Falling star: fast-moving Mira leaves a 'wake' behind it.Christopher Martin

Astronomers have found an unexpected treat on a star first described more than 400 years ago - the streak of a 13-light-year-long tail.

The tail, the first seen of its kind, could provide clues about how celestial bodies are formed from the material spat out by such ageing stars.

The streak was spied by the NASA space telescope Galaxy Evolution Explorer (GALEX) as part of a survey of the ultraviolet spectrum of the sky that the telescope began in 2003 and is expected to complete later this year. According to Mark Seibert, a co-author on the paper in Nature¹ this week and an astronomer with the Observatories of the Carnegie Institution of Washington in Pasadena, California, other telescopes missed the special feature of the star — called Mira — because they were either looking in the wrong wavelength of light, or simply peering too closely.

"Mira has been studied in every conceivable wavelength by the Hubble Space Telescope," Seibert says. "But Hubble didn't see the tail because it only looks at a very small area of the sky, so it missed all the stuff around the star itself."

Mira, also known as Omicron Ceti, is actually a binary star system, made up of a large Mira A and a smaller Mira B. It's the standard example in astronomical texts of a class of pulsating stars in a late stage of evolution, when much of its mass is spat out in a dusty wind. In this case, Mira happens to be moving quite quickly; its stellar wind streams out behind it in a tail that can be seen in the ultraviolet range.

Seibert says they have looked at other stars of this type without spotting tails, but he suspects there are more out there. "There's an enormous number of [similar] stars in the galaxy, so it would be shocking to me if there weren't others like this," he says.

The team, led by Christopher Martin of the California Institute of Technology in Pasadena, also spotted a 'bow wake' in front of Mira, similar to that of a boat pushing through water. Such a wake has only been seen once before on a star of this type, weakly, in the infrared spectrum around a star called R Hydrae.

Tails are sometimes seen on neutron stars as they are spat out at high speed from supernovae explosions, but the phenomena are short-lived.

Seibert says that Mira's "shockingly huge" tail gives scientists valuable insight into the chemical evolution of galaxies. Stars such as Mira generate most of the midweight, but exactly how the elements are produced isn't clear. Mira's tail spreads out this process by putting the older material at the back of the tail and the newer material at the head, with the oldest part of the tail some 30,000 years old. "Now we'll be able to tell at what rate these elements are being generated," says Seibert. "It's going to take years to decipher. But this discovery gets us started."

"I think it's incredibly cool for about half a dozen different reasons," says Michael Shara, a curator of astrophysics at the American Museum of Natural History in New York. "First, it really shows the power of GALEX as a kind of discovery machine. Second, it makes Mira one of the largest coherent structures known in the Milky Way. But most importantly, the tail lays the star's material out for us in a nice, linear fashion, so we can do systematic studies of it."

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American Museum of Natural History

• References

  1. Martin, D. C. et al. Nature 448, 780-783 (2007). | Article |