The center of our galaxy

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The supermassive objects we have discussed thus far exhibit blackhole activity in spectacular ways. Echoing their presence with unmitigated power from early in the universe's expansion, for example, quasars are difficult to supplant as the most unusual entities in existence. But size is not everything. Known as Sagittarius A*, our very own black hole at the center of the Milky Way may not be the most massive, nor the most energetic, but it is by the far the closest. And what a difference a few million light-years can make!

Looking at a photograph of a galaxy such as NGC 4258 (Fig. 2.6), one could understandably be duped into thinking that this aggregate of stars is packed together rather tightly. Yet over most of a galaxy's extent, stars account for an infinitesimal fraction of its volume. For example, if we were to think of a star as a cherry, we would need to commute between the major cities in Europe to simulate a typical distance (of several light-years) between stellar neighbors in space. At the center of our galaxy, however, some 10 million stars swarm within a mere light-year of the nucleus.

The brightest members of this crowded field are captured in Fig. 2.7, an infrared photograph of unprecedented clarity produced recently with the 8.2-meter VLT YEPUN telescope at the European Southern Observatory in Paranal, Chile.5 The image we see here is sharp because of a technique known as adaptive optics, in which a mirror in the telescope moves constantly to correct for the effects of turbulence in the Earth's atmosphere. This motion of the air produces a twinkle in far-away objects and distorts and blurs their appearance on photographs such as this. Adaptive optics can in principle create images with a clarity that is even greater than that of the Hubble Space Telescope, whose primary mirror has an aperture three times smaller than that of the VLT YEPUN.

The laws of planetary motion deduced by Kepler and Newton dictate that objects move faster the closer they orbit about the central source of gravity. Mercury, for example, the planet closest to the Sun, completes one orbit every 88 Earth-days, whereas Pluto, meandering about the farthest reaches of the solar system, takes a full 90 465 Earth-days to accomplish the same feat. This is understandable, of course, in terms of how quickly the Sun's gravitational pull diminishes with distance - this is the whole point of the Newtonian "balance" we have been using to weigh supermassive black holes in the nuclei of their host galaxies.

Sagittarius A* is so close to us compared to its brethren elsewhere in the universe, that on an image like that in Fig. 2.7 we can identify individual stars orbiting a mere seven to ten light-days from

5 Each of the four telescopes in the Very Large Telescope array has been assigned a name based on objects known to the Mapuche people, who live in the area south of the Bio-Bio river, some 500 kilometers from Santiago de Chile. YEPUN, the fourth telescope in this set, means Venus, or evening star.

the source of gravity.6 In the nucleus of Andromeda, the nearest major galaxy to the Milky Way, the best we could currently manage is about two light-years.

With this proximity to the supermassive black hole, stars orbit at blistering speeds of up to 5 million kilometers per hour, allowing us to see their motion in real time - even at the 28 000 light-year distance to the galactic center. They are zipping along so fast, in fact, that astronomers can easily detect a shift in their position on photographic plates taken only several years apart. Their incredible rate of advance makes it possible now to unambiguously trace their orbits with startling precision, revealing periods as short as 15 years! Compare this with the 220 million years it takes the Sun to encircle the galactic center just once.

The most spectacular identification7 to date of a star orbiting about the black hole was announced in October 2002 by an international team of astronomers using the unparalleled light-gathering capability of the VLT YEPUN telescope that produced the stunning image shown in Fig. 2.7. If one looks closely at the middle of this photograph, it appears that one of the fainter stars - designated as S2 - lies right on top of the position where the black hole is inferred to be. S2 is an otherwise "normal" star, though some 15 times more massive and seven times larger than the Sun.

That in itself is not very surprising, since a chance coincidence in the projected position of two objects along the line of sight is not unusual in a crowded field such as this. What is amazing, however, is that the star S2 has been tracked now for over ten years and the loci defining its path trace a perfect ellipse with one focus at the very

6 This effort has benefited from the contributions made by several observatories around the world. The principal investigators leading the effort to use these techniques for imaging the galactic center have been Andrea Ghez, Mark Morris, Eric Becklin, and their collaborators at UCLA, and Reinhard Genzel, Andreas Eckart (now at the University of Koln), and their collaborators at the Max Planck Institut in Garching, Germany.

7 This discovery was reported in Nature (2002) by a large team of astronomers led by Schodel, at the Max-Planck-Institut fur Extraterrestrische Physik.

position of the supermassive black hole. This photograph, taken near the middle of 2002, just happens to have caught S2 at the point of closest approach (known as the perenigricon, as opposed to the point farthest away, known as the aponigricon), making it look as though it was sitting right on top of the nucleus.

At this position, the star S2 was a mere 17 light-hours away from the black hole - roughly three times the distance between the Sun and Pluto, while traveling with a speed in excess of 5000 kilometers per second, by far the most extreme measurements ever made for such an orbit and velocity. Indeed, when this photograph was taken, the astronomers realized that the star S2 had just performed a rapid swing-by near the center, creating an unprecedented opportunity of determining not only the precise position of the source of gravity, but also its strength, and thereby its mass.

Using Newton's universal law of gravitation, we infer that the mass required to harness the motion of stars such as S2 seen closest to Sagittarius A* at the galactic center is 2.6 million Suns, compressed into a region no bigger than about seven light-days, and possibly just 17 light-hours given this latest discovery. For this reason, Sagittarius A*, and its cousin in the nucleus of NGC 4258, whose maser-emitting disk betrays its 40 million solar-mass heft, are considered by astronomers to be the most precisely "weighed" supermassive black holes thus far discovered.

In the coming chapters, we will probe more deeply into the nature of these objects and why they are increasingly being viewed as fundamental building blocks of structure in the universe. It appears that as much as half of all the radiation pervading the void of space may have been produced by these megalithic entities. Some were here near the dawn of time, perhaps even before galaxies as we know them formed, and all will be here toward the end.

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