The Mysterious Maelstrom in the Milky Way
As astronomers get clear views of the cosmic energy in our galaxy's core, the underlying cause continues to elude them
TAKE a good look at the picture at the top of this page. It shows our Milky Way galaxy as it might appear to an incoming alien space ship. It lets us, for the first time, see our galactic home as others might see it. It also marks a new era in galactic exploration.
The picture was taken by a spacecraft sensor - an infrared telescope on board the Cosmic Background Explorer (COBE) satellite circling Earth. As the National Aeronautics and Space Administration (NASA) Goddard Spaceflight Center in Greenbelt, Md., noted in releasing the picture, this is the ``first ... very clear picture of our galaxy'' that anyone has ever seen.
Even though the solar system is on the Milky Way's fringe - some 28,000 light-years from the galaxy's center - astronomers have, until recently, been handicapped in seeing our galaxy as a whole. Interstellar gas and dust obscure the comprehensive view that Earth's position allows - that is, they obscure the view in visible light.
Other types of electromagnetic radiation, however, do shine through. Gamma rays and X-rays, for example, reflect highly energetic processes such as star explosions. Radio waves trace the outlines and motions of clouds of dust and gas. Infrared radiation, among other things, shines from individual stars. Like visible starlight itself, it shows our galaxy as we would expect to see it with our own eyes if the dusty veil were lifted.
Except for radio waves and some infrared wavelengths, Earth's atmosphere blocks the non-optical view. But instruments on high-flying aircraft and balloons - and especially on spacecraft - have begun to make the full electromagnetic spectrum available to astronomers. That is why the historic picture shown here has quickly become a symbol of a new era in the study of our galaxy. And the bright central bulge is the focus of the challenge for the galactic explorers.
``It's a place very different from any other place in the galaxy. ... It's clear that very peculiar things are happening there,'' says astrophysicist E. Sterl Phinney of the California Institute of Technology in Pasadena, Calif. He compares the galaxy's central region - a few light-years wide - to the core of a large city.
Dr. Phinney calls it ``a crowded and dangerous environment'' where there is far more opportunity for stars and other objects to interact, sometimes violently, than in the quiet neighborhood of our sun.
Several million stars are packed into this galactic ``inner city'' within a volume only 3.3 light-years across. For comparison, the star nearest us, Alpha Centauri, is 4.3 light-years away.
There are signs that a massive explosion occurred in the crowded center within the past 100,000 years. Streamers of gas and dust move around the center. A surrounding clumpy irregular disk of clouds rich in various molecules such as hydrogen cyanide has a central clear volume 10 light-years across.
Variable outbursts of gamma rays suggest matter-antimatter annihilation on a massive scale. The radiation's intensity is consistent with action in which electrons and their antimatter twins, positrons, meet and turn into gamma rays at a rate of roughly 10 billion tons of positrons a second.
These are some of the phenomena that lead galactic astronomers such as Phinney to consider the galaxy center to be the site of ``peculiar'' happenings.
There also is the mystery of the unseen mass. The motions of stars and of dust, gas, and molecular clouds suggests that they are moving in a gravitational field far stronger than even the large mass of visible stars and other matter could generate. The visible mass amounts to at least several million times the mass of our sun. Some astronomers think an unseen mass also on the order of a million solar masses lurks in the galaxy's center.
This raises visions of that most exotic of cosmic objects, a black hole. This is an object that has collapsed to the point where its gravity is so concentrated that nothing can escape it, not even light. Such an object can accrete mass indefinitely, packing millions of solar masses into a volume less than that encompassed by the orbit of Jupiter. As it sucks in mass, the in-falling matter can undergo violent actions that could account for some of what is seen in the galaxy core.
Reviewing the relevant knowledge last month in Nature, Phinney noted that tiny regions in the cores of some nearby galaxies do emit enough energy to ``imply that they contain accreting black holes.'' So, he said, some astronomers are tempted to think our galaxy may have a black hole too. But there is no solid evidence for this supposition.
Elaborating on this in a telephone interview, Phinney explained that there are ``sort of various hints'' such as star motions that ``one can interpret according to one's inclination.'' ``But,'' he said, ``there is no evidence to convince a skeptic there is a black hole.''
One of the most interesting prospects for a concentrated mass is an intense radio source called Sagitarius A*. This seems to be so massive it moves little, if at all, under the gravitational forces at the galaxy center.
As Farhad Yusef-Zadeh of Northwestern University in Evanston, Ill., and colleagues noted last month in Nature, this object is ``unique in our galaxy'' and resembles, in weaker form, the kind of intense radio source seen in some other galaxies. They were reporting the possible discovery of four radio-emitting clouds associated with the Sagitarius object. If the existence of these blobs is confirmed, their motions might yield clues to that object's true mass.
Meanwhile, Phinney says that, black hole or not, it is reasonable to believe there is much unseen mass concentrated at or near the galaxy's core. It could be in the form of stars not yet observed.
Whatever it is, he points out that it is unlikely to be spread out more or less evenly over a region of even only a few light-years diameter. Phinney expects that the next generation of ground-based telescopes with infrared detectors will have the resolution to pinpoint sources 10 times more sharply than has been possible so far.
He adds that one of the ``most interesting things'' to look for are signs of disrupted stars - stars torn apart in mutual interactions or by an unseen massive object. Dismemberment and digestion of stars is a key process in galaxy evolution. If it is going on in our galaxy, the ``inner city'' core is the most likely place to find it. Such studies, together with the more detailed infrared observations of of what resides in that core, should mark the next phase in this new era of galactic exploration.