is longer than the
interval between ticks
on his own clock
2. Is shorter than the
interval between ticks
on his own clock
3. Is the same as the
interval between ticks
on his own clock
:10
6E: Whose clock is keeping the “right”
time?
1. Graciela’s
2. Dimitris’
3. Both clocks
4. 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 person’s point of view,
your watch is running slow relative to the clock.