How do we find a Black Hole?
There is evidence for the existence of black holes by observing how matter disappears when it falls beyond the "event horizon," the boundary between a black hole and the outside universe. Astronomers found this evidence by watching the fading and disappearance of pulses of ultraviolet light from clumps of hot gas swirling around a massive, compact object called Cygnus XR-1. This activity suggests that the hot gas fell into a black hole. Diagram of accretion disk Interview 1 Interview 2 Artist's concept movie of matter falling into a Black Hole
Highly Redshifted Objects
Astronomers can locate black holes using their accretion disk in another
fashion: redshifting, similar to the Doppler
effect.. Astronomers can measure the redshift of an object and then calculate how
fast it must be moving. By measuring the redshift of one side of a disk
and the blueshift of the other side, they can get an idea of how fast the
disk is rotating. If they know how fast the disk is rotating, and they
can estimate how large the disk is (which in itself is quite a
tricky and complicated task), they can guess the mass of the thing
about which the disk rotates. If perchance some accretion disk should be
going very fast around something that has at least 3 solar masses, could
that be a black hole?
Wobbling Stars
Sometimes,
astronomers find a star that behaves like it is part of a binary system,
but they can't seem to find its partner. How do they find only one
partner of a binary system? The visible binary member would appear to
wobble OR the star is a single-lined spectroscopic binary. It would be noticed that the
star is following an orbit around another star, just like a binary system.
The only problem is that there is no visible partner.
That in itself is not enough to say that the partner is a black hole. Many binary systems involve neutron stars. So, how do scientists tell whether the invisible partner is a white dwarf, a neutron star, or a black hole? They calculate its mass by watching the movements of the partner. If the visible star holds a loose orbit, perhaps the partner is only a white dwarf. However, if the partner orbits tightly, it could be moving around a black hole. Say the mass is measured at greater than 5 solar masses.. This could be a binary system with a black hole.