NEWS FEATURE NATURE|Vol 446|5 April 2007 A long time ago, in a galaxy not so far away Vast stellar nurseries, clouds that dwarf the Solar System and lurking swarms of black holes. Jeff Kanipe probes the unfolding mysteries at the heart of the Milky Way. efore it was seen, it was heard. In gazing elsewhere in the sky had discovered to explore — from echoes of outbursts to the the early 1930s, a Bell Labs engineer quasars, bodies so bright and yet so small silhouette of the event horizon itself. named Karl Jansky was given the job that it seemed possible they were powered by One problem with observing the very centre Bof sorting out where the static interfer- vast black holes sucking up dust and gas at an of the Galaxy, though, is that it simply isn’t very ence in radio transmissions came from. With incredible rate. In 1969, the British astronomer bright — a firefly to the searchlight of a full- an ungainly but ingenious steerable antenna Donald Lynden-Bell suggested that our Galaxy blown quasar. The obvious explanation for its he tracked a number of sources. Most lack of luminosity, says Andrea Ghez, were thunderstorms, but one wasn’t. As principal investigator of the Galactic Jansky tracked it across the sky from Center Group at the University of Cali- day to day he realized that it was far fornia, Los Angeles, is that even though beyond Earth’s atmosphere, and indeed Sgr A* lies at the heart of a galaxy of OGIES’ BELL LABS beyond the Solar System — an abiding hundreds of billions of stars, it may be hiss from somewhere in the constella- a bit isolated. The radiation from black CHNOL tion of Sagittarius1. holes comes not from the holes them- CENT TE U As constellations go, Sagittarius is selves, but from matter falling onto the L modest both in size and in brightness. accretion disks that swirl around them. What sets it apart, on dark, moonless There may just not be much matter nights, are its background contrasts: around to fall onto the accretion disk brilliant, billowy clouds of stars that at the centre of the Galaxy. Or the disk are punctuated by dusky rifts and may be generating a wind of radia- RBIS voids. No other place in the sky looks tion strong enough to stop any more O C this compelling. gas and dust flowing into it. By the time of Jansky’s discovery, the behav- Karl Jansky (right) used MANN/ iour of other objects in the sky had already a rotating antenna to Unexpected echo BETT provided good evidence that something spe- detect radio waves in the Nevertheless, the flow sometimes cial lay within those beguiling clouds. In 1918, Milky Way. increases and the central engine a study of star clusters by Harlow Shapley, an heats up. For a couple of years in the astronomer at the Mount Wilson observatory and its neighbours could 1950s, for example, Sgr A* looked above Los Angeles, showed that ‘open’ star all have ‘dead quasars’ at perhaps a hundred thousand times clusters were spread throughout the plain of their hearts4. By this line brighter in the X-ray spectrum the Milky Way, whereas globular clusters were of reasoning, Sgr A* had than it is today, probably because concentrated in the direction of Sagittarius2 — to be a vast black hole. it swallowed a planet’s worth of gas in a gulp. some above the Milky Way, and some below it, Even with a compelling theory, seeing the Unfortunately, humankind had no X-ray tele- drawn by some unseen immensity. The globu- details was hard. Teles copes using visible light scopes in the 1950s — the devices only work in lar clusters were like moths batting about a lamp — even the mighty Hubble — could not see space — which might seem a serious impedi- hidden in Sagittarius’s dense folds of dust. through the clouds of dust. But in the past ten ment to learning from the event. But it is not, years, powerful radio arrays, new infrared and it turns out, an insurmountable one. Hidden behind the clouds X-ray telescopes, detectors in orbit and adap- At the January 2007 meeting of the Jansky’s observations provided the first hint tive-optics systems on Earth have revealed American Astronomical Society in Seattle, of what lay behind those shrouds, but it took strange new structures in and around the Washington, Michael Muno of the California decades for further details to become clear. Galaxy’s central engine: magnetic arcs and fila- Institute of Technology (Caltech) in Pasadena It was not until 1968 that the radio source at ments, giant clumps of massive stars and whorls announced that his team had managed to see the centre of the Galaxy, now called Sgr A* (or of gas. Analysis of the motions and masses of part of the 1950s outburst reflected off clouds ‘Sagittarius A-star’), was detected in the infra- the stars within the central two light years of on the far side of the Galactic Centre5. X- red, showing that it was 1,000 times brighter Sgr A* have shrunk the known heart of our rays that had started off heading away from than the radio emission had led astronomers Galaxy down to a region of space no larger Earth had bounced back to us, and because to suspect3. At shorter infrared wavelengths — than Earth’s distance from the Sun, and prob- the clouds were a few tens of light years away which like their longer brethren pass through ably much smaller, containing the mass of four from the centre, the reflected X-rays took half dust much more easily than visible light — it million Suns. For all this insight, the heart of a century longer to get to Earth than did those was even brighter. By this time, astronomers the Galaxy still has mysteries for astronomers that had taken the direct route. Half a century 600 NEWS FEATURE ARTWORK BY LYNETTE COOK LYNETTE BY ARTWORK For the first time, astronomers can say with almost certainty that at the centre of the Galaxy lies a black hole with a mass of some 4 million Suns. The black hole is surrounded by an accretion disk of gas and dust (yellow and pink rings) and orbited by dozens of massive, young stars (white and blue). Farther out are clouds and curtains of interstellar dust, which reflect the light from flares occurring when gas passes into the black hole. The region is also home to some very young, massive clusters (the blue stars at the upper left). isn’t much in terms of a journey across the and Astrophysics. The intermittency of such rial were there: it’s hard for a cloud of gas to Hawaii, Ghez’s Galactic Center Group, has could have occurred if the density of the moment, most astronomers seem to favour 26,000 light years that separate Earth from the events could imply that the disk of material contract into a star under its own gravity when almost seen them make complete circuits of gases in the centre of the Galaxy was much the first scenario. Galactic Centre, but it’s enough to make the swirling about the black hole is both meagre something that weighs as much as four million the centre and return to where they started: higher in the past. Higher density would But although young stars may not be difference between astronomers stuck under- and unstable, only occasionally dropping a stars is sitting next door. “We should see S0-2 close [its orbit] in allow clumps in the clouds to collapse to migrating into the central zone, very neath Earth’s X-ray-absorbing atmosphere and gobbet of matter into the black hole’s maw. Nevertheless, says Ghez, at the Galaxy’s 2010,” she says. form stars, even in the presence of a strong old ones probably are. Theorists at the astronomers who, like Muno’s team, can use But if the black hole’s neighbourhood is by core is a swarm of about 40 massive young stars; The orbits of the central stars of the gravitational field. Galactic Center Workshop described NASA’s Chandra X-ray telescope to monitor and large empty, how can we account for the they are called ‘S stars’ because they belong to Galaxy can be used to further refine the The alternative explanation is that the recent simulations that bolster a striking the centre of the Galaxy. family of bright young stars that swarms about the Sgr A* cluster. One, called S0-2, has a mass mass of the central black hole and to con- stars formed outside the adverse con- prediction first made by Mark Morris of “This is the first X-ray echo that we have it? This apparent paradox received prominent that is some 15 times that of the Sun and orbits strain the distribution of mass in the neigh- ditions of the central region and migrated the University of California, Los Angeles, seen propagating through space after an event attention at the Galactic Center Workshop held Sgr A* with a period of just over 15 years. At bourhood. And their motions might also there later on as a single massive cluster. in 1993. Morris postulated that the inner that we had not originally seen,” says Muno. in Bad Honnef, Germany, in 2006. The black its closest, it comes within 17 light hours of the reveal something about how they got there However, for this to work the core of the three light years of the Galaxy’s centre The observations allowed his team to say that hole’s inactivity suggests that the central few supermassive black hole6 — as close as the edge in the first place.