NOTE: This section is about stellar-mass black holes. For
information about black holes that measure in the billions of solar masses, see
Active Galaxies & Quasars .
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 less mysterious, and no easier to understand.
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| black hole is located deep in the Milky Way galaxy |
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 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." Around the singularity is a region where the force of
gravity is so strong that not even light can escape. Thus, no information can
reach us from this region. It is therefore called a black hole, and its surface
is called the "event horizon."
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, Earth's orbit around the Sun would be unchanged. Of course, 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 are there?
Since stellar black holes are small (only a few to a few
tens of kilometers in diameter), 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 in the visual spectrum.
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
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 its 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, and we see it from
certain angles, 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 black hole candidates.
Cygnus X-1 (Cyg 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 3,000 kilometers. This is one fourth of Earth's diameter.
So the region emitting the X-rays around Cyg 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
oscillate 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 it would not exert enough
gravitational pull to cause the wobble in the spectral lines of HDE 226868.
Other estimate put the mass of Cyg X-1 to as much as 16 solar masses. Since 7
solar masses is too large to be a white dwarf or neutron star, it must be a
black hole.
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| These black holes can suck in nearby star |
An illustration of Cygnus X-1, showing the companion star
HDE 226868,
the black hole, material streaming from the companion to the
black hole,
and the emission of X-rays near the black hole.
There are now about 20 X-ray binaries (as of early 2009)
with known black holes (from measurements of the black hole mass). 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. There are an additional 20 X-ray binaries which are
likely to contain black holes - their behavior is the same as the confirmed
black holes, but mass measurements have not been possible.
