Binary Star Evolution

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Binary Star Evolution
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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.

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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 stars
  Roche Lobe. If a stars surface extends further
  than its Roche Lobe, it will lose its mass.

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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).
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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.
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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.
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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.

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Accretion

• According to Keplers laws, matter close to a
  star will orbit faster than material further
  away. If theres 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 very hot.

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Accretion

• According to Keplers laws, matter close to a
  star will orbit faster than material further
  away. If theres 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 very 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!
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Looking for Accretion Disks in X-rays
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.
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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.
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?.
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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 dwarfs 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 doesnt leave
behind a remnant. Models suggest that the star
is totally destroyed.
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Two Types of Supernovae
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Accretion onto Neutron Stars 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!
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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
stars atmosphere, and heat it up so much that
the star literally evaporates. All that remains
may be some rubble around a bare millisecond
pulsar.
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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.
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Summary Binary Star Evolution

• Half of all stars are in binary systems, so
  stellar evolution in binaries is important
• Roche Lobe 3-D boundary where the gravity of 2
  stars is equal if a star expands beyond this
  boundary some of its matter accretes onto the
  other star
• Matter that transfers from one star to another
  spirals onto the other star through an accretion
  disk
• As the matter gets closer to the object, it moves
  faster and gets hotter because of friction, and
  produces X-rays
• Nova the detonation of accumulated hydrogen in
  an accretion disk around a white dwarf
• Type 1a Supernova collapse and explosion of a
  white dwarf that has accreted enough mass to go
  overcome electron degeneracy

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Black Holes
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Outline Black Holes

• Escape velocity
• Definition of a black hole
• Event horizon
• Gravitational redshift time dilation
• Tidal forces of black holes
• Warping of space time
• Making black holes

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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.
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Escape Velocity
The escape velocity from any body depends on its
mass, and on the starting distance. The escape
velocity is larger for larger mass and smaller
distance.
Escape velocity ? (M/R)0.5
R
R
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Escape Velocity
At the surface of the Earth, the escape velocity
is 11 km/s.
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Escape Velocity
If Earth is compressed to a radius of 1
cm Escape velocity 300,000 km/s speed of
light
If the Earth is compressed any more, the escape
velocity would be greater than the speed of
light. So nothing could escape its surface, not
even light. This is the definition of a black
hole. The radius at which the escape velocity is
greater than the speed of light is called the
event horizon. Anything inside of the event
horizon will never return to our universe. This
is the point of no return.
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Black Hole Sizes

• The size (i.e., the radius of the event horizon)
  of a black hole depends only its mass.

2 x 10-26 cm
1 cm
3 km
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Black Holes Dont Suck
Force of gravity from a black hole is the same as
from any other object with the same mass at the
same distance.
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Black Holes Dont Suck
The orbit of the Earth would not change if the
Sun was replaced with a black hole with the same
mass as the Sun.
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But they have extremely strong gravity near them
because the mass is concentrated in a small area
Force on the rocket
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But they have extremely strong gravity near them
because the mass is concentrated in a small area
Force on the rocket
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But they have extremely strong gravity near them
because the mass is concentrated in a small area
Force on the rocket
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But they have extremely strong gravity near them
because the mass is concentrated in a small area
Force on the rocket
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Effect of Extreme Gravity on 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).
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Effect of Extreme Gravity on Light

• Even light feels the effect of gravity. Objects
  with stronger surface gravity bend the path light
  takes. Black holes bend light so much that it
  cant escape and falls back to the black hole.

white dwarf
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Effect of Extreme Gravity on 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
  space-time. Orbits (and light) just follow the
  curve.

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Effect of Extreme Gravity on 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
  space-time. Orbits (and light) just follow the
  curve.

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Effect of Extreme Gravity on Space-Time

• The warping of space-time by the Sun causes light
  to bend around the Sun.

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Warping of Space-Time

• A black hole represents the extreme case where
  gravity punches a hole in space-time.

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Tides near Black Holes

• 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!

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Tides near Black Holes

• 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!

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Making Black Holes

• Anything can become a black hole if it is
  compressed enough.
• One way that nature makes black holes is through
  the death of massive stars. These black holes
  have masses gt3 M?.

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Making Black Holes

• Black holes can sink to the center of galaxies,
  where they merge into one supermassive black hole.

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Making Black Holes

• Black holes can sink to the center of galaxies,
  where they merge into one supermassive black hole.

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Making Black Holes

• These black holes have masses of gt1,000,000 M?!

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Summary

• Escape velocity
• Definition of a black hole escape velocity is
  greater than the speed of light
• Event horizon radius at which the escape
  velocity the speed of light (size of a black
  hole)
• Gravitational redshift time dilation
• Near a black hole, light is redshifted and clocks
  slow down
• Tidal forces of black holes
• Warping of space time
• Making black holes
• Deaths of massive stars make less massive black
  holes
• Mergers of those black holes make supermassive
  black holes