Black Hole Demystified


History of the term ‘black hole’

In 1915, Albert Einstein in his Theory of General Relativity described that matter causes space and time (which he called as the spacetime) to “bend”, the same way a cricket ball would dimple the surface of a trampoline. And the reason for this distortion which he gave was ‘gravity’.

The scientific community was yet to accept this idea when a few months later, a German scientist Karl Schwarzschild tweaked some of the equations of Einstein and came up with a bewildering conclusion. He found that if an object was dense enough, it would ultimately punch the spacetime fabric of the universe and create a bottomless pit, which he called as a “singularity”.

In the 1960s, these singularities were named as the “black holes”. They are essentially wells in which the force of gravity is so strong that once you enter, there is no way you can turn back. In fact, the force of gravity is so high inside that even light is not able to escape.

This idea of singularity didn’t go well with Einstein as he found it to be absurd and thus wrote an entire research paper to debunk it. However, with the passage of time, more and more researchers took up this topic and found more clues that indicated that Schwarzschild’s mad and wild idea might be real.

An important role in this transformation was played by the astrophysicist Subrahmanyan Chandrasekhar. His expertise on stellar evolution led him to explain the scientific community that some stars catastrophically collapse after they die. This collapse would continue until they reach the kind of singularity Schwarzschild had written about. This is later explained by physicists Hartland Snyder and J. Robert Oppenheimer.

But stellar-mass black holes, the type which these physicists explained are not the only black holes in the universe.

How are black holes classified?

There are basically three types of black holes classified according to their mass and size. The Stellar black holes have mass greater than 10 times the mass of the sun and radius of about 30 km. These are formed from the death of massive stars.

The next category of black holes is Supermassive black holes which are mainly found at the centres of galaxies. They are humongous and have mass about 105 to 1010 times the mass of the sun. And this huge mass is squeezed into a size bracket of 105 to 108 km. Unfortunately, we have no information on how they are formed, however, some scientists have speculated that these black holes form from the collapse of large stellar clusters.

Lastly, there are micro black holes, which have never been observed yet. It is estimated that they would weigh less than the moon and would be of a size less than 0.1 mm.

Why were we not able to capture a black hole before?

This is because of two reasons. First, the black holes, even the supermassive ones, aren’t that massive. For example, there is a supermassive black hole at the center of our Milky Way galaxy which is thought to be about 4 million times the size of our Sun. Now, this is huge relative to the objects that we see on Earth. In terms of the cosmic distance, this is no big deal. It is the same as you making an effort to take a picture of a DVD on the surface of the moon.

Secondly, observing black holes have been such a difficult job because of the extreme gravitational pull due to which even radiation (light) cannot escape out and hence, we cannot detect them directly. As an alternative, scientists infer to the motions of nearby objects around the hidden mass. For instance, if a black hole passes through a cloud of interstellar matter, it would draw matter inwards (as would have seen in the movies). This attracted and accelerated matter would then heat up and emit high energy X-rays which would then radiate into space. It is these high energy X-rays that our telescope catch and we are able to predict the existence of black holes. This is what our current understanding of black holes is all about.

How did scientists capture the image?

Over 200 astronomers around the globe came together and built a network of radio telescopes that, in effect, becomes an Earth-size dish capable of detecting the event horizon of a black hole. These telescopes are typically located at high-altitude sites such as volcanoes in Hawaii and Mexico, mountains in Arizona and the Spanish Sierra Nevada, the Atacama Desert and Antarctica. The network of these 8 ground-based radio telescopes is known as the Event Horizon Telescope (EHT).

The supermassive black hole that was captured by EHT belongs to the largest galaxy we know as of now, Messier 87 and is located at a distance of about 54 million light years. The black hole itself is a “monster” having a mass about 6.5 billion times that of the Sun and is spread in an area of 40 billion km across – three million times the size of the Earth.

What are we actually seeing in the image?

In image, we get to see a donut-shaped structure whose centre is dark. The dark centre actually is the “shadow” of the black hole which is illuminated by the glowing gases that are present near the event horizon.  But the colours that we are seeing are not the actual colours of the glowing gases. These are appearing orangish is colour because the radio astronomers involved in the projected chose orange colour to show the brightness of emissions. The yellow tone depicts the most intense emissions, while the red ones depict emissions of low intensity. The black area represents no emissions.

If this was not done by the astronomers, colour of emissions that would be visible to us would have been white or maybe a slightly tainted with blue or red.



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