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NEWS FEATURE |Vol 446|5 April 2007

A long time ago, in a not so far away Vast stellar nurseries, clouds that dwarf the and lurking swarms of black holes. Jeff Kanipe probes the unfolding mysteries at the heart of the .

efore it was seen, it was heard. In gazing elsewhere in the had discovered to explore — from echoes of outbursts to the the early 1930s, a Bell Labs engineer , bodies so bright and yet so small silhouette of the event horizon itself. named Karl 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 . 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 ’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 , 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 go, Sagittarius is selves, but from matter falling onto the L modest both in size and in brightness. disks that swirl around them. What sets it apart, on dark, moonless There may just not be much matter , 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 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 clusters by , an heats up. For a couple of years in the astronomer at the 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 . it swallowed a ’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 and of part of the 1950s outburst reflected off clouds ‘-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 , and prob- the clouds were a few tens of light years away which like their longer brethren pass through ably much smaller, containing the of four from the centre, the reflected X-rays took half dust much more easily than visible light — it million . 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 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 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. original cluster would have needed a mass might contain as many as 20,000 star- the burst must have been 1,000 times brighter light years doesn’t contain enough raw material of our Solar System is to Earth. After 12 years There are two explanations for the that was ten million times greater than sized black holes. These are the remnants and 1,000 times longer than the contemporary to make stars. And the enormous gravitational of monitoring the motions of these stars using stars’ presence. One theory is that the that of the Sun, packed into a volume of of previous generations of bright young ones seen with the Chandra telescope or the tidal forces around the black hole would seem the infrared capabilities and of stars formed more or less where they are no greater than 3 light years, which is more stars, which have sunk slowly in towards Japanese Advanced for to prohibit stars from forming even if the mate- the W. M. Keck Observatory on Mauna Kea, today, near the black hole. In principle, this compact than any cluster known. At the the central and much larger black hole over

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billions of years. The presence of a blur the image of Sgr A*, close-knit cluster of dead stars is much like frosted glass blurs

supported by Chandra’s discovery an image,” says Bower. NRAO/AUI of four bright but variable X-ray Bower and his peers J. CHUMACK/SPL J. sources — within 3 light years of Sgr hope that new generations A* (ref. 7). The sources’ variability is of interferometers working a characteristic of systems in which at millimetre and submilli- matter from a normal star is sucked metre wavelengths, which onto a black hole or an ultradense are less subject to interven- . Four fairly easily dis- ing distortion, might in the cerned X-ray sources of this type in long run actually reveal the such a confined region, say astrono- black hole’s event horizon8. mers, provide strong circumstantial How would that look to evidence that tens of thousands of outside observers? Depend- black holes and neutron stars have ing on its orientation, settled in and around Sgr A*. astronomers think that the relativistic effects of the black Smouldering stars hole’s intense gravitational To see some real clusters, we have field would make the event to step back a little. A hundred light horizon appear as a large years from Sgr A* lie the Arches and shadow or a silhouette cast

ESO Quintuplet clusters — two of the on a background of bright most massive young clusters and plasma, in which the shadow unlike either the open or globular is the boundary where light clusters seen elsewhere. The stars passes into the throat of the in the Arches cluster are 50 times black hole itself. closer together than are those in the At radio wavelengths, shells of gas surround Sgr A* (point source at centre). “This image can be made neighourhood of our Solar System. with a network of milli- At the density seen in the Arches cluster, the ters seen in Spitzer images are diminished; metre- and submillimetre-wavelength tele- space between the Sun and its nearest neigh- the cores of cooler clouds blossom with light, scopes distributed around the Earth,” Bower bour would contain 100,000 stars. The other indicating that they have yet to collapse into says. Some of these telescopes already exist. cluster, the Quintuplet, is a bit older and more massive protostars. These regions, which Others, including the largest, the Atacama dispersed, but it has one of the biggest and form a ridge-like structure in images taken at Large Millimeter Array, are under construc- potentially most volatile stars known, called millimetre wavelengths, might one day form tion. Lashing them together into an ad-hoc At the centre of the Milky Way (above) is a supermassive black the (so named for the pistol-shaped a chain of clusters like those in the Arches or interferometer the size of Earth, though, is a hole called Sgr A* (right), which is surrounded by both hot in which it lies). Quintuplet, unless gravitational forces from daunting technological challenge. That, says (blue) and cold (red) stars. The stars form in ‘nurseries’ (below) The source of these mammoth star the black hole disrupt their formation. Such Bower, “is part of the thrill of the chase”. as bright cores embedded in giant molecular clouds. clusters are giant molecular clouds — cool, structures, says John Bally, the principal Such an observation would, of course, be a dense complexes of dust and gas investigator of the survey, “are unique to the milestone — the first direct proof that an event up to 130 light years in breadth and contain- Galactic Centre region.” horizon, and therefore a black hole, exists. ing the mass of between 10,000 and 500,000 And observing ‘hot spots’ orbiting the black Suns. Instruments such as those on NASA’s The Galaxy’s dark heart hole would allow astronomers a qualitative can see the parts that The one structure that is absolutely unique, way to test the effects of relativity in a strong contain young stars, which glow brightly in though, is the itself. gravitational field, an endeavour that has so the infrared. Astronomers working at longer Given its apparent size and proximity to far yielded ambiguous results9. The Galactic wavelengths can see even earlier stages of the Earth, says Geoffrey Bower of the University Centre may no longer be the mystery it was in star-birth process. New maps of the Galactic of California, Berkeley, it affords astronomers Shapley or Jansky’s day, but the better known Centre made at the Caltech Submillimeter their best chance to image the black hole’s it is, the more remarkable it looks — and it Observatory (CSO) on Mauna Kea, Hawaii, event horizon — the boundary beyond which promises to become even more remarkable reveal objects so early on in their development no light can escape. before too long. ■ that they can’t yet be called stars. “What we At the moment, the best observations Jeff Kanipe is a science writer based in see are usually cores of clouds that have not of the black hole and its accretion disk are Maryland. necessarily begun to form stars yet, or are in those made by a technique that links radio 1. Jansky, K. Proc. Inst. Rad. Eng. 232, 1158–1163 (1935). the early stages of doing so,” says Elisabeth telescopes around the world, called very long 2. Shapley, H. Contributions from the Mount Wilson Solar Observatory No. 152 (1918). J. BALLY & THE BOLOCAM/CSO SURVEY TEAM PLANE SURVEY GALACTIC & THE BOLOCAM/CSO BALLY J. Mills, who is a member of CSO’s Bolocam baseline interferometry. At a distance of 26,000 3. Becklin, E. E. & Neugebauer, G. Astrophys. J. 151, 145–161 Galactic Plane Survey. “With millimetre-wave light years, an interferometer working at radio (1968). data you get a more unbiased census of where wavelengths with a baseline the size of a planet 4. Lynden-Bell, D. Nature 223, 690–694 (1969). 5. Muno, M. P., Baganoff, F. K., Brandt, W. N., Park, S. & begins to occur. You see all of should be able to resolve details as small as the Morris, M. R. Astrophys. J. 656, L69–72 (2007). these nurseries, and whether or not they have orbit of the Earth. Unfortunately, the radio 6. Ghez, A. M. et al. Astrophys. J. 586, L127–132 (2003). a ‘baby’ in them yet.” waves from Sgr A* pass through intervening 7. Muno, M. et al. Astrophys. J. 622, L113–116 (2005). 8. Bower, G. C. J. Phys. Conf. Ser. 54, 370–376 (2006). A mosaic of these maps reveals a different regions of highly ionized gas, which scatter its 9. Broderick, A. E. & Loeb, A. J. Phys. Conf. Ser. 54, 448–455 side to the Galactic Centre. The bright clus- radio emissions. “These random distortions (2006). 604 © 2007 Nature Publishing Group