and required between five and 10 minutes to run a collision scenario with only 350 particles. By the late 1980s, an IBM desktop computer equipped with compiled BASIC could perform the same feat.

The last decade of the 20th century saw significantly more sophisticated computer models that could simulate galaxy interactions over a billion years of time. Initial results of these simulations revealed an unexpected finding. If two spiral galaxies of nearly equal mass passed each other with a low enough velocity, they would essentially fall into each other and, over hundreds of millions of years, merge into a single, massive elliptical galaxy. With this insight, astronomers were not only able to forge an evolutionary link between galaxies of various forms, but also at different epochs in the history of the universe.

The most powerful computers today—supercomputers—have revolutionized the field of galaxy interactions. In one recent example, astronomer Fabio Governato of the University of Washington, Seattle and colleagues used several U.S. supercomputing facilities to solve a long-standing mystery in galaxy formation theory: why most galaxies do not have more stars at their cores. This predicament is most pronounced in a class of diminutive galaxies called dwarf galaxies, the most common type of galaxy in the neighborhood of the Milky Way. Governato’s computer simulations showed that supernova explosions in and around the core of a developing dwarf galaxy generate enormous winds that sweep huge amounts of gas away from the center, preventing millions of new stars from forming there.

Governato’s project consumed about one million CPU hours, the equivalent of 100 years on a single desktop computer. “This project would have not been possible just a few years ago,” he says, “and, of course, completely unthinkable in the 1980s.” The enormous growth in computing power, Governato adds, makes the early results obtained by the Toomre brothers using rudimentary computers “even more amazing as their results still hold true.”

searching for extrasolar Planets

Since 1995, astronomers have discov-
ered more than 400 planets orbiting
other stars. Because stars are about
a million times brighter than typical
planetary bodies, most have been de-
tected using indirect means. One of
the most successful of these employs a
highly sensitive spectrograph that can
detect minute periodic changes in the
star’s velocity toward and away from
Earth—motions caused by changes
in the system’s center of gravity as
the planet orbits the star. These back-
and-forth velocity shifts, however, are
incredibly slow, amounting to as little
as a few meters per second. Still an-
other search method looks for slight
wobbles in the star’s position in space,
a motion also induced by an orbiting
body. The extent of the wobble allows
astronomers to determine the exo
planet’s mass. The positional change
is very fine—within 20 milliarcseconds
or so, which is about the diameter of
a golf ball seen at the distance of the
Moon. Such fine-tolerance measure-
ments as these would be impossible
without computers.

the last decade
of the 20th century
saw significantly
more sophisticated
computer models
that could simulate
galaxy interactions
over a billion years
of time.

team of astronomers associated with a planet-hunting space mission called COROT (for COnvection ROtation and planetary Transits), led by the French space agency CNES, announced that they had detected a planet only 1. 7 times Earth’s diameter. But the planet, called CoRoT-7b, is hardly Earthlike. It lies only 1. 5 million miles from its host star and completes one orbit in a little over 20 hours. Observations indicate the planet is tidally locked, so that it keeps one hemisphere perpetually turned toward its sun. Being so close to its host star, the dayside temperature is some 2600° K, or 4220° F—hot enough to vaporize rocks.

Astronomer Bruce Fegley, Jr. of Washington University and research assistant Laura Schaefer used a computer program called MAGMA to model CoRoT-7b’s atmospheric structure. MAGMA was originally developed to study the vaporization of silicates on Mercury’s surface, but has since been used, among other things, to model the vaporization of elements during a meteor’s fiery passage through Earth’s upper atmosphere.

Fegley and Schaefer’s results showed that CoRo T-7b’s atmosphere is virtually free of volatile compounds, such as water, nitrogen, and carbon dioxide, probably because they were boiled off long ago. But their models indicate plenty of compounds normally found in a terrestrial planet’s crust, such as potassium, silicon monoxide, and oxygen. They concluded that the atmosphere of CoRoT-7b has been altered by the vaporization of surface silicates into clouds. Moreover, these clouds may “rain out” silicates, effectively sandblasting the planet’s surface.

“Exoplanets are greatly expanding the planetary menagerie,” says Fegley. “They provide us with all sorts of oddball planets that one could only imagine in the past.”

citizen science

In 1999, the Space Sciences Laboratory at the University of California, Berkeley, launched a program called SETI@ home, which enabled thousands of home computers to sift through unprocessed data collected by radio telescopes in the search for artificial signals from other worlds. The program, which today comprises of more