The Stars in the Centre of our Galaxy Michael Burton University of New South Wales PO Box 1, Sydney, NSW 2052, Australia What lies at the heart of our Milky Way Galaxy? This question has puzzled astronomers since the time of Herschel, who first demonstrated our Sun inhabited a huge flattened system of stars we call the Galaxy. We have learnt much about the nature of our Galaxy since HerschelÍs day. For instance, we now know we live in the outer suburbs of a spiral disk over 100,000 light years across but just 4,000 light years thick, and containing at least one hundred thousand million stars. We lie 28,000 light years from its centre and spend 200 million years just to travel around it once. Within the inner 3,000 light years the stars are no longer confined to the disk but spread out to form a spheroidal nuclear bulge. Whereas the stars of the disk are relatively youthful, those in the Bulge are nearly as old as the Galaxy, typically some 11 to 14 billion years of age. But what lies in the very centre of this vast system? Popular speculation has it that a massive black hole resides in the nucleus, perhaps as heavy as a million suns. Certainly talk of a massive black hole always grabs the publicÍs attention. It is, however, a controversial interpretation of the available data and one that many astronomers do not accept. In this essay I do not intend to go over the debate on the nature of the very nucleus, but rather to describe a facet we have more confidence of our understanding. This is the nature of the stars and gas in the central ten light years of the Galaxy. In the past few years, by applying the new technology of infrared detector arrays, astronomers have learnt that a cluster of massive, young and extremely luminous stars reside in our GalaxyÍs core. Understanding how they came to be there and whether they will remain so is now at the heart of the debate over the nature of the Galactic nucleus. A debate whose outcome will likely determine how massive a black hole might reside there? Our knowledge of the nuclear regions of the Galaxy has till recently been very limited. Visible light from the centre is absorbed by the extensive clouds of intervening gas and dust lying between us. Only high energy gamma and X-rays, and low energy radio waves, have been readily detectable. They tell us something of the nature of the gas in the nuclear regions, but nothing of the stars which might be there. The coming of the age of infrared astronomy has changed this picture. Visible light from the Galactic Centre is attenuated by a factor of around 10,000 million compared to infrared light of wavelength 2 microns. What is impossible to see in the visible becomes readily apparent in the infrared. The technology that has recently brought us the ability to take images in the infrared now lets us view these stars directly. We have found that the central few hundred light years is inhabited by a cluster of relatively cool, old stars with a stellar density some 300 million times greater than our solar neighbourhood. It is the inner part of the Galactic Bulge. Nevertheless, simply taking infrared pictures of the Galactic Centre, however spectacular they may be, doesnÍt reveal much about the nature of these stars by itself. In optical astronomy we can use colour to discriminate between the different types of stars present, and thus to spectrally classify them. Determining colour can be regarded as measuring the difference between the amount of red and blue light emitted by a star. Infrared colours can be similarly defined for the infrared wavebands. The colours so measured, however, are dominated by the differential obscuration between the emitting wavelengths. They are of little use in determining the spectral type of the star. Astronomers have to rely on spectroscopic techniques to classify the infrared stars, looking for the emission signatures from elements such as hydrogen and helium in the stellar radiation. This is not an easy task as infrared spectrometers are considerably less sensitive than their optical counterparts. Moreover, only the short wavelength range from 1.5 to 2.5 microns is readily accessible, further limiting our diagnostic abilities. Only in the last three years have there been instruments capable of the job. One of the worldÍs great telescopes has been put to the task, the Anglo Australian Telescope in the state of New South Wales in Australia. Two groups in particular have used the telescope for this project, a German group from the Max- Planck Institut in Munich lead by Arthur Krabbe, and an Australian group lead by David Allen from the Anglo Australian Observatory. Using their own instruments they have made maps in the infrared emission lines of several elements and molecules which have revealed a new picture of activity in the heart of our Galaxy. A cluster of some dozen stars has been found emitting in the infrared light of a helium line at 2.05 microns within the central light year of the Galaxy. Surrounding these stars are streamers of ionized gas glowing in hydrogen light at 2.16 microns, embedded in a cavity some 5 light years in radius. At the edge of this cavity is a ring of hot molecular gas, shining in the light of molecular hydrogen at 2.12 microns. The helium line we saw is the signature of emission from very hot, luminous stars. They are extremely massive, around 20 solar masses each, and most likely Wolf-Rayet stars. Such stars are also exceedingly rare and to find a cluster of them is unprecedented. They have short lifetimes, which means they must have formed recently and raises the question of how did they get where they are in the first place? Yet these helium stars are only the tip of the iceberg when it comes to classifying the stars in the Galactic Nucleus. Searching for other infrared spectral diagnostic lines, such as iron, has shown us that there are lower mass members present from the same cluster of hot, young stars in the Galactic Centre. We cannot yet say how many stars are in the cluster, but there must be at least several hundred. Their combined radiation field and stellar winds can create the ionized cavity in the central few light years and heat the surrounding ring of molecular gas. Their total emission, some 10 million solar luminosities, supplies the energy which powers the far-infrared emission measured from the region, the thermal radiation from hot dust grains heated by UV- photons. The cluster of stars is the luminosity source at the centre of our Galaxy. It comprises a population of young massive stars in the core of the old, cool stars making up the Galactic Bulge. What brings such stars to the centre of the Galaxy? They must have formed recently which begs the question of why are we seeing them now? Are we living at a special time in the life of the Galaxy, soon after a burst of massive star formation in its core? Or is such activity the norm, fueled perhaps by the continual inward flow of gas down the central bar of the galaxy onto the nucleus? There are two principle explanations for their existence being touted. The first is that the cluster is the core of the old stars of the Galactic Bulge, whose members have been rejuvenated through stellar collisions and mergers. The second is that a group of massive stars formed in the nucleus some ten million years ago, probably the result of a collision between two molecular clouds. In other words, that we live in a mild starburst galaxy. This is a debate which will be settled only by further advances on both the observational and theoretical fronts. The exact nature of the members of the stellar cluster can be determined by a concerted campaign of spectroscopic observations at both high spectral and spatial resolution. We need, for instance, to determine the distribution of stellar masses in the cluster. Such a project will be well-suited to the new generation of 8-m telescopes now being built. The theorists need to tackle the question of the dynamics of the region. How indeed can you supply the core of our Galaxy with a tight cluster of massive stars? If you can, is that compatible with the presence of a million solar mass black hole at the very centre? 1400 Words Possible Colour Pictures 1. 'True-colour' infrared image of the stars in the central 30 light years of our Galaxy. The image has been put together by assigning the colour red to 'long'-wavelength near-infrared light at 2.2 microns, green to light at 1.65 microns and blue to 'short'-wavelength light at 1.25 microns. It shows how a person with near-infrared eyesight would perceive the Galactic Centre. The Galactic plane is clearly revealed extending NE-SW (top left - bottom right), through the Galactic nucleus. Several dark clouds of molecular gas can be seen in a ring around the nucleus. Relatively fewer stars are seen to the SE of the image due to increased general obscuration to those regions rather than there being less stars present there. 2. Spectral line infrared image showing the gas and hot stars within the central 10 light years of our Galaxy. Red shows emission from molecular hydrogen at 2.12 microns, revealing a ring of hot molecular gas. Green denotes emission from hydrogen gas at 2.16 microns, showing the ionized central cavity. Blue shows emission from a helium line at 2.05 microns, arising from the most massive stars in the young central star cluster. This three colour image clearly shows the close relationship between the hot stars, the ionized gas, and the ring of hot molecular gas. Both pictures taken using the Infrared Imaging Spectrometer (IRIS) on the Anglo Australian Telescope (AAT).