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January 15, 2002

Fountains and Bubbles: New Cosmic Mysteries


A brown dwarf star, at 7 o'clock in both images, was found orbiting a star called 15 Sge, top. In the center image, the primary star's light has been removed. The primary star is 58 light-years from Earth. Bottom: A 400-by-900 light-year mosaic of images of the central Milky Way. Incandescent gas bathes white dwarf stars, neutron stars and black holes.

With the whole cosmos as its agenda, the meeting of the American Astronomical Society last week in Washington was a sounding board for scientists with new findings and ideas about nearly everything from mysterious gamma ray bursts in deep space to revealing images penetrating the turbulent heart of the Milky Way, Earth's home galaxy.

Two new discoveries described at the meeting underscored the growing and bewildering realization that planetary systems abound in the nearby universe and that they come in all shapes and sizes, bearing little apparent resemblance to the Sun's family of planets.

Both discoveries revealed unexpected configurations of stars and orbiting companions.

In observations by telescopes in Hawaii, astronomers were surprised to detect an object that is no ordinary planet orbiting a sunlike star 58 light- years from Earth. The object is a brown dwarf, the name given to what are thought to be failed stars that lacked the mass to ignite their nuclear furnaces.

Dr. Michael Liu, an astronomer at the University of Hawaii, reported that the brown dwarf was estimated to have a mass about 65 times as great as that of Jupiter, the solar system's largest planet. The brown dwarf is orbiting the star called 15 Sge at a distance of 1.3 billion miles 14 times the distance between Earth and the Sun.

Of the more than 80 giant objects that have recently been found orbiting nearby stars, most have been estimated to be Jupiter-size planets.

But several are so massive that they are suspected of having starlike origins and thus of being brown dwarfs.

"This companion is probably too massive to have formed the way we believe that planets do, namely from a circumstellar disk of gas and dust when the star was young," Dr. Liu said. "This finding suggests that a diversity of processes act to populate the outer regions of other solar systems."

Other astronomers said the brown- dwarf discovery was important because it was the first of these extrasolar companion objects to be detected by direct observations, not indirectly by measuring the effects of their gravity on the motions of the host stars.

A new technology called adaptive optics, a spinoff of military research, uses computers to compensate for atmospheric distortions and produce sharper images of faint cosmic objects.

Dr. Alan Boss, an astrophysicist at the Carnegie Institution of Washington and a theorist who specializes in the new planetary systems, said such discoveries were "really just tantalizing appetizers for things to come."

In a report of a related discovery, astronomers said they had made the first detection of a planet orbiting an extremely large star one that had already burned its hydrogen fuel and was dying.

The star, iota Draconis, has expanded to a radius that is 13 times as great as that of the Sun, which is an average-size star.

"Until now, it was not known if planets existed around such giant stars," said Dr. Sabine Frink of the University of California at San Diego, a member of the discovery team. "This provides the first evidence that planets at Earth-like distances can survive the evolution of their host star into a giant."

Sometimes, coming down to Earth, astronomers speak an almost ordinary language, as when they talked at the meeting about cosmic bubbles, fountains and halos.

The bubbles are in this case ghostly cavities in huge galactic clusters. The clusters of 100 or more galaxies stretch millions of light-years across and are considered the largest stable objects in the universe.

From the centers of the clusters, cavities virtually devoid of X-ray and radio emissions rise like bubbles in a glass of soda pop. Some bubbles are as large as 60,000 light-years wide, almost as big as the Milky Way.

Dr. Brian McNamara, an astronomer at Ohio University in Athens, described the discovery and analysis of the bubbles with data from the Chandra X-ray Observatory in Earth orbit.

The bubbles, Dr. McNamara said, were most likely created by extremely powerful explosions, probably involving material plunging toward grasping black holes.

Observations show that the bubbles are not completely empty, but contain some hot gas, high-energy particles and magnetic fields. The bubbles may play a major role in transporting magnetic fields through the clusters.

"We think magnetism in some locations of the universe could have been as important as gravity in shaping" the galaxies and even larger- scale cosmic structures, Dr. McNamara said.

Dr. Edward M. Murphy, an astronomer at the University of Virginia, reported on other spacecraft observations that have yielded evidence for a towering fountain of gas in a nearby galaxy, NGC 4631, which in telescopes bears a resemblance to a whale.

The spacecraft is the Far Ultraviolet Spectroscopic Explorer, or FUSE, operated for NASA by Johns Hopkins University.

In describing the phenomenon at a news conference, Dr. Murphy managed to use all three evocative terms bubbles and halos, as well as fountains. He did that in proposing a possible solution to the longstanding mystery of the ultimate fate of energy from exploding stars, or supernovas.

A profusion of stellar explosions, Dr. Murphy suggested, would probably start the circulation of hot gas, rising in a fountain of bubbles out of the galaxy and into the halo of diffuse gas that surrounds the galaxy. Gas remaining in the halo for perhaps a million years would presumably cool and fall back into the galaxy, bringing with it material for the next generation of new stars.

Dr. Murphy said the FUSE data provided the "first good evidence" for the long-theorized "galactic fountain" cycle of circulating gas.

Some of the same ultraviolet observations have enabled astronomers to detect clouds of hydrogen gas raining onto the Milky Way galaxy and glowing on contact with the intergalactic medium, like showers of meteors that streak through Earth's upper atmosphere.

From the FUSE data, the astronomers found that beyond the previously known and denser halo around the Milky Way there was another one less dense and hot, possibly one million degrees Fahrenheit. That outer halo, invisible in ordinary light, can be seen in ultraviolet light as a blue, football-shape envelope around the entire galaxy.

Astronomers said the outer halo could be left over from the formation of the Milky Way or it may have been created by early episodes of star formation in which the hot gas was heaved by supernovas and expelled from the galactic disk perhaps in some kind of fountain of bubbles. In any event, because astronomers are still detecting the hydrogen clouds that are falling into the galaxy, they think the Milky Way is still pulling in new material.

The new findings, said Dr. Kenneth Sembach of the Space Telescope Science Institute in Baltimore, are "going to help astronomers understand better how galaxies are assembled and hopefully how they evolved over time."

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