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There’s a hole at the centre of our galaxy (Image: http://www.iamciara.co.uk/

The centre of the Milky Way is darker than you’d expect – and not just because it’s home to a supermassive black hole

A LITTLE over 25,000 light years away lies the most mysterious place in the nearby universe. Jam-packed with colliding stars and cloaked in dust, it is the centre of our galaxy. At its very heart, we suspect, lurks a monstrous black hole more than 4 million times as massive as the sun. Known as Sagittarius A*, it is thought to rip stars apart, orchestrating stellar mayhem as it warps the very fabric of space and time.

Similar supermassive black holes are thought to exist at the centre of every galaxy. It is only now, by observing stars whirling about the monster closest to home, that we stand on the verge of confirming their existence once and for all. Not only that, we could also test Einstein’s general theory of relativity in the most extreme environment yet.

While the centre of our galaxy could serve as a lab for studying processes that occur in other galaxies, the first tantalising glimpses of it are throwing up surprises about our own. Recent observations have revealed that the heart of our galaxy harbours a second kind of hole – a region of space containing only a few young stars and mysteriously empty of older ones.

Previous scans of the Milky Way’s heart showed a few dozen young stars whose bright blue light is intense enough to shine through the shroud of dust. Astronomers expected them to be the tip of the stellar iceberg, their light overwhelming the faint glow emitted by vast numbers of more ancient stars.

That all changed when three teams independently got their hands on sensitive infrared telescopes capable of penetrating the dust shrouding the galactic centre. As they scanned the Milky Way, they counted thousands of old stars. But when they got very close to the galactic centre, the numbers plummeted, revealing a patch of space 3 light years across that was seriously lacking in stars ().

The hole story

This is a big surprise because it goes against our ideas of what ought to be happening at the galactic centre. The gravitational field around Sagittarius A* is thought to be strong enough to have herded stars into its neighbourhood over billions of years. So why aren’t there more ancient stars at the galactic centre?

The most mundane explanation is that even the latest infrared telescopes are not sensitive enough to pick up their faint light. But there is also a more exciting possibility: that the centre of the galaxy is composed of super-dense bodies that are hard to see, such as neutron stars and stellar-mass black holes left behind in supernova explosions. If this idea is correct, it suggests that most of the stars that form at centre of the galaxy are massive ones that end their lives as supernovae. “This would make the region different from all other places we have observed,” says of the Rochester Institute of Technology in New York.

There are problems with this scenario, however. The main one, says Merritt, is that these massive stars would not grow up alone: a small number of less massive stars should also have formed here. At the end of their life these would have grown into red giants, luminous stars that should be easy to observe. So why haven’t we seen any? One possibility is that the stellar-mass black holes ate all the red giants, but “it is hard to make this scenario work”, Merritt says. “We would need more stellar-mass black holes than can be accounted for by the 1 million solar masses of matter known to exist in the innermost part of the galaxy.”

An even more exotic explanation is that sometime in the past, the Milky Way merged with another galaxy whose own supermassive black hole swallowed some of the Milky Way’s stars. Alternatively Sagittarius A* itself could be responsible for the stellar void surrounding it. Anything straying within about 5 light minutes of a supermassive black hole would be ripped apart, a fate that could have befallen the missing stars.

Eyeing the monster

Merritt also blames Sagittarius A*, but favours a slightly different scenario. He has calculated that the orbits of stars circling Sagittarius A* will become longer and narrower over time. Eventually the stars will venture close enough to the black hole to be sucked in. Alas, this theory, too, has problems. As stars are continually forming, in order to create a void “you would have to send stars towards the supermassive black hole, then stop further stars being resupplied” to the central region, Merritt says. But it is hard to see just what could stop stars arriving at the galaxy’s heart.

So while there are plenty of ideas on the table, the mystery persists. “The observational results are not sufficient to really determine which of these scenarios is most likely, or even rule one out completely,” says of the University of Cologne in Germany, who helped discover the void at the galactic centre. “For now, we can assume the hole is there, though we do not know for certain why.” To find an answer we will need to get closer to the monster at the heart of the Milky Way.

Luckily, a number of techniques are allowing astronomers to do that. Those same techniques could also help us achieve something even more profound: putting general relativity – Einstein’s theory of gravity – to the test. Its effects in the vicinity of planets, stars and galaxies have been probed, and the theory has passed with flying colours every time. Where relativity hasn’t been checked is in the extreme gravitational field of a black hole, where space and time are warped to an extraordinary degree. By watching exactly how matter falls into a black hole, astronomers hope to tell whether black holes are anything like the picture of them painted by general relativity.

Up till now the most promising technique has been (VLBI), which combines the signals from radio telescopes scattered across the globe to simulate a radio dish as big as the Earth. This virtual dish can resolve fine detail in astronomical objects, but even so its vision isn’t yet acute enough to discern the most distinguishing feature of the supermassive black hole: its event horizon. The point of no return for in-falling matter, it is about 15 million kilometres across, or one-tenth of the distance between Earth and the sun – minuscule in astronomical terms. Even the best picture of Sagittarius A* to date, taken by a team led by Shep Doeleman at the Massachusetts Institute of Technology’s in Westford, is still too blurred by a factor of 3.

There is, however, a way to boost VLBI’s resolution: observing at wavelengths shorter than the centimetre-long ones studied so far. By looking at wavelengths of 1.3 millimetres and maybe even 0.87 millimetres, the technique should finally be able to pick out what is happening near the event horizon (91av, 23 May 2009, p 28).

Even so, it won’t be easy. The radio waves we are trying to observe are emitted by electrons inside “hotspots” in the electrically charged gas swirling into the supermassive black hole. To test general relativity in the vicinity of the black hole, we would first have to run computer simulations of the swirling gas, predict its radio emission, and compare it with the observations. “VLBI is a promising technique but it is unlikely to give us a clean signal,” says Merritt. “It’s messy.”

Two groups of astronomers have a far cleaner way of probing Sagittarius A*: observing the individual stars orbiting it. Teams led by of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, and of the University of California, Los Angeles, have been observing 20 super-bright stars orbiting within 100 light days of our .

One star is pre-eminent in their studies: a heavyweight called S2 that is 20 times as massive as the sun. S2 is the only star to have been observed making a complete orbit of the galactic centre, a journey that takes it 15 years. From this, Genzel and Morris’s teams have calculated the mass of the central supermassive black hole to be 4.3 million times that of the sun, which is slightly higher than previous estimates ().

Let’s not forget that, until now, there has only been indirect evidence for a black hole at the centre of our galaxy. We know that something massive lurks there because its gravity affects the motion of nearby stars, and the most likely culprit is a black hole. But we need direct evidence to be sure. Now the hope is that stars like S2 will not only provide that evidence but also allow us to test our most cherished ideas about black holes.

“We know that something massive lurks at the galaxy’s centre because of its gravitational effects, but we need direct evidence to be sure that it is a black hole”

Among them is the idea, known as the no-hair theorem, that black holes are essentially so simple that they can be described adequately by their mass and how fast they spin. Theorist Clifford Will of Washington University in St Louis, Missouri, suggests that we could test the theorem, and therefore general relativity, by examining the orbits of stars close to the supermassive black hole. One way to do this would be to watch a star complete many orbits around the galactic centre. Einstein’s theory predicts that the star’s point of closest approach to the centre should progressively shift from one orbit to the next. If the no-hair theorem is correct, the rate of this “precession” depends on the mass and spin rate of the black hole, and nothing else. Even better, says Will, would be to track two stars (). That way, you can use the relationship between both stars’ orbits to cancel out the mass of the black hole, so the precession depends only on its spin. If it turns out that the precession depends on something more complex, then the no-hair theorem is wrong. And if that is true, then general relativity is also wrong. So the stakes are high.

“If a black hole’s gravity doesn’t depend on just its mass and spin rate, then general relativity is wrong”

Another way to test relativity is to use pulsars. These super-dense remnants of supernova explosions spin very rapidly, sweeping a lighthouse beam of radio waves across the sky once every turn. This makes them fantastically precise timekeepers. If any exist in the centre of the galaxy, then we might be able to pick up another relativistic effect – gravitational time dilation, where the passage of time slows down in the warped space-time surrounding a massive object. Spot this and we would have evidence of a massive black hole.

Star-spotting

Unfortunately, pulsars are intrinsically faint, making them difficult to detect in the dusty galactic centre. But astronomers have just embarked on an attempt to detect all the pulsars in the Milky Way, and they are hopeful of observing pulsars in the centre of the galaxy (91av, 17 March 2010, p 30).

General relativity isn’t under threat just yet. So far S2 is the only star we know of that comes within 1 light day of Sagittarius A* during its orbit. To really probe the space-time around the supermassive black hole, we will need to observe many more stars this close to the galaxy’s centre.

That is the aim of a team led by Andrea Ghez at the University of California, Los Angeles, which is currently upgrading the infrared interferometer at the twin 10-metre Keck telescopes in Hawaii. Meanwhile, Genzel’s team is building an instrument called that will combine near-infrared light collected by the four telescopes at the in Cerro Paranal, Chile, to measure faint objects with unprecedented resolution. They hope it will let them watch stars moving within a region just a few times the diameter of the supermassive black hole’s event horizon. The instrument could be in operation by 2013.

For billions of years the Milky Way has kept its best secrets hidden from view. Waiting a few more years before we finally uncloak its supermassive black hole isn’t too much to ask, is it?