There are many popular myths concerning
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
NOTE: This section is about what are called "stellar-mass black holes". For information about black holes with the mass of billions of Suns, see
Active Galaxies & Quasars
Black Holes: What Are They?
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
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
what is known as a "
". 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 only
thing that would change would be the Earth's temperature. 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
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
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
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.
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
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
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.
Can You Give Me Some More References?
There is quite a bit of black hole theory out there. For more information
on it, try these books:
- Black Holes and Warped Spacetime - William J. Kaufmann, III
- Lonely Hearts of the Cosmos - Dennis Overbye
- Black Holes and Time Warps, Einstein's Outrageous Legacy - Kip S. Thorne
- The Mathematical Theory of Black Holes - S. Chandrasekhar
- Black Holes and Baby Universes and other Essays - Stephen Hawking
- Universe - William J. Kaufmann, III
- Black Holes and the Universe - Igor Novikov