Get sucked in: The ultimate guide to black holes

Scientists on Thursday unveiled the first-ever photo of a black hole.

It took eight telescopes, 200 astronomers and two years of number-crunching to produce the historic image, which looks exactly as astronomers predicted it would, thanks to theories developed by Albert Einstein a century ago.

But what is a black hole? And why did astronomers choose to look at this one in particular?

The idea of an object whose gravity is so strong not even light can escape was first formulated in the 18th century, but it wasn't until the 1910s that scientists began to understand how they could exist.

Despite their name, black holes aren't empty space - quite the opposite. They're the densest things in the universe.

Most are created when stars at least three times bigger than our sun die - if there's enough mass left over after it's exploded, the sheer strength of the gravity pulls everything together so tightly, Einstein's equations break down. This is called a singularity.

It's estimated there could be as many as 1 billion of these types of black holes in our galaxy, the Milky Way, but they're hard to spot. 

It's not yet clear how supermassive black holes are formed, but the leading theory is they're the result of several smaller black holes joining together.

Usually found at the centres of galaxies, supermassive black holes can be billions of times as massive as our sun. The object in the photograph released on Thursday is a supermassive black hole found at the centre of another galaxy - Messier 87 - 55 million light years (520,340,176,000,000,000,000km) away.

The infamous image of Messier 87's black hole.
The infamous image of Messier 87's black hole. Photo credit: National Science Foundation/Getty

The dark circle in the photo isn't the black hole itself, since not even light can escape its gravity - it's the shadow cast by the black hole on light and gas that's curving around it.

The supermassive black hole at the centre of our galaxy - Sagittarius A* - has the mass of 4 million suns, but could fit inside the orbit of Mercury. Messier 87's black hole is more than 1000 times bigger than Sagittarius A* - 6.5 billion times the mass of our sun. The entire solar system could fit inside it.

The orange glow around Messier 87's black hole is called the accretion disk. Much like how stars catch things like planets and comets in their orbit, black holes do too. Except the sheer strength of black holes' gravity causes captured gases to rotate so fast they heat up due to friction, and emit huge amounts of X-rays and other electromagnetic radiation.

Some black holes - including Messier 87's - also let off jets of energy from their north and south poles that appear to - but don't - travel faster than the speed of light. The one in the centre of our galaxy appears to be firing a jet right at Earth, a study earlier this year found.

Black holes that aren't fed new mass will eventually die, according to the late Stephen Hawking. Pairs of particles that come into existence on the edge of the black hole's event horizon are split - one falls in, the other out. Because the energy required to create the particles came from the black hole, the fact one falls away from the black hole means it will, over time, lose mass.

In December, scientists came up with the theory that black holes eventually turn into 'white holes', spewing everything back out. Because black holes are predicted to last much, much longer than the age of the universe to date, it's unlikely we'll ever find out if this is true.