Download 11. Dead Stars

Astronomy 110: SURVEY OF ASTRONOMY
11. Dead Stars
1. White Dwarfs and Supernovae
2. Neutron Stars & Black Holes
Low-mass stars fight gravity to a standstill by becoming
white dwarfs — degenerate spheres of ashes left over
from nuclear burning. If they gain too much mass,
however, these ashes can re-ignite, producing a titanic
explosion. High-mass stars may make a last stand as
neutron stars — degenerate spheres of neutrons. But
at slightly higher masses, gravity triumphs and the result
is a black hole — an object with a gravitational field so
strong that not even light can escape.
1. WHITE DWARFS AND SUPERNOVAE
a. Properties of White Dwarfs
b. White Dwarfs in Binary Systems
c. Supernovae and Remnants
White Dwarfs in a Globular Cluster
Hubble Space Telescope Finds Stellar Graveyard
The Companion of Sirius
Sirius weaves in its path; has a companion (Bessel 1844).
1800
1810
1820
1830
1840
1850
1860
“Father, Sirius is a double star!” (Clark 1862).
P = 50 yr, a = 19.6 au
a3
=
M
+
M
A
B
2
P
3 M⊙
MA = 2 M⊙, MB = 1 M⊙
1870
Why is the Companion So Faint?
Because it’s so small!
ρB
LB ≈ 0.0001 LA
DB = 12000 km < D⊕♁
2 × 106 g/cm3
The Dog Star, Sirius, and its Tiny Companion
Origin and Nature
A white dwarf is the degenerate carbon/oxygen core
left after a double-shell red giant ejects its outer layers.
Degeneracy Pressure
Electrons are both particles and waves.
The wavelength λ of an electron is
e
Rules for electrons in a box:
1. Waves must fit evenly.
2. Each wave must be different.
(Note: the same rules apply to
electron orbits around nuclei.)
h
λ=
me v
Degeneracy Pressure
Electrons are both particles and waves.
The wavelength λ of an electron is
e
Rules for electrons in a box:
1. Waves must fit evenly.
2. Each wave must be different.
Suppose we compress the box:
• all λ decrease all v increase!
• energy cost implies pressure
h
λ=
me v
Sizes of White Dwarfs
Planet Earth
1 M⊙ White Dwarf
1.3 M⊙ White Dwarf
surface gravity: 1 G
surface gravity: 3.8×105 G
surface gravity: ~2×106 G
The higher a white dwarf’s mass, the smaller its radius!
This trend continues to lower masses; Jupiter
is about as large as degenerate objects get.
White Dwarf Structure
Visible surface: normal gas
~50 km thick; pure H or He
Interior: degenerate matter
typically C/O mixture
Center: degenerate matter
C/O nuclei in crystal form
Galaxy's Largest Diamond
Star gradually crystalizes from inside out as it cools.
White Dwarfs in Binary Systems
A white dwarf orbiting another star can become active
when the other star becomes a red giant...
Animation of Interacting Stars
Accretion Disks
Mira: "Wonderful" Star Reveals its Hot Nature
Because it has angular momentum, the transferred gas
orbits around the dwarf, forming an accretion disk.
Friction in the disk moves angular momentum outward
and mass inward. The disk becomes incandescent.
Classical Novae
H and He from the companion build up on the white
dwarf’s surface.
When enough has accumulated, the H burns violently,
producing a thermonuclear explosion.
Explosions from White Dwarf Star RS Oph
Classical Novae: RS Ophiuci
Classical Novae: RS Ophiuci
16 Feb, 2006
Recurrent Nova RS Ophiuci
Explosions repeat every ~20 yr; about 10% of accreted
mass is retained. Current mass: MWD 1.35 M⊙.
The Limits of Degeneracy
c
Smaller wavelengths imply higher
velocities, v.
As v gets near the speed of light,
c, electrons behave like photons.
velocity
We can add electrons if they
have smaller wavelengths, λ.
Light (radiation) pressure makes
stars unstable, so no white dwarf
can weigh more than ~1.4 M⊙.
0
White Dwarf Supernovae
If a white dwarf’s mass reaches 1.4 M⊙, carbon ignites
in its degenerate center, and a thermonuclear explosion
completely destroys the star.
Illustration of Mira System
White Dwarf Supernova Simulation
3-D Simulation of Type Ia Supernova
Origin of the Elements
Explosion makes ~0.8 M⊙
of Fe-group elements.
Iron-Group Elements
Most of the iron in the universe is made by white-dwarf
supernovae.
Supernovae Compared
Two different scenarios produce titanic explosions:
Massive Star SN
White Dwarf SN
Evolved star with initial
mass > 8 M⊙.
White dwarf in binary
with nearby giant star.
Degenerate Fe core
reaches 1.4 M⊙.
Degenerate C/O star
reaches 1.4 M⊙.
Gravitational collapse
2
yields ~ 0.2 M⊙c ;
99% escapes as neutrinos.
Nuclear burning of C/O
2
yields ~ 0.002 M⊙c .
Supernovae Light Curves
Ni56 → Co56:
half-life 6 days
Co56 → Fe56:
half-life 77 days
Massive-star and white-dwarf supernovae reach similar
peak luminosities and fade gradually with time.
Supernovae Remnants
Inner remnant
(iron lines)
X-ray
Outer shock wave
(hydrogen lines)
Visible
White-Dwarf Supernova Remnant
DEM L71: Supernova Origin Revealed
Debris from supernova explosions expand at thousands
of km/s in all directions, slamming into interstellar gas.
Remnant of Tycho’s Supernova (1572)
supernova
debris
relatavistic
electrons
Shock velocity
3000 km/s ~ 0.01 c
Tycho's Supernova Remnant Provides Shocking Evidence for Cosmic Rays
Remnants accelerate cosmic rays to near light-speed.
The Crab Nebula (1054)
The Crab Nebula from Hubble
The Veil Nebula
The Veil Nebula Unveiled
2. NEUTRON STARS AND BLACK HOLES
a. Neutron Stars and Pulsars
b. A Brief Introduction to Black Holes
Nearest Known Neutron Star
Hubble Sees a Neutron Star Alone in Space
Origin and Nature
Animation of Star Collapse
A neutron star is the degenerate sphere of neutrons
left after the iron core of a massive star collapses.
Birth of a Neutron Star
In the core, nuclei are smashed
into protons & neutrons; the
protons combine with electrons
to make neutrons & neutrinos.
At birth, the temperature of a neutron star is ~1011 K,
6
7
but neutrino emission cools it to ‘only’ 10 to 10 K.
Sizes of Neutron Stars
Google Maps: Oahu
Sizes of Neutron Stars
~20 km
surface gravity: ~1011 G
density: ρ ≈ 1015 g/cm3
Artist's impression of a neutron star
Why Are Neutron Stars So Small?
White dwarfs are supported by electron degeneracy;
the electron wavelength is
h
λ=
me v
Neutron stars are supported by neutron degeneracy;
the neutron wavelength is
h
λ=
mn v
Now, mn = 1840 me, so we expect neutron stars to be
about 1840 (say, 2000) times smaller than white dwarfs.
Why Do Neutron Stars Spin So Fast?
Conservation of angular momentum: before collapsing,
the star’s core probably rotates once every few hours.
Collapse by a factor of x decreases
the rotation period by a factor of x2.
The core collapses by roughly a
factor of 1000, so it spins about
10002 = 106 times more often.
Final rotation period is a few hundredth’s of a second!
Pulsars
A pulsar is a spinning neutron star with a magnetic field
tilted at an angle to its rotation axis.
Pulsars
Particles accelerated
by the spinning field
create two beams of
radiation aligned with
the magnetic axis.
As the pulsar turns,
these beams sweep
through space.
Discovery of Pulsars
• Using a radio telescope in 1967, Jocelyn Bell noticed
very regular pulses of radio emission coming from a
single part of the sky.
• The pulses were coming from a spinning neutron star
—a pulsar.
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s
A
l
l
a
m
As S
Why Pulsars Must Be Neutron Stars
Circumference of Neutron Star = 2π (radius) ~ 60 km
Spin Rate of Fast Pulsars ~ 1000 cycles per second
Surface Rotation Velocity
~ 60,000 km/s
~ 20% speed of light
~ escape velocity from NS
Anything else would be torn to pieces!
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The Crab Nebula Pulsar
The Crab Nebula from Hubble
The Crab Nebula Pulsar
The Crab Nebula and Pulsar
Crab Nebula: a Dead Star Creates Celestial Havoc
Neutron Stars in Binary Systems
X-Ray Bursts
• Matter accreting
onto a neutron star
can eventually
become hot enough
for helium to fuse.
• The sudden onset
of fusion produces
a burst of X rays.
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Degeneracy’s Limits, Again!
c
A neutron star which somehow
gains more mass presumably
collapses to form a black hole.
velocity
Just as with white dwarfs, there’s
a maximum allowed mass for a
neutron star, roughly 3 M⊙.
0
A Brief Introduction to Black Holes
Black Hole Images
A Brief Introduction to Black Holes
A black hole is an object with a gravitational field so
strong that not even light can escape.
Black Hole Images
Thought Question
What happens to the escape velocity from an object if
you shrink it?
A. It increases.
B. It decreases.
C. It stays the same.
Hint:
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Escape Velocity
initial kinetic
energy
(escape velocity)2
2
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=
=
final gravitational
potential energy
G × (mass)
(radius)
The Schwarzschild Radius
Let’s insert the speed of light, c, into the escape-velocity
equation:
c2
GM
=
R
2
The result is a relationship between the mass, M, and
radius, R, of a black hole. Solving for R, we get
2GM
R=
c2
R is called the Schwarzschild Radius. Any object
of mass M becomes a black hole if its radius is less than
or equal to than R, because light is unable to escape.
Sizes of Black Holes
18 km
M = 3 M⊙
Google Maps: Oahu
A black hole’s mass
strongly warps space
and time in the
vicinity of the event
horizon.
Note: ‘event horizon’
is another term for the
Schwarzschild Radius.
These diagrams show
how space becomes
warped near a massive
object or black hole.
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No Escape
• Nothing can escape from within the event horizon
because nothing can go faster than light.
• No escape means there is no more contact with
something that falls in. It increases the hole’s mass,
changes its spin or charge, but otherwise loses its
identity.
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Singularity
• Beyond the neutron star limit, no known force can resist
the crush of gravity.
• As far as we know, gravity crushes all the matter into a
single point known as a singularity.
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Evidence for Black Holes
Black Holes Are Black
Some X-ray sources are unusually faint — evidence that
accreting matter falls into a black hole instead of falling
onto a neutron star.