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Black holes: What you need to know

The first photograph of a black hole is set to be revealed by scientists on Wednesday. And that may leave you with some deep, dark questions about black holes. Here are some answers.

What is a black hole?

A black hole is an object in space that is so dense and has such strong gravity that no matter or light can escape its pull. Because no light can escape, it is black and invisible.

There's a boundary at the edge of a black hole called the event horizon, which is the point of no return — any light or matter that crosses that boundary is sucked into the black hole. It would need to travel faster than the speed of light to escape, which is impossible.

Anything that crosses the event horizon is destined to fall to the very centre of the black hole and be squished into a single point with infinite density, called the singularity.

If black holes are invisible, how can we detect or photograph them?

By looking for the effects of their extreme gravity, which sucks stars and gases toward it.

Also, while anything past the event horizon is invisible, outside that boundary there is sometimes a spiral disk of gas that the black hole has pulled toward — but not yet into — itself.

The gases in that accretion disk are heated up as they accelerate toward the black hole, causing them to glow extremely brightly. The colours they glow are invisible to us, but are detectable with an X-ray telescope.

With some supermassive black holes, a fraction of the matter falling toward the black hole doesn't get sucked right in. Instead, paradoxically, it gets pushed away from the black hole at close to the speed of light in two narrow beams or "jets." Like other objects in space, black holes rotate, and the jets form along the black hole's axis of rotation. Those jets are thought to be the sources of high-energy particles called cosmic rays, and they also emit light.

A supermassive black hole with millions to billions times more mass than our sun is seen in an undated NASA artist's concept illustration. The black hole is surrounded by an accretion disk and has an outflowing jet of energetic particles, believed to be powered by the black hole's spin. (NASA/Reuters)

In theory, with a good enough telescope, you should be able to see light surrounding the black hole right to the edge of the event horizon, which is the goal of the Event Horizon Telescope. There, scientists expect to see a ring of light emitted by matter orbiting the black hole at almost the speed of light before crossing the event horizon. That ring of light is called the black hole's shadow or silhouette.

How big are black holes?

Small black holes are called stellar-mass black holes. They have masses similar to those of larger stars — about five to 20 times the mass of the sun.

The other kind is supermassive black holes, which are millions to billions of times more massive than the sun. That's the kind the Event Horizon Telescope has been trying to photograph, as bigger objects ought to be easier to see.

There is some evidence that black holes between these two sizes exist, but that has yet to be confirmed.

While black holes are very massive, that doesn't mean they take up a lot of space. Because they're so dense, they're actually quite small.

According to NASA, a black hole 20 times the mass of the sun could fit inside a ball 16 kilometres wide — the width of the Island of Montreal at its widest point.

Where are black holes found?

Supermassive black holes are found at the centre of most galaxies, including our own Milky Way. The one in our galaxy is called Sagittarius A* and is one of those the Event Horizon Telescope has been attempting to photograph (besides it being easier to try to see something that's big, it's also obviously easier to try to see something that's nearby).

This artist’s impression depicts a rapidly spinning supermassive black hole surrounded by an accretion disc. This thin disc of rotating material consists of the leftovers of a sun-like star that was ripped apart by the tidal forces of the black hole. Shocks in the colliding debris as well as heat generated in accretion led to a burst of light, resembling a supernova explosion. (ESO)

Sagittarius A* isn't the only black hole in our galaxy, though. Earlier this year, astronomers discovered another 12 within three light-years of it, suggesting there could be upwards of 10,000 black holes around the galactic centre.

Where do black holes come from?

Supermassive black holes are believed to form at the same time as the galaxy that surrounds them, but astronomers aren't sure exactly how.

Stellar mass black holes form when a star with a mass greater than three times that of our sun runs out of fuel. It explodes into a supernova and collapses into an extremely dense core that we know as a black hole — something predicted by Albert Einstein's general theory of relativity.

Einstein's theory also predicts the size and shape of the black holes that the Event Horizon Telescope is trying to photograph.

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