Black holes: picturing the heart of darkness
Astronomers are poised Wednesday to unveil the first direct image of a black hole and the surrounding whirlwind of white-hot gas and plasma inexorably drawn by gravity into its ravenous maw, along with the light they generate.
The picture will have been captured by the Event Horizon Telescope (EHT), a network of eight radio telescopes scattered across the globe.
Paul McNamara, an astrophysicist at the European Space Agency and project scientist for the LISA mission that will track massive black hole mergers from space, helped AFP put what he called an “outstanding technical achievement” into context.
How do we know black holes exist?
“We think, of course, of a black hole as something very dark. But the mass it sucks in forms a so-called accretion disk that gets so hot it glows and emits light.
Over the years, we accumulated other indirect observational evidence -– X-rays coming off objects, for example, in other galaxies.
In September 2015, the LIGO gravitational wave detectors in the US made a measurement of two black holes smashing together.
All the evidence we have from around the universe -– X-rays, radio-waves, light -– points to these very compact objects, and the gravitational waves confirmed that they really are black holes, even if we have never actually seen one.”
What is an ‘event horizon’?
“At the centre of a black hole is something we call a ‘singularity’ -– a huge amount of mass shrunk down to an infinitely small, zero-dimensional point in space.
If you get a certain distance away from that singularity, the escape velocity drops under the speed of light. That’s the event horizon.
It is not a physical barrier -– you couldn’t stand on it. If you’re on the inside of it, you can’t escape because you would need infinite energy. If you are on the other side, you could escape—in principle.”
How big is a black hole?
“The diameter of a black hole depends on its mass but it is always double what we call the Schwarzschild radius.
If the Sun were to shrink to a singularity point, the Schwarzschild radius would be three kilometres, and the diameter would be six.
For Earth, the diameter would be 18 millimetres, or about three-quarters of an inch. The event horizon of the black hole at the centre of the Milky Way, Sagittarius A*, measures about 24 million kilometres across.
For Earth, the diameter would be 18 millimetres, or about three-quarters of an inch. The event horizon of the black hole at the centre of the Milky Way, Sagittarius A*, measures about 24 million kilometres across.
Source:https://phys.org/news/2019-04-black-holes-picturing-heart-darkness.html