Black holes the most efficient energy sources in the universe

Astronomers observing the core region of a galaxy 1.2 billion light years away have detected what could be the buildup to an extremely high-energy event—the collision of two black holes.

Two supermassive black holes, weighing in at possibly millions of times the mass of the Sun, are closely circling each other, spiralling inwards toward a meeting that could happen some time this year.

Black holes are excellent candidates for being most bizarre things in the universe, and being able to observe a pair of them colliding is attracting a lot of astronomical interest.

Despite being bizarre, black holes are conceptually quite simple things. The Earth, Sun and other objects in the universe are defined by a balance of gravity pulling inwards, trying to compress them, and the resistance of the materials making up those bodies to that compression.

As a body is compressed, its gravity increases, and there is a threshold where as bodies get smaller, the gravity gets stronger faster than the resistance to compression. The result is a catastrophic collapse to a very small size. At some point in the shrinkage, the gravity gets strong enough to stop anything, even light, from getting out. We have a black hole.

Fortunately, this runaway collapse threshold is hard to get to. To make our Earth into a black hole would require some external force squeezing it down to a diameter of about 1.75 cm.

To make the Sun into a black hole would require compressing it, without losing any of its material, down to a diameter of about six kilometres.

The forces needed to form black holes existed in the early universe; some might have been formed in the Big Bang itself, just under 14 billion years ago. Younger black holes have been formed in high-energy events such as the explosive deaths of really big stars. When they run out of fuel, giant stars collapse and then explode. This explosion is not in the middle, so the result is tremendous shock waves going out into space and downward to the centre, where the core can get squeezed down enough to make a black hole.

Once formed, the extreme gravitational fields of black holes can pull in anything passing too close, such as gas, dust, planets, stars, or even other black holes. As material falls in, the mass of the black hole increases, its gravitational pull grows stronger, and more distant bodies get pulled in. When that material finally disappears into the black hole, a tremendous amount of energy is released. Black holes are probably the most efficient energy sources in the universe.

Our telescopes can detect the bright flashes of light, X-rays and radio emission as distant black holes have their meals.

Most galaxies have really big black holes in their cores. These can have masses millions of times that of the Sun. We know they are still growing because we have an easily observable supermassive black hole in the centre of our galaxy, and we can see it snacking on stars and neighbouring gas and dust clouds.

These black holes probably formed along with their host galaxies, possibly under the force of “infalling” material in the core of the galaxy. However, as yet we don't know how big they were when they were born and how much they have grown since.

The distant galaxy currently fascinating astronomers seems to have two supermassive black holes in its core, which are closely orbiting around each other. This situation is not stable. Moving black holes, with their high masses and densities, distort space as they move through it, forming a bow wave and wake, made up of gravitational waves. Making these takes energy, so the two black holes are slowly spiralling together.

Over three years, the time taken for one to swing round the other has shrunk from one year to about a month. Their collision is expected to take place this year, and we really want to observe it, from a safe distance of 1.2 billion light years.

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• Venus is conspicuous in the southeast before dawn, with Mars, Mercury and Saturn low in the glow.

• The Moon will be new on Feb. 2.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.