Black Holes : Key Facts and Types of Black Holes

Each object has its own gravity which depends on its size and mass. The objects which have highest mass in smallest size would exert most gravity. The ultimate combination of large mass and small size is called a black hole. A black hole exerts such massive gravitational force that its escape velocity is equal or more than speed of light. The idea came in 18th century that such objects were so small and massive that particles of light could not escape from them and this would be black. When the general theory of relativity was confirmed, scientists started to explore the implications of gravity as the curvature of space by matter. Scientists realized that there could be locations in the universe where space was so severely curved that it would actually be “ripped” or “pinched off.” Anything that fell into that location would not be able to leave. This idea of an inescapable spot in space—a hole where not even light could leave—led physicists to coin the term “black hole.”

How Black Holes are detected?

One way of finding black holes is to observe the matter moving around in a orbit at much higher speed than expected. By carefully mapping this motion, the third law of Kepler and Newton’s law of gravitation can be applied without seeing the actual object at the centre of the orbit.

Another way of finding black holes is to look at them as sources of X-ray radiation. The tremendous gravitational field of the black hole can produce huge amount of light nearby and around itself even if it itself is black. Just as a meteorite or spacecraft gets hot as it enters Earth’s atmosphere, the in falling matter gets hot from the frictional drag too, sometimes reaching temperatures of millions of degrees. That hot material glows brightly and emits far more X-ray radiation and radio waves than would normally be expected from such a small volume of space. The scientists search for such small spaces with abonormally high X-Ray radiation to look for black holes.

Types of Black Holes

There are two categories of black holes viz. low mass and super massive are known to exist, and a third kind (primordial blank hole) has been hypothesized but not yet detected.

Low mass / Stellar Black Hole

Stellar black hole or low-mass black hole is found wherever the core of a very massive star (usually 20 or more times the mass of the Sun) has collapsed.

Super massive Black Hole

Super massive black hole is found at the centres of galaxies and is millions or even billions of times more massive than the Sun. Our Galaxy also has a black hole at its centre.

Promordial Black Hole

The third kind of black hole called a primordial black hole is found at random locations in space. It is hypothesized that these black holes were created at the beginning of cosmic expansion as little “imperfections” in the fabric of space-time. However, no such black hole has yet been confirmed to exist.

How many black holes are known today?

Today, thousands of black holes are known to exist, and the total population of black holes may number in the many billions.

Structure of a Black hole?

The centre of the black hole is called the singularity. It is a single point that has no volume but infinite density. The laws of physics as we understand them simply do not work at the singularity of a black hole the way they do in the rest of the universe. Surrounding the singularity is a boundary called the event horizon. This is the place of no return, where the escape velocity for the black hole is the speed of light. The more massive the black hole is, the farther the event horizon is from the singularity, and the larger the black hole is in size.

The singularity at the centre of any black hole has no volume. The size of the event horizon however, varies depending on the black hole’s mass. The mathematical relationship between the  mass of a black hole and the size of its event horizon was derived by the German astrophysicist Karl Schwarzschild (1873–1916), and in his honour, the radius of a black hole’s event horizon is called Schwarzschild radius.

The Radius of a stellar black holes is few hundred miles while that of super massive black holes is few million to billion miles. Further, if Sun is squeezed small enough to become a black hole; its radius would be around three miles only. If earth is squeezed into a black hole, its radius would be  about three-quarters of an inch.

Properties of Black holes

Black holes have huge densities and the key properties they have include mass (weight), rotation (spin) and electric charge.

Wormholes and Cosmic Strings

Wormholes and cosmic strings are theoretical imperfections in space-time. While a Black hole has one point of singularity, the wormhole may have two points – one where matter can only enter and another where matter can only exit. No worm hole has been detected so far, so they are only in theory and science fiction.

A cosmic string is a theoretical, vibrating stand that is like a black hole but instead of being a point or sphere, it is a long but very think crease left in otherwise smooth universe. Cosmic string is also theoretical and no such string has been detected so far.