Black Holes

If you had read my article on ‘The Life of the Star’, you would have known that a Black Hole is created when a huge and massive star (more than 2.5 times the mass of the sun) runs out of hydrogen and collapses into a Supernova explosion. If the core of the star remains the explosion it becomes a black hole. But what exactly is a Black Hole? Why is it different from other dead stars?

This image shows how a Black Hole may look like in the Milky Way.

When such a massive star is crushed into a smaller volume, the gravitational attraction of the star increases and hence the escape velocity. The gravitational attraction of a Black Hole is very high that it pulls out anything that is closer to it. The escape velocity of a Black Hole is more than the velocity of light i.e. an object in a Black Hole should travel faster than the speed of Light to escape the gravity of Black Hole. And hence even light cannot escape its gravity. Eventually nothing can escape out of a Black Hole as nothing can travel faster than light. This is the same reason that the Black Hole is invisible. You can only see an object when it reflects light. But since Black Hole does not reflect light it is invisible.

NOTE: None of the images of the Black Holes are real. They are artistic works to show how a Black Hole may look like. A Black Hole can not be photographed. It can only be detected.

There are three types of Black Holes – Stellar Black Holes, Supermassive Black Holes and Miniature Black Holes.

A Stellar Black Hole is a Black Hole created by a single star as explained above. It can be seen in almost every other galaxy.

This is an image of a Stellar Black Hole.

A Supermassive Black Hole is a Black Hole whose mass is between 105 and 1010 times the mass of the sun. It is known that most of the galaxies, including the Milky Way contains a Supermassive Black Hole in the center. It could have formed when a Stellar Black Hole start growing by pulling stars of huge mass by means of gravity. Supermassive Black Holes are only found in the center of the galaxy.

Image of a Supermassive Black Hole.

A Miniature Black Hole has not been identified but some theories say that Miniature Black Holes might have formed a short while after the Big Bang.

Questions may arise such as ‘How a Black Hole was discovered if it is invisible?’ and ‘Who discovered the first Black Hole?’ Here are the answers.

Before answering them let me explain about Binary Star System. A binary star system consists of two stars very close to each other and also moves around each other. Most of the stars we see in the night sky are binary stars though it looks like one twinkling star. Sirius, the brightest star in the sky is a binary star. Sirius A is bigger and brighter than Sirius B which can be seen only through a telescope. Sun, of course is a single star. Distance between two binary stars may be about 20 to 50 times the distance between Sun and Earth. But the distance between two single stars will be in Light years.

Image of Sirius A (bright one) and Sirius B (smaller one above 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. 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 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.