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The Weirdest Objects
in the Universe
General Relativity
Premise: acceleration and gravity are equivalent (i.e., are
you accelerating or falling in a gravitational field?)
Since the speed of light is always the same, this means that
the path of light can be bent by gravity.
Implication: Gravitational Lensing
One consequence of
general relativity is that
light must bend when it
goes through a
gravitational field.
Escape Velocity
According to Newton, the greater the gravity,
the faster an object must go to escape into
space. This is called the escape velocity.
Escape Velocity
Obviously, the escape velocity from any body depends on its
mass, and on the starting distance. The further away you are, the
weaker the gravity; the closer you are, the greater the gravity.
Black Holes
If the gravitational
attraction is large
enough (either
through high mass
or small distance),
there will be a place
where the escape
velocity is greater
than the speed of
light. This is a
Black Hole.
Gravity and Light
Remember: the force of
gravity around black holes
is the same as that on Earth.
G M1 M2
F = 
r2
At large distances, nothing
changes. If the Sun were a
black hole, the Earth’s orbit
would be the same as it is now.
The only difference is that
you can get a lot closer to a
black hole. (In other
words, r can get real small.)
4000 mi
Black Hole Sizes
2 x 10-26 cm
The size (i.e., the radius of
the event horizon) of a
black hole depends only
its mass.
1 cm
3 km
Event Horizon
The radius at which the
escape velocity is greater
than the speed of light is
called the event horizon
(sometimes called the
Schwarzschild radius).
Anything beyond the event
horizon will never return to
our universe.
Gravitational Redshift of Light
Close to a black hole, gravity is
strong. According to relativity:
 High Gravity  Large Acceleration
 Large Acceleration  High Speed
 High Speed  Time Dilation
Time slows down (as you measure
it) for someone close to a black
hole). This includes atoms – the
frequency of emitted light gets
smaller. Thus produces a
gravitational redshift.
It also means that for an object at the event horizon, time
stands still (at least, as you measure it).
Warping of Space-Time
Another way to look at the relation
between black holes and light is to
assume that light travels in straight
lines, but that mass warps spacetime. Orbits (and light) just follow
the curve.
Warping of Space-Time
Another way to look at the relation
between black holes and light is to
assume that light travels in straight
lines, but that mass warps spacetime. Orbits (and light) just follow
the curve.
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
A black hole represents
the extreme case where
gravity punches a hole
in space-time.
Black Holes, Neutron Stars, and Tides
Remember – gravity depends on mass and distance. Objects
such as neutron stars and black holes are very small, yet very
massive. So if you get close, the tides may get you!
Binary Star Evolution
About half the stars in the sky are binaries. These stars may
begin life as separate entities, but often times this does not last.
Roche Lobes
Between any two stars are gravitational balance points, where the
attraction of one star equals the attraction of the other. The point
directly between the stars is called the Lagrange point. The
balance points in general map out the star’s Roche Lobe. If a
star’s surface extends further than its Roche Lobe, it will lose its
mass.
Binary Star Classification
Detached: the stars are separate
and do not affect one another
Semi-detached: one star is spilling
mass (i.e., accreting) onto the other
Contact: two stars are present
inside a common envelope (i.e., it
is a common-envelope binary).
Common Envelope Evolution
If a red giant overflows its Roche lobe so that it engulfs the
companion, its outside may be stripped away, leaving only its hot
core.
QuickTime™ and a
Motion JPEG A decompressor
are needed to see this picture.
Common Envelope Evolution
If a red giant overflows its Roche lobe so that it engulfs the
companion, its outside may be stripped away, leaving only its hot
core.
This object will look like an (odd-shaped) planetary nebula
Accretion
If a star overflows its Roche
lobe through the Lagrange
point, its material will simply
go into orbit about the
companion. The material will
stay in the plane of the system
and form an accretion disk.
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
Accretion
According to Kepler’s laws, matter close to a star will orbit
faster than material further away. If there’s a lot of material in
a disk, this will cause the atoms will rub up against each other.
There will be friction! So
 The material will lose orbital energy and spiral in
 The disk will get real hot.
Accretion
According to Kepler’s laws, matter close to a star will orbit
faster than material further away. If there’s a lot of material in
a disk, this will cause the atoms will rub up against each other.
There will be friction! So
 The material will lose orbital energy and spiral in
 The disk will get real hot.
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
The faster the gas moves,
the greater the friction,
and the hotter the disk. If
the companion star is
compact (white dwarf,
neutron star, or black
hole), then near the center,
the disk will emit x-rays!
Accretion
According to Kepler’s laws, matter close to a star will orbit
faster than material further away. If there’s a lot of material in
a disk, this will cause the atoms will rub up against each other.
There will be friction! So
 The material will lose orbital energy and spiral in
 The disk will get real hot.
The faster the gas moves,
the greater the friction,
and the hotter the disk. If
the companion star is
compact (white dwarf,
neutron star, or black
hole), then near the center,
the disk will emit x-rays!
X-ray Identifications
Because accretion disks around compact objects can get much
hotter than stars, x-ray surveys can identify them!
Optical Picture
X-ray Picture
The more compact the object, the hotter the accretion disk, and
the more (very high energy) x-rays that are produced.
Novae
By definition, white dwarfs are what they are because they have
no more fuel to burn. But if a white dwarf accretes hydrogen,
it suddenly will have fuel, and can burn it – explosively.
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This is called a nova. Outbursts can occur once every few
years, or once every 50,000 yr, depending on the system.
When in outburst, a nova will be as bright as 500,000 L.
Type Ia Supernovae
Recall that white dwarfs are held up by electron degeneracy.
Their masses must therefore be less than 1.4 M. Over time,
accretion may push a white dwarf’s mass over this limit. If this
happens, the star will collapse, and become a Type Ia Supernova.
A Type Ia supernova is just as
bright as a regular (Type II)
supernova, but it doesn’t leave
behind a remnant.
Models
suggest that the star is totally
destroyed.
Millisecond Pulsars
When a star explodes as a supernova, the neutron star that is left
behind rotates about once a second. However, if a star accretes
onto this neutron star, it can cause it to spin 1000 times faster!
Millisecond Pulsars
When a star explodes as a supernova, the neutron star that is left
behind rotates about once a second. However, if a star accretes
onto this neutron star, it can cause it to spin 1000 times faster!
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
Evaporated Stars
Accretion disks around neutron stars (or black holes) emit large
numbers of very energetic x-ray photons. These x-rays can
strike the companion star’s atmosphere, and heat it up so much
that the star literally evaporates. All that remains may be some
rubble around a bare millisecond pulsar.
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
Accretion Disks and Black Holes
The accretion disk around a black hole can extend very close
to the event horizon. The gas speed there is very close to the
speed of light, so the friction in the disk is extremely intense.
This type of disk will produce the most-energetic x-rays.
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
But note: in the optical,
the disk may be faint –
much fainter than the
companion (donor) star.
Finding a Black Hole
• Identify the optical counterpart of an x-ray binary
• Observe the optical component of the binary
 Estimate the total mass of the system using Kepler’s and
Newton’s laws
 Estimate the mass of the visible star from its spectral
type, etc.
• Subtract to estimate the mass of the unseen companion
• Exclude possible stellar types based on visibility and
knowledge of stellar astrophysics
If all possibilities are excluded, you have a black hole!
Next time -- Star Formation