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Black
Holes
There are many popular myths concerning black holes, many
of them perpetuated by Hollywood. Television and movies have portrayed them as
time-traveling tunnels to another dimension, cosmic vacuum cleaners sucking up
everything in sight, and so on. It can be said that black holes are really just
the evolutionary end point of massive stars. But somehow, this simple
explanation makes them no easier to understand or less mysterious.
What Are Black Holes?
Black holes are the evolutionary endpoints of stars at
least 10 to 15 times as massive as the Sun. If a star that massive or larger
undergoes a supernova explosion, it may leave behind a fairly massive burned out
stellar remnant. With no outward forces to oppose gravitational forces, the
remnant will collapse in on itself. The star eventually collapses to the point
of zero volume and infinite density, creating what is known as a " singularity
". As the density increases, the path of light rays emitted from the star are
bent and eventually wrapped irrevocably around the star. Any emitted photons are
trapped into an orbit by the intense gravitational field; they will never leave
it. Because no light escapes after the star reaches this infinite density, it is
called a black hole.
But contrary to popular myth, a black hole is not a
cosmic vacuum cleaner. If our Sun was suddenly replaced with a black hole of the
same mass, the earth's orbit around the Sun would be unchanged. (Of course the
Earth's temperature would change, and there would be no solar wind or solar
magnetic storms affecting us.) To be "sucked" into a black hole, one has to
cross inside the Schwarzschild radius. At this radius, the escape speed is equal
to the speed of light, and once light passes through, even it cannot
escape.
The Schwarzschild radius can be calculated using the
equation for escape speed.
vesc =
(2GM/R)1/2
For photons, or objects with no mass, we can substitute c
(the speed of light) for Vesc and find the Schwarzschild radius, R,
to be
R = 2GM/c2
If the Sun was replaced with a black hole that had the
same mass as the Sun, the Schwarzschild radius would be 3 km (compared to the
Sun's radius of nearly 700,000 km). Hence the Earth would have to get very close
to get sucked into a black hole at the center of our solar system.
If We Can't See Them, How Do We Know They're
There?
Since black holes are small (only a few to a few tens of kilometers in size),
and light that would allow us to see them cannot escape, a black hole floating
alone in space would be hard, if not impossible, to see. For instance, the
photograph above shows the optical companion star to the (invisible) black hole
candidate Cyg X-1.
However, if a black hole passes through a cloud of interstellar matter, or is
close to another "normal" star, the black hole can accrete matter into itself.
As the matter falls or is pulled towards the black hole, it gains kinetic
energy, heats up and is squeezed by tidal forces. The heating ionizes the atoms,
and when the atoms reach a few million degrees Kelvin, they emit X-rays. The
X-rays are sent off into space before the matter crosses the Schwarzschild
radius and crashes into the singularity. Thus we can see this X-ray emission.
Binary X-ray sources are also places to find strong black hole candidates. A
companion star is a perfect source of infalling material for a black hole. A
binary system also allows the calculation of the black hole candidate's mass.
Once the mass is found, it can be determined if the candidate is a neutron star
or a black hole, since neutron stars always have masses of about 1.5 times the
mass of the sun. Another sign of the presence of a black hole is random
variation of emitted X-rays. The infalling matter that emits X-rays does not
fall into the black hole at a steady rate, but rather more sporadically, which
causes an observable variation in X-ray intensity. Additionally, if the X-ray
source is in a binary system, the X-rays will be periodically cut off as the
source is eclipsed by the companion star. When looking for black hole
candidates, all these things are taken into account. Many X-ray satellites have
scanned the skies for X-ray sources that might be possible black hole
candidates.
Cygnus X-1 is the longest known of the black hole candidates. It is a highly
variable and irregular source with X-ray emission that flickers in hundredths of
a second. An object cannot flicker faster than the time required for light to
travel across the object. In a hundredth of a second, light travels 3000
kilometers. This is one fourth of Earth's diameter! So the region emitting the
x-rays around Cygnus X-1 is rather small. Its companion star, HDE 226868 is a B0
supergiant with a surface temperature of about 31,000 K. Spectroscopic
observations show that the spectral lines of HDE 226868 shift back and forth
with a period of 5.6 days. From the mass-luminosity relation, the mass of this
supergiant is calculated as 30 times the mass of the Sun. Cyg X-1 must have a
mass of about 7 solar masses or else it would not exert enough gravitational
pull to cause the wobble in the spectral lines of HDE 226868. Since 7 solar
masses is too large to be a white dwarf or neutron star, it must be a black
hole.
However, there are arguments against Cyg X-1 being a
black hole. HDE 226868 might be undermassive for its spectral type, which would
make Cyg X-1 less massive than previously calculated. In addition, uncertainties
in the distance to the binary system would also influence mass calculations. All
of these uncertainties can make a case for Cyg X-1 having only 3 solar masses,
thus allowing for the possibility that it is a neutron star.
Nonetheless, there are now about 10 binaries for which
the evidence for a black hole is much stronger than in Cygnus X-1. The first of
these, an X-ray transient called A0620-00, was discovered in 1975, and the mass
of the compact object was determined in the mid-1980's to be greater than 3.5
solar masses. This very clearly excludes a neutron star, which has a mass near
1.5 solar masses, even allowing for all known theoretical uncertainties. The
best case for a black hole is probably V404 Cygni, whose compact star is at
least 10 solar masses. With improved instrumentation, the pace of discovery has
accelerated over the last five years or so, and the list of dynamically
confirmed black hole binaries is growing rapidly.
What about all the Wormhole Stuff?
Unfortunately, worm holes are more science fiction than
they are science fact. A wormhole is a theoretical opening in space-time that
one could use to travel to far away places very quickly. The wormhole itself is
two copies of the black hole geometry connected by a throat - the throat, or
passageway, is called an Einstein-Rosen bridge. It has never been proved that
worm holes exist and there is no experimental evidence for them, but it is fun
to think about the possibilities their existence might create.
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