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Lecture

L15.docx

7 pages92 viewsWinter 2013

Department
Science
Course Code
SCI238
Professor
Mike Fich

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L15
- when the supernova explodes it expels heavy elements into space, and
produces new elements (heavier than iron) in the ejecta. This is the only known
process that makes
these elements.
all are: small, dense, no fusion
- masses: from <1MSun to >10MSun
- radii: ~constant => gravity-pressure balance
- pressure support needed depends on mass
- white dwarf (WD) -> neutron star (NS) -> black hole =>
sequence of mass, pressure support limits, density, radius
White Dwarfs
the exposed core of a star that has died and shed its outer layers in a
planetary nebula. It is quite hot when it first forms, because it was the
inside of a star, but it slowly cools with time.
no internal energy source -> white dwarfs cool. > black dwarf
o Its size will never change, because its electron degeneracy
pressure will forever keep it stable against the crush of gravity.
if they were gaseous they would contract as the gas pressure would
decrease.
but, white dwarfs are supported by electron degeneracy pressure
Pe not dependent on temperature; depends only on density and number
of electrons
o this has an interesting and important effect on the stars structure:
the more massive the star is, the smaller it is.
o masses like those of stars
o sizes (radii) like that of Earth
o can shine quite brightly in high-energy light such as ultraviolet
and X rays
o starlike mass and a small size makes gravity very strong near its
surface.
o
leads to an upper mass limit for white dwarfs: must be less than 1.4 Solar
Masses. What happens to more massive dead stars?
o electron speeds are higher in more massive white dwarfs
o electron speeds would reach the speed of light in a white dwarf
with a mass of about 1.4 times the mass of the Sun (1.4MSun).
o
the pressure that opposes gravity must come from some other source.
The source is degeneracy pressurethe same type of pressure that
supports the ―failed stars‖ that we call brown dwarfs and that arises
when subatomic particles are packed as closely as the laws of quantum
mechanics allow
o electron degeneracy pressure: closely packed electrons
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o outward push of electron degeneracy pressure matches the inward
crush of gravity.
What can happen to a white dwarf in a close
binary system?
o When a clump of mass first spills over from the companion to the
white dwarf, it has some small orbital velocity.
o law of conservation of angular momentum dictates that the clump
must orbit faster and faster as it falls toward the white dwarf’s
surface.
o infalling matter therefore forms a whirlpool-like disk around the
white dwarf.
o material falls onto another body is called accretion, this rapidly
rotating disk is called an accretion disk.
o The inward-spiraling gas in the accretion disk becomes quite hot
as its gravitational potential energy is converted into thermal
energy
o shine with intense ultraviolet or X-ray radiation
o dramatic events can occur as fresh hydrogen gas from the
companion star accumulates on the surface of a white dwarf.
more massive star evolves first to giant or supergiant
mass in outer envelope of expanded star transferred
to less massive companion
...which can eventually become the more massive
component...
o
Novae
hydrogen spilling toward the white dwarf from the companion
gradually spirals inward through the accretion disk
white dwarf’s strong gravity compresses this hydrogen gas into a
thin surface layer.
pressure and the temperature rise as the layer builds up with more
accreting gas.
bottom of the layer reaches about 10 million K, hydrogen fusion
suddenly ignites
This thermonuclear flash causes the binary system to shine for a
few glorious weeks as a nova
o less luminous than a supernova, but still can shine as
brightly as 100,000 Suns.
o generates heat and pressure, ejecting most of the material
that has accreted onto the white dwarf.
o material expands outward, creating a nova remnant that
sometimes remains visible years after the nova explosion
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