What Does a Black Hole Look Like?

by Nicholas Mee on April 21, 2019

Black holes were once just the outlandish speculations of theoretical physicists. Now we know they really are out there. Gravitational waves produced by the collision and merger of black holes have been detected as faint ripples in the fabric of space. And now we even have a picture of a distant supermassive black hole.

So what does a black hole look like?

Black holes are sometimes represented as giant plug holes in the sky. This is incorrect and rather misleading.

The planet Saturn offers a surprising analogy to the geometry of a black hole. There are obvious differences, but first we will consider the similarities.

The geometry of Saturn and its rings resembles a black hole more than the plug hole image that is often seen.

Black holes are defined by their event horizon. Once inside the event horizon nothing can escape, not even light. The event horizon of a black hole is spherical so, like Saturn, black holes are spherical. Material in the vicinity of the black hole, such as rock or dust or gas, will accumulate in a disc known as an accretion disc, and this material swirls around the black hole in the plane of the black hole’s equator. This disc—or strictly speaking annulus, as it is a disc with a hole in the middle—is reminiscent of Saturn’s rings. Between the event horizon of the black hole and the inner edge of the accretion disc there is a gap, just like the gap between Saturn and its rings.

Computer generated black hole sphere surrounded by plasma disc.

Now we come to the differences. The gravitational field of a black hole is so intense that nothing can escape, not even light, which is why they are black, so the sphere that defines the black hole is completely black. The accretion disc orbits the black hole at a ferocious pace approaching the speed of light, which is far from the serene orbital dance of Saturn’s majestic rings. And it is not formed of ice and dust like Saturn’s rings, it is composed of material that fell towards the black hole releasing huge amounts of gravitational binding energy, so it is hot – very hot. It is a violently swirling cloud of plasma.

X-ray Specks

This is an artist’s impression of a black hole known as Cygnus X-1, which was the first good candidate for a black hole to be discovered. The artist has given the black hole an accretion disc that is much larger than the black hole and added jets of material emanating from the region close to the black hole. The accretion disc swirling around the black hole is so hot it emits X-rays and this is the origin of Cygnus X-1’s name.

Artist’s impression of Cygnus X-1. Credit: NASA/CXC/M.Weiss.

Cygnus X-1 was discovered in 1964 and it is one of the most powerful X-ray sources in the night sky. X-ray emission is an unmistakable signature of black holes, as only in such extreme environments is matter heated to the enormous temperatures required for X-ray emission.

This image released in 2017 might not look much, but it is the X-ray equivalent of the more well known Hubble Ultra Deep Field image. It was produced by the Chandra X-ray satellite staring for eleven and a half weeks at a small region of sky.

Ultra Deep X-ray image produced by the Chandra space X-ray telescope. Credit: NASA/ CXC/ Penn State/ B.Luo et al.

Every dot and smudge in the image is an X-ray source and about 70% of them are believed to be the accretion discs of supermassive black holes at the centres of distant galaxies. If this image covered a region of sky the size of the Moon’s disc it would contain about 5000 X-ray sources, each the accretion disc of a black hole. There are hundreds of billions of galaxies in the visible universe and each is believed to contain a supermassive black hole at its centre.

The Supermassive Black Hole and the Jets

On 10 April 2019 the first photograph of a black hole was published by the Event Horizon Telescope. The target of the project was a supermassive black hole situated at the centre of a giant elliptical galaxy known as M87. It lies at the heart of the Virgo cluster of galaxies at a distance of 55 million light years. M87 probably achieved its enormous size through the merger of numerous galaxies over the course of its 13 billion year history. The supermassive black hole at the centre of M87 is also a giant with a mass estimated as 6.5 billion times that of the Sun.

The large diffuse sphere is the giant elliptical galaxy M87. Several other much smaller galaxies are also visible in the image. Credit & Copyright: Adam Block, Mt. Lemmon SkyCenter, U. Arizona.

The M87 galaxy is known for a huge jet emanating from its core, which was discovered just over 100 years ago. The jet consists of material travelling towards us at close to the speed of light. We now know it was emitted from the region around the supermassive black hole. There is also a jet pointing in the opposite direction, but it is very much fainter. The great difference in brightness of the two beams is due to an effect known as relativistic beaming or Doppler beaming.

Greatly enlarged image of the galaxy M87 showing the plasma jet emanating from its centre. Credit: NASA and the Hubble Heritage Team (STScI/AURA).

The jets are believed to be due to the huge magnetic fields generated by the rapidly spinning plasma of the accretion disc. Plasma is accelerated in these magnetic fields and shoots out along the spin axis of the supermassive black hole, so the jets are aligned with this axis. The spin axis of the M87 supermassive black hole is inclined just 17 degrees away from pointing directly towards us.

The Event Horizon Telescope

The image published by the Event Horizon Telescope is the result of simultaneous observations by radio telescopes at eight sites across the globe acting together like one giant receiver almost as large as a hemisphere of the Earth. This has enabled an incredibly high resolution image of the supermassive black hole at the centre of M87 to be produced.

Visible light from the vicinity of the black hole is blocked by intervening gas clouds, so the telescopes are tuned to detect microwave radiation, which can penetrate the murk much better. As microwaves are invisible to us, the image has a false orange colouring so we can see it.

In the centre we see a sphere, which is the event horizon of the black hole. This sphere has a diameter of 40 billion kilometres, which is about four times the size of Neptune’s orbit around the Sun. (Neptune is the outermost planet, since the relegation of Pluto.) And over 125 times the size of Earth’s orbit.

The first ever image of a black hole produced by the Event Horizon Telescope. Credit: Event Horizon Telescope Collaboration.

The orange ring shows the accretion disc. It is superheated plasma with a temperature of around 6 billion degrees. As we look at the sky this ring of plasma is swirling around the black hole in a clockwise direction. But even at close to the speed of light it will take several days for the plasma to complete one circuit of the huge black hole.

The image is actually flipped left to right, so it is revolving anti-clockwise in the image. (This is the conventional orientation when showing astronomical images, as images are usually flipped when viewed through a telescope.)

The spin axis of the black hole can’t be seen, of course, but if it were projected onto the image it would point to the right and slightly upwards. The brighter part of the orange ring at the bottom of the image is the region of the plasma disc that is moving towards us. The fainter part of the ring at the top is the region of the plasma disc that is moving away from us. Just as with the jets, the plasma moving towards us at close to the speed of light looks much brighter than the plasma moving away from us.

What else will the EHT see?

In the near future we can expect more images from the Event Horizon Telescope. The next target will be the supermassive black hole at the centre of our own galaxy. This is much smaller than the M87 supermassive black hole with a mass of just four million Suns, but it is also much closer at a distance of about 25,000 light years.

Further Information

There is more about black holes and the other mysteries of the universe in my new book from Oxford University Press: The Cosmic Mystery Tour.

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