Home ›› 04 May 2023 ›› Opinion
The first image taken of a black hole, the picture that finally turned artists’ impressions into a reality, was of the supermassive black hole at the centre of the galaxy Messier 87. Most supermassive black holes are found in the centres of galaxies. They sit in the gravitational driving seat as hundreds of billions of stars in the surrounding systems happily orbit them, just like the planets orbit the Sun at the centre of our Solar System.
The black hole at the centre of Messier 87 lies at the more massive end of the supermassive scale, cramming a mass that’s six billion times that of the Sun (six billion solar masses) into an area the size of Neptune’s orbit.
But as huge as that might sound (especially when compared to the black hole at the centre of the Milky Way, which is a mere four million solar masses), it’s by no means the most massive black hole that we know of.
That title goes to TON 618, which is an astonishing 66 billion solar masses. It’s so big that astronomers had to invent a new term to describe it; hence, TON 618 is what’s become known as an ultramassive black hole.
To give you an idea of just how mind-bogglingly big TON 618 is, imagine taking all the stars in the Milky Way and squishing the matter in them down to create a black hole. Even if you did that, you would still be a few billion Suns’ worth of matter shy. So how did TON 618 become such a behemoth?
Black holes are made of vast amounts of matter that have accumulated in one spot and been packed together as densely as possible, to the point where the gravitational pull from the accumulation is so strong that not even light can escape it.
The reason material is able to cross the event horizon is collisions between particles in the disc of material orbiting it. Atoms of hydrogen gas collide and transfer energy, just like balls on a snooker table.
If you’re a talented snooker player you might be able to hit the cue ball against a coloured ball and get the cue ball to stop dead, transferring all of its energy to the coloured ball, which shoots off across the table.
A similar exchange in energy can happen to colliding particles in the disc of material orbiting a black hole; the particle that gains all the energy will get a boost, pushing it further away from the black hole.
But the particle that loses its energy will have nothing to resist the gravity pulling on it, and gradually slide down to cross the event horizon and become part of the black hole.
Not every collision will result in such a drastic energy exchange; on average there’ll be an equalling out of the energy. But eventually, one particle will have enough collisions and be unlucky enough to lose energy in every one, bringing it ever closer to the black hole.
It’s only when the particle has a collision in the region known as the ‘innermost stable circular orbit’ (ISCO) that it will finally cross the event horizon to become a part of the black hole.
The ISCO doesn’t exist in Newton’s theory of gravity, which we all learn at school. Newton’s theory says that all perfectly circular orbits, no matter their distance, are very stable.
That means if an object on a circular orbit is unsettled slightly, it’ll remain largely on the same orbital path. For instance, if something was somehow orbiting near the Sun in a perfect circle and was nudged by another object, its orbit might become more oval, but it would continue orbiting the Sun.
Science Focus
