Odd Star Sheds Cometlike Tail

John Roach
for National Geographic News
August 15, 2007
The star Mira sheds a cometlike tail of rich material as it streaks through space—something that has never been seen before—astronomers announced today.

Acting sort of like a cosmic Johnny Appleseed, the star is leaving behind carbon, nitrogen, oxygen, and other important "seed" elements needed for new stars, planets, and potential life to form.

The phenomenon is "completely new and unexpected," Chris Martin, an astronomer at the California Institute of Technology in Pasadena, told reporters today in a telephone briefing.

"We believe that the tail is made up of material that is being shed by the star, which is heating up and then spiraling back into this turbulent wake," he said.

The star's tail is 13 light-years long—three times the distance from our sun to its nearest star neighbor.

Martin and colleagues describe the star's cometlike tail in tomorrow's issue of the journal Nature.

Interstellar Wake

Scientists have studied Mira, which lies about 420 light-years away in the constellation Cetus, for more than 400 years.

Mira is one of the first discovered "variable" red giant stars—that is, it expands and contracts over a 332-day period, at times growing bright enough to be seen with the naked eye. The star will next be visible in mid-November.

The tail, however, is invisible to the naked eye and was discovered using NASA's Galaxy Evolution Explorer space telescope, which observes in ultraviolet light. (See a photo gallery of space telescope snapshots.)

Billions of years ago, scientists say, Mira was like the sun. Over time it swelled into a variable red giant and today sheds massive amounts of surface material.

This material forms a tail because Mira—like all stars—is moving through space.

In fact, Mira is speeding through the galaxy at 291,000 miles (468,000 kilometers) an hour—an unusually fast clip that may be the result of gravitational boosts from other passing stars.

Mira's supersonic speed causes a type of shock wave known as a bow shock to form in front of the star, study co-author Mark Seibert, from the Observatories of the Carnegie Institution of Washington in Pasadena, explained at the briefing.

"It's compressing the interstellar medium like a boat moving through the water ... and the gas in this bow shock is actually very hot," he said.

Cool wind blowing off the star mixes with this hot bow-shock gas, and the mixture flows around and behind Mira, creating the long wake, he added.

Eventually, Mira will eject all of its remaining gas into space, forming a shell called a planetary nebula.

The nebula will fade with time, leaving only the burnt-out core of the original star, an extremely dense object called a white dwarf. (Related: "Solar System's Fate Predicted by Nearby White Dwarf?" [December 21, 2006].)

Common Process?

The material in Mira's tail was shed over the past 30,000 years. Studying the tail will allow scientists to understand how stars like the sun die and seed new solar systems in the process, Caltech's Martin said.

Mira is a common type of star, Carnegie's Seibert added, and even though the cometlike tail phenomenon has never been seen before, the behavior is likely widespread.

Michael Shara is a curator at the American Museum of Natural History in New York and a professor of astronomy at Columbia University.

He was not involved in the study but participated in the briefing to offer perspective on the discovery.

"It's giving us this fantastic insight, I think, into the death processes of stars and their renewals—their phoenix-like revivals as their ashes get cycled back into the next generation of stars," Shara said.

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