THE centre of our galaxy is a place of extremes. “It has the highest density of stars, the fastest-moving stars, the most concentrated reservoir of gas and the strongest magnetic fields in the galaxy,” says , an astronomer at the University of California, Los Angeles. And lurking at its very heart is the most enigmatic object of all: our galaxy’s very own supermassive black hole.
Known as Sagittarius A* – SgrA* for short – this dark presence whirls stars around at speeds approaching 20 million kilometres per hour, and is as massive as 4 million suns. Yet it is a docile monster. It merely snacks on the tenuous interstellar gas, which emits a faint glow of radio waves before disappearing into the gravitational maw.
Its character is about to change. SgrA* has in the past been responsible for mega eruptions that shaped the Milky Way into the galaxy it is today. Later this year, we are due to get our first glimpse of how a black hole springs into life, when a gas cloud called G2 nears its edge. It will give us an unprecedented insight into what makes a galaxy’s dark heart tick.
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It’s not easy to study the Milky Way’s centre. Vast quantities of dust absorb almost all the visible light emanating from it before it reaches Earth. If we dimmed our noonday sun by the same amount, it would appear fainter than the Pole star and the daytime sky would be unfathomably black. So to get a good view we must turn to radiation that penetrates the dust: radio, infrared and X-rays.
Observations from ground and space-based telescopes working at these wavelengths over the past couple of decades have revealed that SgrA* is, in cosmic terms, not a particularly outstanding beast. It is only 100 times brighter than the sun; a mere star such as Betelgeuse outshines the sun 100,000 times. Black holes can weigh in at billions of suns, with the gas falling into them heating and emitting radiation that can erupt as a quasar.
Discovered 50 years ago last month, quasars are the brightest objects in the universe. Their distance, however, makes them difficult to investigate. “Careful study of our own galactic centre, including the G2 encounter, is probably our best hope of understanding these phenomena in detail,” says of Northwestern University in Evanston, Illinois.
The latest instrument that we have turned on SgrA* is NASA’s , which launched in June 2012. It takes detailed views of the sky as seen in the highest-energy X-rays – the kind of radiation that shrieks out from searing hot gas on the edge of a black hole.
Last year, NuSTAR detected a burst of X-rays from SgrA*. That might be down to magnetic fields threading through the gas that’s swirling around the black hole: as fields wind up tighter and tighter, they may suddenly short-circuit to produce a brilliant flash, like a flare on the sun. Alternatively, the fields might spin out of the gas disc altogether to produce a narrow jet of energy. Many quasars proudly display vast jets of energy that stretch thousands of light years, and the minor eruptions of SgrA* may be showing us how these jets are born.
Or perhaps they mean something else entirely. at the University of Leicester in the UK thinks the X-ray burst has all the hallmarks of coming from a lump of gas much smaller than G2 heated to 100 million degrees in the vicinity of the black hole.
Doomed planets
His idea is that this blob of superheated gas is the remains of an ill-fated asteroid 10 kilometres across. Falling inwards from a huge cloud of space rocks that might surround the galactic centre, , and was ripped apart by immense gravitational forces. “An asteroid’s orbit can change if it ventures too close to a star or planet near SgrA*,” says Nayakshin. “If it’s thrown toward the black hole, it’s doomed.”
Every 100,000 years or so, Nayakshin reckons, an unlucky planet ends in the same way, in an even more spectacular blaze. Such a demise could account for an outburst of SgrA* as bright as a million suns a century ago, echoes of which still bounce around the galactic centre. Such echoes, in which radiation reflects off nearby gas clouds, were first discovered in visible light in the area around exploding stars, long after the original star had faded. Two X-ray satellites have monitored our galactic centre over the past decade, and have seen a wave of X-ray brightness spreading across the cool gas clouds there. “We can work back from these observations to glean that a very powerful emission was coming from SgrA* as recently as 100 years ago,” says Haggard.
At the moment, it’s impossible to tell if this outburst was indeed the funeral pyre of a planet, or a giant magnetic burp. With future eruptions we should be able to tell, thanks to the imposingly named
Seeing SgrA* in its most intimate detail requires the biggest possible telescope, and the Event Horizon Telescope fits the bill: it is a “virtual” telescope that links up radio dishes around the world, making it as wide as our planet. In its first outing, three linked radio observatories stared at SgrA* hard enough to make out a blob of gas right on the margin of the black hole, its event horizon. As extra radio dishes in Chile and Antarctica are linked in to provide additional sensitivity, the telescope should be able to see if the outbursts of SgrA* are falling inwards – the mark of a disrupted asteroid or planet – or shooting outwards as a jet would.
The outburst 100 years ago was small beer on the scale of quasars, which can outshine a trillion suns. But further galactic archaeology has unearthed evidence of a more awesome outburst in the Milky Way’s heart that pushed it to the edge of quasar brilliance. It comes in the form of two that now tower 25,000 light years high on either side of the galaxy’s centre (see diagram). These “Fermi bubbles” were detected in 2010 by NASA’s , which picks out gamma rays from space. What could have inflated such vast structures?
A clue comes from three clusters of massive young stars, one buzzing around SgrA* and two orbiting a little further out. One of the latter is the Quintuplet cluster, which contains the Pistol star, one of the most massive and luminous stars in the entire Milky Way. The birth of these clusters was a major event in itself and must have had wider ramifications. “Star formation is a very sloppy business, and 50 per cent of the gas would have been dumped onto SgrA*,” says , also of Northwestern University. With a glut of gas to gorge itself on, SgrA* would have erupted with the power of 100 billion suns, making it shine like a quasar – albeit a mild-mannered one – and perhaps blowing the two huge bubbles of hot gas.
There are other explanations. “Everybody has a personal bias when it comes to the Fermi bubbles,” says Yusuf-Zadeh. “Mine is that it is due to the energy of the starburst.” But Fermi also shows tantalising hints of a straight “jet” within the bubbles that might mark a track of energy beaming out from the black hole to inflate the bubbles.
Whatever the details, the existence of the Fermi bubbles points to a huge disturbance at the galaxy’s centre, with vast quantities of gas in motion, either falling into the black hole or collapsing to form the massive star clusters we see today. The quantity of gas is easy to explain: though there’s little gas immediately around SgrA*, a few hundred light years out a necklace of huge, dense gas clouds orbits the galactic centre. One of these, known as SgrB2, weighs as much as 3 million suns, and contains over 100 different kinds of molecule, including enough alcohol to fill a glass the size of Earth. The question is how in the past similar clouds could have been kicked out of their stable orbits to form the new star clusters and feed the voracious SgrA* black hole.
Last month, Kelly Holley-Bockelmann from Vanderbilt University in Nashville, Tennessee, and colleagues suggested that the culprit was a dwarf galaxy. As this interloper smashed into the Milky Way’s heart, it squeezed gas clouds, causing them to collapse into a rash of bright new stars, including monsters like the Pistol star. The rest was dumped onto the central black hole.
Taking the plunge
Haggard has looked at twins of the Milky Way billions of light years away, and reckons such disturbances repeat over a galaxy’s lifetime, perhaps every 10 to 100 million years. That suggests a long wait for the next “big one” – and means that astronomers are looking forward even more eagerly to events expected to unfold later this year.
Stefan Gillessen of the in Garching, Germany, found the gas cloud G2 while examining images of SgrA* that he and his colleagues had amassed over the past 10 years. It was a complete surprise, he says. Instead of orbiting the black hole at a safe distance, like its neighbouring high-speed stars, G2 is plunging almost straight in. Gillessen’s best bet is that the cloud was created by streams of gas from nearby stars, known as stellar winds, colliding and stalling.
Perhaps the most extreme suggestion describes G2 as gas boiling away from a system of planets forming around a young star. In that case, we are witnessing a blighted solar system swinging by a giant black hole more closely than Neptune orbits the sun – surely a scenario for a future science fiction movie.
“We could be witnessing a blighted solar system pass the black hole closer than Neptune orbits the sun”
Much more will be revealed when G2 passes closest to the black hole later this year and some of its gas begins to be sucked in. “It’s a great, great event,” says Yusef-Zadeh. For a start, it will tell us something about the atmosphere of a black hole. If very little gas surrounds SgrA*, G2 will simply warm up as the black hole’s gravity squeezes it into a long spaghetti-like shape. Infrared telescopes will see a surge in its brightness, but other instruments won’t detect much at all.
On the other hand, if there’s a so far undetected dense disc of gas tucked in closely around the black hole, G2 will generate a burst of X-rays and radio waves as it smashes through at high speed. The collision may even show if SgrA* is emitting a jet of energy too small for astronomers to have detected with ordinary telescopes.
Gillessen and his colleagues started watching last month and expect to keep their eyes glued until March 2014. Haggard, too, is in for the long haul. “The encounter between G2 and SgrA* could unfold over the course of several years – or even longer,” she says. If the remains of the gas cloud swirl into a persistent disc around the black hole, we could even see a miniature version of a quasar, powering more serious jets shooting off into space.
Should we be worried? Probably not. G2 is a weakling on the galactic scene – it weighs only as much as three Earths – so no one is expecting a fully-blown quasar to flare up in the galaxy’s centre. An outburst of that size is perhaps 10 million years away, says Morris. Even then, it is unlikely that our descendants, if still on the scene, would need to head for the nearest bomb shelter. “While the explosions and quasar episodes that take place at the galactic centre are a fantastic opportunity for astronomers, they are very unlikely to have a major effect on Earth,” says Morris. “At a distance of 25,000 light years, we are pretty far from the danger zone.”
It would be quite a spectacle if we could make the 25,000-light-year trip to the galactic centre to see the action at first hand. “Our vision of the ‘night-time’ sky would be breathtaking,” says Morris. “For every star we can see in the terrestrial sky, you would see a million.” But with high levels of background radiation, and an uncertain monster lurking nearby, it is probably somewhere we would not want to consider going, says Yusef-Zadeh. “No way! I’m not crazy, I know what it would do to me and I value life too much.”
This article appeared in print under the headline “The giant awakes”
