Become an Expert: Black Holes Part 1

The infamous, mysterious and unknown: Black Holes. We’ve all thought about them before especially with the late and great Stephen Hawking, what are they, how do they form and most mysteriously of all, what happens if you enter a black hole? These are the questions I will be answering in this 2 part series

So, for starters, what are back holes? Well, there are two different types of black hole: a stellar black hole and a supermassive black hole (We’ll be covering that one in part 2). 

A stellar black hole emerges when a massive star (at least 20 times the mass of the sun) comes to the end of its life and collapses in what is called a supernova. This amount of mass in such a small area would create a black hole. Also during a supernova, after the star collapses, it explodes part of the mass and creates some spectacular displays for anyone watching. Additionally, the leftover remains of the supernova create some of the most beautiful objects in the universe: nebulae but that’s for another day. Okay then, let’s look at the anatomy of a black hole. There’s the singularity (an infinitely small infinitely dense dot), then there’s the event horizon (the point of no return, anything that goes past this line is never coming back) , there is the photon sphere (where lots of photons are emitted by all the friction going round just outside the event horizon), then there is the innermost stable orbit (the closest an orbit can be without risking falling into the event horizon.  Penultimately, there is the accretion disk (all the matter the black hole has collected spinning round just outside of the event horizon) and lastly the relativistic jets (the jets that zoom out of the poles of the black hole at near light speed that occurs when the black hole feeds on the matter). Here is a diagram to sum it all up.

Credit: Nicholas Rajan on Art Station

The first people to propose an idea that something would have an escape velocity (the speed at which one has to travel to escape the gravitational field of an object) that was higher than the speed of light (so not even light could escape the gravitation field) was John Michell and Pierre Simon Laplace in the 18th century. While Karl Schwarzschild had the first modern solution to the black hole using Einstein’s general relativity in 1916 (a rumor is that he developed this while in a WWI trench). Schwarzschild was never able to publish his work as he died serving in the German Imperial Army during WWI. His work did get published though, only by David Finkelstein in 1958. Schwarzschild didn’t believe in the existence of black holes yet gave a lot to the understanding of black holes. Even the Schwarzschild radius (which we’ll be talking about in part 2 but is basically the radius of an event horizon) was discovered by him. They were long a mathematical mystery and curiosity until theoretical work showed that black holes were a generic prediction of general relativity in the 1960s. But in 1971 the first black hole was detected called Cygnus x-1. This is a stellar black hole (meaning it was produced by a dying star). The x stands for x-ray. So we discovered the black hole by detecting large amounts of x-rays coming from the star Cygnus A. The amount of x-rays was so high that astronomers predicted that it was a black hole next to the star and taking away its mass. This mass then orbited around the black hole creating lots of friction and therefore producing x-rays. Later evidence proved this. Now astronomers predict that there are about 10 million to 1 billion stellar black holes in our galaxy.

Credit: Astronomy Magazine

This is it for part one, so keep on the lookout for part 2. To end part 1 I would like to share one of my favourite Stephen Hawking quotes: “Look up at the stars and not down at your feet. Try to make sense of what you see, and wonder about what makes the universe exist. Be curious.”

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