Long-haul flight: Mira appears as a small white dot in this bulb-shaped structure, and is moving from left to right in this view. The shed material can be seen in light blue. The star's comet-like tail stretches a startling 13 light-years across the sky. For comparison, the nearest star to our sun, Proxima Centauri, is only four light-years away.

Credit: NASA

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SYDNEY: Astronomers have spotted a vast comet-like tail trailing billions of miles through space behind a red giant star.

NASA's Galaxy Evolution Explorer observatory has imaged the tail emanating from a star called Mira – a fast-moving, and aged giant that is shedding massive amounts of surface material as it moves through space at supersonic speeds.

The U.S. space agency telescope discovered the tail on the already well-known star during an ongoing ultraviolet light survey of the entire night sky. Project scientists noticed what looked like a comet with a gargantuan tail.

Further examination revealed that material blowing off Mira is forming a wake 13 light-years long, or about 20,000 times the average distance between Pluto and the Sun. Nothing like this has ever been seen before.

Unique opportunity

"I was shocked when I first saw this completely unexpected, humongous tail trailing behind a well-known star," said Christopher Martin project leader for the Galaxy mission at the California Institute of Technology in Pasadena, USA. "It was amazing how Mira's tail echoed on vast, interstellar scales the familiar phenomena of a jet's contrail or a speedboat's turbulent wake."

Martin is also lead author of a paper detailing the discovery in today's edition of the British journal Nature.

The experts believe that Mira's tail offers a unique opportunity to study how stars like our Sun die and ultimately seed new solar systems. As Mira hurtles along, its tail sheds carbon, oxygen and other important elements needed to form new stars, planets and possibly even life. This tail material, visible for the first time, has been released
during the past 30,000 years.

"This is an utterly new phenomenon to us, and we are still in the process of understanding the physics involved," said co-author Mark Seibert of the Observatories of the Carnegie Institution, also in Pasadena. "We hope to be able to read Mira's tail like a ticker tape to learn about the star's life."

Billions of years ago, Mira (pronounced 'my-rah', Latin for "wonderful") is thought to have been similar to our Sun. Over time, it began to swell into what is called a variable red giant - a pulsating, puffed-up star that periodically grows bright enough to see with the naked eye. Mira eventually will eject all its remaining gas into space, forming a colourful shell called a planetary nebula, said the experts. This will fade with time, leaving only the burnt-out core of the original star, a white dwarf.

Choppy wake

Compared to other red giants, Mira is travelling unusually fast, at 468,000 kph, possibly due to gravitational boosts from other passing stars. Racing alongside Mira is a small, distant companion star thought to already be a white dwarf. The binary pair, also known as Mira A and Mira B, orbit slowly around each other as they travel together in the constellation Cetus, 350 light-years from Earth.

In addition to Mira's tail, the Galaxy Evolution Explorer also discovered a bow shock – a buildup of hot gas – in front of the star, and two sinuous streams of material emanating from the star's front and back. Astronomers think hot gas in the bow shock is heating the gas blowing off the star, causing it to fluoresce with ultraviolet light. This glowing material then swirls around behind the star, creating a turbulent, tail-like wake. The process is similar to a speeding boat leaving a choppy wake or a steam train producing a trail of smoke.

Mira's tail only glows with ultraviolet light, which might explain why other telescopes have missed it before now. The Galaxy Evolution Explorer is very sensitive to ultraviolet light and also has an extremely wide field of view, allowing it to scan the sky for unusual ultraviolet activity.

"It is amazing to discover such a startlingly large and important feature of an object that has been known and studied for more than 400 years," said astronomer James D. Neill with the project team at the California Institute of Technology.