Some stars in double-star systems have found a quick way to lose weight by dumping their extra pounds onto their companions. Astronomers using NASA's Hubble Space Telescope have discovered such a case in the double-star system Phi Persei. A "rapid diet" program has trimmed an aging, once massive star to a lean one- solar mass, while the once mild-mannered, moderate-sized companion has bulked up to a hefty nine-solar masses and is spinning so violently that it's flinging gas from its surface. This observation has allowed astronomers to catch a glimpse of an unusual, fleeting moment in the life of a massive star in a double-star system.

"We have seen massive star binaries at the end of their lives, when one star has collapsed to become a neutron star," says Douglas Gies of the Center for High Angular Resolution Astronomy at Georgia State University, Atlanta. "There are about six dozen of those known in our galaxy. But what we hadn't seen before is the phase just prior to the collapse of the aging star. The Hubble observations dramatically show how severely a star's material can be removed through the gravitational influence of a nearby companion."

What Gies and his team have seen is an opportunistic star that has taken advantage of an aging, ailing partner. After consuming most of its hydrogen -- the fuel that keeps its thermonuclear furnace running -- the aging star swelled up and began jettisoning its mass until only its bare core was left. The companion star cannibalized the discarded material, thereby increasing in size. Gies calls the stripped-down star a subdwarf, a type of aging star that has passed the expansion phase -- by swelling and puffing away its outer layers -- and is on its way to becoming a fading white dwarf. Yet this aging, stripped-down star, which has the same mass as the Sun, is nine times hotter than the Sun at 95,000 degrees Fahrenheit and is very bright.

The subdwarf would be the brightest object of its class in the sky (a sixth-magnitude star) if it could be seen alone. If placed at the Sun's distance, it would appear 200 times brighter than the Sun. However, the beefed-up companion is ten times brighter in visible light than the subdwarf, which is lost in its glare and eluded detection for many years.

The subdwarf was detected by the Hubble telescope's Goddard High Resolution Spectrograph (which was removed from Hubble last February during the second servicing mission), allowing scientists to identify its spectral signature. Gies' analysis of these observations will appear in the Jan. 20, 1998, issue of the Astrophysical Journal. With the Hubble data, he pieced together a better picture of life in this double-star system, especially how the beefed-up companion gained its extra mass.

By puffing away most of its mass, the aging, stripped-down star has given new life and a new identity to its companion. The roughly 10-million-year-old companion has potentially doubled its lifetime because it has gained a vast amount of hydrogen fuel, which is needed to maintain its thermonuclear furnace. By beefing up, the companion also has changed its identity from a normal, moderately massive star to a "Be" star, a type of hot star with a broad, flattened disk of hydrogen gas swirling around it, much like the rings of Saturn. Based on measurements taken by astronomers at the U.S. Naval Observatory, the disk is eight times wider than the star.

The disk formed from gas spun off the rapidly rotating "Be" star. What causes the fast spin of "Be" stars has been a mystery to astronomers. Now the Hubble telescope observations of Phi Persei offer at least a partial explanation: The gas discarded from a nearby swelling star strikes the companion off-center, causing it to spin faster.

"The companion is now rotating so fast (one million mph or 450 kilometers per second at its equator) that the star is distorted into a flattened oblate figure in which gravity can barely maintain its hold on the star's outer layers," Gies explains.

This new information about the stripped-down star and its companion leads scientists to speculate about their past. Before the exchange of material, the stripped-down subdwarf was the more massive of the two, about six times more massive than the Sun. Its companion was slightly less bulky, about five solar masses. Such massive stars usually race through life at a faster pace than most stellar objects, ending their lives in one big supernova explosion. However, stars in binary systems live differently. When the once massive subdwarf entered its twilight years about one million years ago, it swelled in size as it began using up its hydrogen fuel. A single massive star would have eventually exploded, but the presence of the companion prevented the once- massive star from suffering such a violent fate. Instead, the once-massive star dumped most of its outer layers onto its companion, and now may be heading to a quiet demise.

A curious destiny may await this pair. As long as the "Be" star doesn't break apart, it will live for another 10 million years because of the hydrogen fuel it acquired from its companion. Then it will swell during the expansion phase and possibly dump some of its mass back onto the subdwarf, which will have evolved into a white dwarf. The subdwarf then might grow in mass and eventually explode as a supernova. Or, the companion might swell up so much that it would engulf the white dwarf, eventually tossing out its material in mix-master action.

"The beefed-up 'Be' star has won a new lease on life, but its ultimate fate will be determined by the corpse of its former companion, which remains in orbit uncomfortably nearby," Gies concludes. Phi Persei is 720 light-years away in the constellation Perseus, visible in the autumn evening sky in the northern hemisphere, just north of the Andromeda Galaxy, M31. The double-star system is visible as a fourth-magnitude star.

The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) for NASA, under contract with the Goddard Space Flight Center, Greenbelt, MD. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency (ESA).

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