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Double star Mira’s comet-like tail amazes astronomers

19 August, 2007

In a phenomenon never seen before, astronomers have discovered a glowing, comet-like tail trailing behind a double star called Mira.

It was the astronomers in the United States who found the long tail, which is visible only in far ultraviolet light, according to a report in the August 16, 2007, issue of the science magazine Nature.

The team of astronomers, led by Christopher Martin of Caltech in Pasadena, California, the United States, chanced upon the tail trailing behind the star using the US National and Aeronautics and Space Administration’s (NASA) Galaxy Evolution Explorer satellite, which was surveying the sky at ultraviolet wavelengths.

The star’s tail extends 13 light years from Mira (‘Mira’ means ‘wonderful’ in Latin). Mira, one of the best-studied star systems in the sky, lies 350 light years from the Earth.

The tail appears to trace the path of Mira’s motion across the sky over the past 30,000 years, based on its size and Mira’s speed, which has been previously measured.

One star in the pair, called Mira A, is a bloated, ageing red giant that sheds large amounts of gas and dust into space, while the other, Mira B, is a dense stellar corpse, called a white dwarf.

The team of astronomers believes that the tail was created as a result of Mira A’s stellar wind, an outflow of gas and dust from the star, which hits ambient gas as it moves through space. Fast-moving electrons generated by the collision then strike hydrogen molecules in surrounding gas, producing ultraviolet light, thus creating a glowing trail behind Mira as it travels through the galaxy at 130 kilometres per second.

Mark Seibert from the Carnegie Observatories in Pasadena, the United States, was quoted as saying: “This is an utterly new phenomenon to us, and we are still in the process of understanding the physics involved. We never would have predicted a turbulent wake behind a star that glows only with ultraviolet light.”

According to the team, the trail could provide a “fossil history” of how Mira shed mass over thousands of years.

Previous studies had shown that some of the material from Mira A’s wind has collected into a disc – which could potentially form planets – around Mira B.

As they age, the cores of some massive stars eventually become unstable, triggering runaway nuclear reactions that tear them apart in supernovae.

But, stars such as Mira A, which start out with a few times the mass of the Sun, avoid this fate by shedding most of their mass in stellar winds to become placid white dwarfs.





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