Discovery of Vast Tail on Dying Star Promises Clues to Solar Birth

By Marc Kaufman
Washington Post Staff Writer
Thursday, August 16, 2007

Astronomers have for the first time found a gargantuan, comet-like tail created by a slowly dying star, a discovery that gives new insights into how old stars seed the galaxies with material that ultimately becomes new stars and solar systems.

Astronomers have long known that dying stars provide the building blocks for future ones, but never before have they seen the process so vividly in action.

The 13-light-year-long tail is made up of molecules of oxygen, carbon and nitrogen shed by the slowly dying but very fast-moving star as it speeds through the Milky Way at almost 300,000 miles per hour. Though it is the first of its kind ever found, astronomers said they expect it will not be the last.

"I was shocked when I first saw this completely unexpected, humongous tail trailing behind a well-known star," said lead investigator Christopher Martin of the California Institute of Technology in Pasadena. His collaborator, Mark Seibert of the Carnegie Observatories in Pasadena, added that "this is an utterly new phenomenon to us, and we are still in the process of understanding the physics involved."

The star, named Mira, has been well-known to astronomers for centuries. Its tail was discovered using NASA's Galaxy Evolution Explorer, or Galex. The probe has scanned the entire sky for ultraviolet light since its launch in 2003; other telescopes apparently missed Mira because the tail glows ultraviolet and astronomers were looking with a different part of the light spectrum.

Astronomers studying the results noticed what looked like a comet with an enormously long tail. As they studied it further, they realized the tail was material that had blown off Mira and now stretched out a distance the equivalent of 20,000 times the average distance between Pluto and the sun.

Mira was born billions of years ago as a star similar to our sun but is now a slowly dying "red giant" -- a pulsating, puffed-up star with about the same mass as our sun but 400 times as large. Because of its great size, the gravitational force keeping it together is weak at the edges, allowing the winds created by the fast-moving star to pull matter off and send it back into the tail.

Martin said the discovery shows not only how matter is spread through the galaxy by fading stars but also points to the likely fate of our own star. The sun is expected to last another 4 billion to 5 billion years and then dissipate in a way perhaps similar to Mira.

Mira, named after the Latin word for "wonderful," has been a favorite of astronomers for about 400 years -- in part because it goes black once every 332 days. Despite the centuries of observation, nobody before saw that it was trailed by such a long and impressive wake.

"It's amazing to discover such a startlingly large and important feature of an object that has been known and studied for over 400 years," said James D. Neill of the California Institute of Technology, who participated in the NASA teleconference that announced the discovery yesterday. News of Mira's tail was published today in the journal Nature.

Mira's tail is believed to be at least 30,000 years old. The researchers said that generally when a phenomenon is discovered by astronomers, they soon find that it is not unique. These solar tails, the researchers said, probably will allow them to learn more about the stars they are formed by. Seibert, the study collaborator, said, "We hope to be able to read Mira's tail like a ticker tape to learn about the star's life."

But equally important: Astronomers will study how the shed remnants of a star collect to become other celestial objects over time -- what NASA called the Johnny Appleseed nature of the star's tail.

The scientists likened the star's movement through space to a bullet traveling at supersonic speed through air. The front end of the star creates a shock wave of sorts as it pushes forward, and that shock causes hot gas to fly back. Cool winds coming off the star mix with the hot gases to form the tail and make it glow, or fluoresce, with ultraviolet light. The process is similar to a speeding boat leaving a choppy wake or a steam train producing a trail of smoke, they said.

Galex orbits about 430 miles above Earth -- above the ozone layer. Earth-based telescopes cannot view objects in ultraviolet because ozone in the atmosphere scatters or absorbs much of it.

Caltech leads the Galex mission and is responsible for its science operations. NASA's Jet Propulsion Laboratory, also in Pasadena, built the scientific instruments.

According to NASA, Mira will eventually eject all of its remaining gas into space, leaving behind only the burned-out core of the original star. The remnant is called a "white dwarf" and can continue in that form for eons.

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