A black hole is a region of space-time that experiences such strong gravitational effects that nothing can escape it, not even light… hence it is a black hole! So, what is space-time? Space-time was a conceptual tool that emerged from Einstein’s General Theory of Relativity (in 1905) to describe the strong gravity regime. It links the three dimensions of space with time to make a four-dimensional space-time continuum. Space-time is often portrayed as a warped coordinate system that is distorted by the presence of mass (think of a snooker ball (mass) being placed on a bedsheet (curvature of space-time), see this great demo).
When the mass is large enough (e.g. stars merging or galaxies colliding) relativity predicts that space-time is warped so much, black holes can be formed. The edge of a black hole, beyond which nothing can return, is called the event horizon. This "point of no return" in a black hole is defined by the Schwarzschild radius (Rs) and is equal to 2GM/c^2 (G = gravitational constant, M = mass of black hole). And whats at the centre of a black hole? Current theory points towards it being a singularity (a point of infinite density), but no one can be sure (for now?); nothing can be sent inside to check and (more importantly!) retrieved to shed light on the issue.
So is the famous picture showing us the edge of the event horizon, or something else?! Well, it’s not the event horizon. So what is it...
The area surrounding a black hole is a rather apocalyptic zone. It is occupied by dust and gas, millions of degrees hot, that is orbiting at a fraction of the speed of light. This is known as the accretion disc and is a flat disc of matter surrounding the black hole. Images of the black hole over time has led scientists to state that the matter in the accretion disc is orbiting clockwise, and completes an orbit every two days.
The innermost stable circular orbit of matter is actually at 3Rs, within that all matter plummets towards the black hole never to be seen again. Light, however, has no mass and can orbit closer. Nonetheless, even light can have momentum and is affected by the warping of the space-time continuum. You may have heard of gravitational lensing? This is when light is ‘bent’ around a large mass, such that even if a galaxy is exactly behind a closer galaxy we would still be able to see the further galaxy (albeit possibly distorted) since light from the distant galaxy can be bent around the closer galaxy, like a lens. Lensing is a great example of the bending of light due to the warping of spacetime by a heavy mass. Here is a great example of gravitational lensing in a recent APOD picture:
Another thing to think about: what plane or tilt is the accretion disc relative to how we are looking at it? If we see the accretion disc flat-on, then we wont see much distortion of the light from the accretion disc itself. However, if the accretion disc is at any other tilt angle (i.e. such that part of the accretion disc is behind the black hole) then it’s light will be warped above and below the black hole. In this case, we will see the type of black hole shown in the Interstellar movie (below) where the accretion disc is side-on, and the light you see above and below the black hole is actually the light from the accretion disc going over-round-and-under the black hole as well as under-round-and-over the black hole. Confusing, I know! It really changes the way we think about light and space; and all dreamt in the inner recesses of the brains of very clever people over 100 years ago. Brilliant.
So, what does the iconic first black hole image actually show us?!? The black circle you see in the centre is not the event horizon; it is the blurry radius beyond which we begin to see light. Surrounding that we see light from the accretion disc which (at this resolution), seems to be relatively flat-on (i.e. not like the Interstellar black hole image).
Well there you have it. A black-hole, imaged, in our lifetime. An awesome achievement of ingenuity. A milestone reached. Next stop (and hopefully very soon): An image of the black hole at the centre of our very own Milky Way… stay tuned.