Download white dwarf supernova

White Dwarfs
Sirius, the brightest star in the sky, is a visual binary.
Sirius A vastly outshines Sirius B in visible and infrared wavelengths.
Sirius A
Sirius B
Infrared
X-ray
However, Sirius B outshines Sirius A in X-ray wavelengths.
Sirius B is a white dwarf,
and therefore much
hotter than Sirius A.
It is fainter in visible and
infrared because it is so
much smaller.
However, Sirius A is not
hot enough to radiate
much in X-rays, so even
a small star that is hot
enough can outshine it in
X-rays.
So you think you know what a white dwarf is?
• Its composition depends on the mass of the star it formed from:
• The smallest stars form helium white dwarfs.
• Sun-sized stars form carbon white dwarfs.
• Larger (but still “low-mass”) stars form white dwarfs with a mix of heavier elements
• A teaspoon of this material – held up against gravity by electron degeneracy pressure – would weigh
several tons on Earth.
More massive white dwarfs are smaller than less massive white dwarfs.
(Degeneracy pressure only depends on density)
Stronger gravity
requires stronger
degeneracy pressure to
balance it.
Which is hotter, a red giant star or a white dwarf?
A) The red giant
B) The white dwarf
C) They are generally the same temperature
D) Can’t answer without more information
Which is more luminous, a red giant star
or a white dwarf?
A) The red giant
B) The white dwarf
C) They are roughly the same luminosity
D) Can’t answer without more information
Which is hotter, a red giant star or a white dwarf?
A) The red giant
B) The white dwarf
C) They are generally the same temperature
D) Can’t answer without more information
Which is more luminous, a red giant star
or a white dwarf?
A) The red giant
B) The white dwarf
C) They are roughly the same luminosity
D) Can’t answer without more information
Which is larger, a 0.8 MSun white dwarf or a 1.3 MSun white dwarf?
A) The 0.8 MSun white dwarf
B) The 1.3 MSun white dwarf
C) They are the same size
D) Can’t answer without more information
Back to strange behavior from close binary stars…
Matter spilling off a partner star onto a white dwarf forms an accretion disk
Accretion disks generate a
lot of heat and can radiate
in UV and even X-rays.
That material eventually falls all the way to the
surface of the white dwarf.
Temperature and pressure increase as more and
more hydrogen accumulates.
When the temperature reaches 10,000,000 K,
hydrogen fusion ignites!
This burst of fusion is called a nova
The nova shines for a few weeks, as brightly as
100,000 Suns at its peak
The nova does not destroy the star.
In fact, accretion resumes shortly after the
nova subsides…
A single white dwarf can go nova more
than once:
T Pyxidis went nova in 1890, 1902, 1920,
1944, 1966 and 2011
HST – VIS
mv : 15.5  7.0 (15,600 ly away)
Over many cycles of accretion – nova – accretion – nova … the white dwarf’s mass can grow.
But there is a limit to how much mass can be supported by electron degeneracy pressure.
When the white dwarf reaches about 1.4 MSun (the Chandrasekhar limit)
it hits that limit and goes BOOM!
This is called a white dwarf supernova
When the white dwarf hits the mass limit, it gets hot enough for carbon fusion to start.
It undergoes carbon fusion everywhere at once, so it’s a HUGE release of energy.
This is called a “light curve”
It plots luminosity as a function of time
Type II – hydrogen lines present
Type I – no hydrogen lines
(WD SN = Type 1a)
Astro-Cash Cab!
Tyler
Zach
Aaron
Katie
1) What is the source of energy in a white dwarf supernova?
Nuclear fusion
Nuclear fission
Gravitational potential energy
Chemical potential energy
2) Which supernova shows no hydrogen lines in its spectrum?
Type I – White dwarf supernova
Type II – Massive star supernova
They both show hydrogen lines
Neither shows hydrogen lines
3) What is the upper mass limit of a white dwarf?
4) True / False
Someday our Sun may end its existence in a white dwarf supernova explosion.