## How are black holes discovered? (Intermediate)

How did the first astronomer discover the first black hole? Who had discovered it and when was it found? Please explain how the first black hole was discovered.

No single astronomer has the credit for discovering a black hole. Before I explain how astronomers found evidence for the existence of black holes, let me give you some of the necessary physics background.

1. Any body which is above absolute zero (-273 Celsius) radiates thermal energy, and the peak wavelength of emission depends on the temperature of the object. For example, the sun's surface is about 6000 Kelvin so that its peak emission is in green light. If an object's temperature is about a million degrees, then its peak emission will be in X-rays.

2. Normally stars are prevented from collapsing from gravity due to thermal gas pressure and radiation pressure. However, if the thermal energy source (nuclear fusion reactions) stop, then the star will collapse. It turns out that there are forces other than gas pressure which counteract gravity when the star becomes more compact (for instance the neutron star is only 10 km across!). But the astrophysicist Chandrasekar proved that there is a maximum mass beyond which nothing can beat gravity. So, if we detect a compact object in space which is more than this critical mass, then we can be confident that it is a black hole.

Now to return to the question of finding black holes: How can one detect a black hole if nothing can escape from it? Consider a binary system of stars where one of the stars is a black hole and the other a normal star. If the normal star's envelope gets close enough to the black hole, then the fierce gravity of the black hole can rip out gas from the normal star which is then swallowed by the black hole.

However, due to the conservation of angular momentum, the gas cannot plunge straight into the black hole, but must orbit it for some time before it gets sucked. Thus, a disc like structure is formed around the black hole from which gas is pulled slowly into the black hole. When the gas orbits the black hole in the disc, its temperature is raised to several millions of degrees which emits radiation in the X-ray part of the spectrum (by the first note that I explained above). Thus, when we detect X-ray sources in the sky, then we know that there is gas which has been heated to several million degrees, and one of the mechanisms to achieve that is the accretion disc around the black hole.

If the system giving out X-rays turns out to be a binary star, then a case can be made that one of the stars is a compact object (a neutron star or a black hole). Binary stars are very useful to astronomers because it allows us to measure the mass of the stars in the system (by Kepler's laws). If the mass of the compact object turns out to be more than the critical mass mentioned above, then one can be sure that it is a black hole. So that is how black holes are discovered.

Now about actual discovery: In the early 1970s, an intense X-ray source was found in the constellation Cygnus called Cygnus X-1. As the years passed, in the spring of 1972, Cygnus X-1 was identified with a star known by its classification number HDE226868 (which is a radio source). Soon evidence was found that it is a binary star system with a period of about 5.6 days.

By the special theory of relativity, no information can travel faster than the speed of light. Hence, a celestial object cannot change its luminosity on a time scale shorter than the time taken for the light to reach from one side of it to the other. Analysis of Cygnus X-1 showed that its emission had luminosity variations on time scales as short as thousandths of a second, suggesting that the object was only a few kilometers wide. Thus evidence was found that one of the stars was a compact object. Finally, astronomers used the binary star system to determine the mass of the compact object and found that it was greater than the critical mass, so that it was most likely a black hole. That is about the discovery of the first black hole in our universe.

Since then, astronomers have detected several black holes in space using several techniques. While one class of black holes have "small" masses (greater than 5 times the mass of the sun), there are others which have gigantic masses (more than a million times the mass of the sun), called supermassive black holes. These black holes are found in the centers of several galaxy, with our own Milky Way harbouring a two million mass black hole in the center. 