The Sciences

The Year in Science: Astronomy 1997

The Pistol Star makes our own sun lookd downright puny.

By David H FreedmanJan 1, 1998 6:00 AM


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Astronomers have long seen our sun as sort of an average Joe among stars. But lately it seems downright puny, thanks to the unveiling in October of the Pistol Star. Detected near the center of our galaxy by the Hubble Space Telescope’s new infrared camera (see story opposite), and named after the gun-shaped nebula that surrounds it, the Pistol is roughly a hundred times as massive as the sun and anywhere from 2.5 million to 15 million times as bright. If it were not veiled by dust, it would be visible to the naked eye, even though it is 25,000 light-years away. If it were placed in the center of our solar system, its surface would extend to Venus. It just may be the brightest, most massive star ever seen.

And this is the lean version of the Pistol. ucla astronomer Don Figer, who led the team that made the Hubble observations, says the Pistol probably started off with twice as much mass. But it burned half of itself off in a ferocious stellar wind and in a couple of extraordinarily violent eruptions a mere 5,000 or so years ago. Those eruptions hurled gas as far as 2 light-years away, creating the smoky nebula. The bigger a star is, the hotter it burns and the faster it runs through its fuel; Figer says the Pistol is only a couple of million years old but is already within a few million years of going supernova—that is, of blowing up. The sun, in contrast, has been around for 5 billion years and has about that long to go.

Although there is a small chance that the Pistol could turn out to be two smaller stars huddled together—Figer’s team is taking a sensitive new infrared detector to the Keck telescope in Hawaii next summer to find out—it could also turn out to be even more massive than the initial estimates. Astronomers are surprised that a star so large could ever have been born. Stars are created when clouds of gas gravitationally contract into a ball and ignite at the core from the compression; the conventional wisdom is that a star would ignite and blow off outer layers of gas long before it had a chance to build up into a Pistol-like monster. We’ll have to come up with new notions of star formation now, says Figer.

Even more intriguing is the possibility that the Pistol is a missing link between ordinary large stars and a handful of exotic, superhot but smaller objects known as Wolf-Rayet stars. Perhaps very large stars become unstable, blow off their outer layers, and end up as brilliant midgets. If so, the debris from the Pistol’s eruptions 5,000 years ago may be a snapshot of the first stages of that process.

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