BOSTON — ANDROMEDA - the closest major galaxy - is a familiar object to astronomers. Yet it still can surprise them.
Images taken by the Hubble Space Telescope July 6 and released today show what looks like a "double nucleus" at the core of this giant spiral of stars, dust, and gas. What astronomers had thought was the bright central nucleus they now see is some five light years from the galaxy's true center. A second, fainter bright spot lies at the galaxy center.
Each of these bright regions contains several million closely spaced stars. The traditional concept of ordinary spiral galaxies such as Andromeda or our own Milky Way is that of a structure with stars arranged in pinwheel-like spiral arms around a densely populated nucleus. Astronomer Tod Lauer said in announcing the discovery that the new images show Andromeda's nucleus "is much more complex than previously thought." He added that the offset star grouping, which astronomers had mistaken for a central nu cleus, may be the remains of an old galaxy that Andromeda had swallowed up.
Andromeda is only 2.3 million light years away. That's right in our own back yard as astronomical distances go. Yet ground-based telescopes had not revealed the complexity at the galaxy's center. It took the orbiting observatory's clearer view - unrestricted by distorting effects of Earth's atmosphere - to reveal that kind of detail. This sharpens astronomers' anticipation of what the Hubble telescope will find when astronauts repair its faulty optics. The repair mission now is scheduled for December.
Astronomer Stephen Maran at the National Aeronautics and Space Administration Goddard Space Flight Center in Greenbelt, Md., says: "If you've never looked before with a certain [telescopic] power, you don't know what to expect." Dr. Maran, who also handles public information for the American Astronomical Society, adds that observing with a repaired Hubble telescope will be venturing into the unknown even when studying normal objects such as Andromeda.
In this connection, he notes that astronomers already have seen something unexpected "in practically every case where Hubble has looked at the nuclei of galaxies." Its finding of a possible double nucleus in Andromeda suggests "there may be no such thing as a `normal' galaxy," he says. Galaxies may all have such peculiarities.
Hubble telescope's abilty to resolve unprecedented detail, even with a flawed main mirror, has opened up a new field of galactic study. This is the study of multiple nuclei at many galaxy centers. The presence of more than one such densely packed star groupings in a galaxy suggests that what astronomers see today may be the result of mergers of galaxies in the past.
Some astronomical theorists now think that such mergers can explain why most galaxies fall into one or the other of two general categories. Some are spiral structures like Andromeda or our Milky Way. Others have a roughly elliptical shape. Theorists suggest that multiple nuclei in elliptical galaxies could be the remains of spiral galaxies that had collided, merged, and lost spiral shape in the process.
If Andromeda has a true double nucleus, this too might be the result of galaxy merger. However, this would have been a case of a larger galaxy swallowing a small galaxy a billion or so years ago in a way that the merger did not destroy the larger structure's spiral shape.
However, in suggesting this possibility, Dr. Lauer notes that it is not the only explanation for the apparent double nucleus. He explains that Andromeda may have a single nucleus that is partly obscured by dust. In that case, the appearance of two nuclei would be an illusion.
This is the kind of problem that a repaired Hubble telescope should help solve. Spherical aberration - the optical defect in the telescope's mirror - smears images so that only about 15 percent of the light gathered by the telescope can be sharply focused. A repaired telescope should be able to sharply focus 70 percent of the light or more, as originally planned. The increased detail the telescope would then see should enable observers to determine whether or not Andromeda really does have two separate n uclei.
Astronauts on the space shuttle Discovery should soon be testing tools and practicing some of the techniques to be used to repair Hubble. This is part of the mission whose launch was postponed at the Kennedy Space Center at Cape Canaveral, Fla., on Saturday when electrical problems were discovered at the launch pad. The launch may take place later this week.