All Eyes on the Centre of the Galaxy

by Nicholas Mee on April 2, 2019

The image below shows a beautiful region of the night sky in the constellation Sagittarius. The asterism known to amateur astronomers as the ‘teapot’ forms part of the constellation. This is rather apt as the many nebulae and gas clouds located towards the centre of the galaxy appear as steam rising from the spout of the teapot. The precise centre of the galaxy is indicated by an ‘X’ in the illustration.

X marks the spot of the centre of the galaxy, a region known to astronomers as Sgr A*.

Where the Action is!

Radio astronomers have named this region Sgr A*, an abbreviation that means the most powerful source of radio signals in the constellation of Sagittarius. The star * is added to emphasise the special nature of this object. It is where the action is in our galaxy. Our immediate cosmic neighbourhood is incredibly quiet. The Sun is surrounded by oceans of space, it is over four light years to the nearest star. By contrast, within one light year of the centre of the galaxy there are, perhaps, a million stars. These include many burnt out stellar remnants such as neutron stars and black holes, as well as many luminous blue supergiants.

The Innermost Heart of the Galaxy

German astronomer Reinhard Genzel studied the innermost heart of the galaxy in the early 1990s using the European Southern Observatory’s 3.5 metre New Technology Telescope in Chile. His observations showed that the stars at the centre of the galaxy are moving extremely fast, and the closer to the centre the faster they are travelling. This suggests that there is a very high concentration of mass right at the centre. Furthermore, the location of the point right at the centre appears to be fixed while all else whirls frantically around it.

The orbits of the stars at the centre of the galaxy as mapped out by Andrea Ghez and her team.
Credit: Keck/UCLA Galactic Center Group.

Genzel’s observations were followed up by the American astronomer Andrea Ghez and her team with the two 10 metre Keck telescopes in Hawaii. The stars right at the centre of the galaxy are moving so quickly that over the course of just a few years they were able to plot out significant segments of their orbital paths. The closest neighbours to Sgr A* are racing around at up to 5 million kilometres per hour. As well as tracking their motion across the sky, it is possible to measure their velocity towards or away from us through the Doppler shift of their light. This has enabled Ghez and her team to calculate accurate trajectories of these stars in three dimensions. One such star designated SO-2, which takes fifteen and a half years to complete its highly eccentric orbit, has been monitored over the course of an entire orbit. It will be watched eagerly as it returns for another close encounter with the central black hole next year. Ghez has also found a star known as SO-102 with an even smaller 11.5 year orbit.

A Supermassive Black Hole

The speed at which SO-2 and these other stars are moving is determined by the mass of the object that they are orbiting. This mass can be calculated using Kepler’s 3rd Law and it turns out to be around 4 million times the mass of the Sun. But the observations show that this object is smaller than the Earth’s orbit around the Sun. There is only one possible conclusion – it is a supermassive black hole. The event horizon of the black hole is thought to have a radius of around 12 million kilometres. Anything that finds itself within this radius, including light, cannot escape the clutches of the black hole. By comparison the radius of the Sun is 700,000 kilometres. So the black hole event horizon has a diameter that is around twenty times that of the Sun.

An artist’s impression of a supermassive black hole.
Credit: ESO/L. Calçada.

The Event Horizon Telescope

A computer generated image showing what the Event Horizon Telescope is expected to reveal.

The ultimate challenge is to image the event horizon of the black hole, but at a distance of around 25,000 light years this is currently beyond the resolving power of even the world’s best astronomical instruments. This could all change within the next few months as Shep Doeleman of MIT (Massachusetts Institute of Technology) is leading an incredibly ambitious international effort to assemble the Event Horizon Telescope (EHT) in order to generate the world’s first image of a black hole. Success will require at least 5,000 times the resolving power of the Hubble Space Telescope. It is comparable to imaging a cricket ball on the Moon. The galactic centre is shrouded in hot gas, which blocks the visible light emitted from the stars in this region of the galaxy. Infra-red radiation is much better at penetrating the murk, however, so the EHT will be an Earth-sized instrument operating in the far infra-red/microwave region of the spectrum. It will combine the data collected by a network of radio telescopes around the world to produce images with an unparalleled resolution. These instruments are located at sites in California, Arizona, Hawaii, Chile, Europe and even the South Pole.

Some of the sites of the telescopes that will form the Event Horizon Telescope. The images from these telescopes will be combined using Very Long Baseline Interferometry.

A second target for the Event Horizon Telescope is the supermassive black hole at the centre of the giant elliptical galaxy M87. This is a huge galaxy at the centre of the nearby Virgo cluster of galaxies. Nearby in cosmological terms, anyway. The distance to M87 is around 53 million light years, so it is about two thousand times as distant as the galactic centre. However, the supermassive black hole at its core is believed to be around 6 billion times the mass of the Sun, so the radius of its event horizon is about 1,500 times that of the supermassive black hole at the centre of our galaxy. This means that imaging its event horizon should be only marginally more difficult.

A jet is emanating from the centre of the giant elliptical galaxy M87 – the diffuse amber sphere in this image. The jet is thought to have been produced by the supermassive black hole at the centre of the galaxy. Credit: Hubble Space Telescope/NASA

What’s more, the M87 supermassive black hole is very active. It has spewed out an enormous jet into intergalatic space, as can be seen in the image above. (It is assumed that there is a second jet in the opposite direction, but we can only see the one pointing towards us.)

We are entering a new era for black hole physics. We might have the first direct image of a black hole some time this year.

Further Information

There is more about the Event Horizon Telescope and black holes in my new book: The Cosmic Mystery Tour.

There is a lot more information about black holes in my book Gravity: Cracking the Cosmic Code.

The official website of the Event Horizon Telescope is at:

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