The mysteries of black holes in our universe explained

Rob Fender, professor of astrophysics at Brasenose College, Oxford, discusses how black holes were 'discovered', how they are formed, how we detect them and if they are a threat to us.

Spiral galaxy NGC 4845, 65 million light years away in the Virgo constellation, captured by the Hubble Space Telescope in January. The dust-mottled disc around the glowing centre signifies a black hole. NASA / ReutersShow less

The odds of detecting the gravitational waves created by two colliding black holes, exactly 100 years after Einstein first proposed the possibility, are probably incalculable.

But until just a few decades ago the very concept of black holes itself teetered precariously on the dividing line between science fact and fiction, says Rob Fender, professor of astrophysics at Brasenose College, Oxford.

“For a long time, debate raged about black holes,” says Prof Fender, who gave a talk about the subject at New York University Abu Dhabi Institute this week.

But thanks to great leaps in technology and in understanding, that debate – in as far as any theoretical discussion can be – it is all but over, he says.

“We are rather convinced that black holes are out there in very large numbers in the universe,” says Prof Fender. In fact, there are “probably 100 million within our own galaxy, and of course we believe there are thousands of millions of galaxies out there”.

The theory of black holes was first proposed not by Einstein or any of the other big names in physics, but by a long-forgotten English clergyman, in an extraordinary paper written more than 220 years ago.

John Michell suggested the existence of what he called “dark stars” in 1783, although it was 1967 before the term “black hole” was finally coined (by US physicist John Wheeler).

Naming black holes and spotting them, however, has proven to be two entirely different things.

The problem has always been that, by definition, black holes cannot be seen. They are formed by the catastrophic collapse of a star, an event that compresses matter into such a relatively small space that nothing, not even light, can escape its massive gravitational pull.

What can be seen, however, is the celestial dance of visible objects orbiting ghostly partners and bursts of X-rays emitting from these so-called binary systems. These are two objects, usually stars, that are close enough that they orbit around a shared mass.

“The first, and probably still [the] strongest, evidence for black holes is that we see binary star systems that go through phases where they are extremely luminous in X-rays,” says Prof Fender.

“They are far more luminous than a normal star, and the most efficient way of releasing such amounts of energy that we can conceive of in the universe is that matter gets extremely hot as it falls down towards a black hole.”

But then there are periods when this bright X-ray emission fades away, and this reveals a further clue – one first proposed by Michell back in the 18th century.

“What you can then see is the star being swung around in an orbit in space and yet you can’t see the companion star which is making it do this. The only satisfactory explanation is that there is essentially a dark star there,” says Prof Fender.

Until relatively recently, the tell-tale light show was not visible from Earth, because our atmosphere absorbs X-rays. It was only when NASA launched the first X-ray-detecting satellite in 1970 that the signature of the first confirmed black hole was spotted.

The position of black hole Cygnus X-1 – in our galaxy, a mere 6,000 light years from the Sun – is also betrayed by the presence of its partner star, HDE 226868, which is about 25 times the mass of our Sun and circles it in a five-and-a-half-day orbit.

Work over the past decade or so has also changed the belief that any matter that crossed the “event horizon” of a black hole – its point of no return – was lost for good.

In fact, says Prof Fender, “what we find is that when matter falls down, as well as getting hot and producing these X-rays, for reasons that we frankly still don’t understand it produces very powerful jets of matter and energy that fire away from the black hole in opposite directions”.

Are we in any danger of being swallowed by a black hole? Almost certainly not, says Prof Fender. There’s a huge one at the centre of our galaxy, about three or four million times the mass of our Sun, and its size and position betrayed by the flock of stars in 10-year orbits around it, but that’s 25,000 light years away from Earth.

“In all probability there is a black hole within about 20 or 30 light years of us that we just haven’t found yet,” says Prof Fender. “But there’s still no threat to us.”

Luckily, our own Sun is simply too small to become a black hole. Instead, it will slowly run out of fuel and shrivel into a so-called “white dwarf” – though not, scientists believe, before expanding dramatically in its death throes and scorching away all life on Earth.

Luckily, we have a while to make plans: about three billion years.

Jonathan Gornall is a regular contributor to The Review.