Bizarre comet-like tail found behind star

时间:2019-02-27 01:10:01166网络整理admin

By David Shiga (Image: C. Martin et al./GALEX) (Image: C. Martin et al./GALEX) A glowing, comet-like tail has been discovered trailing behind a double star called Mira, a phenomenon never seen before. It may contain clues about the star’s activity over the past 30,000 years. Mira, which means “wonderful” in Latin, is one of the best-studied star systems in the sky and lies 350 light years from Earth. 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. 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 (see Dying star’s wind creates planetary nursery). Now astronomers led by Christopher Martin of Caltech in Pasadena, US, have discovered the long tail, which is visible only in far ultraviolet light. They happened upon it using NASA’s Galaxy Evolution Explorer (GALEX) satellite, which was surveying the sky at ultraviolet wavelengths. The tail extends 13 light years from Mira – if it were visible in the sky, it would span the width of four full Moons. It 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. Martin’s team believes the tail is created as a result of Mira A’s stellar wind – an outflow of gas and dust from the star – hitting ambient gas as it moves through space. Fast-moving electrons generated by the collision then strike hydrogen molecules in surrounding gas, producing ultraviolet light. This creates a glowing trail behind Mira as it travels through the galaxy at 130 kilometres per second. Watch an animation of Mira creating the tail (QuickTime, 6 MB). “This is an utterly new phenomenon to us, and we are still in the process of understanding the physics involved,” says team member Mark Seibert of the Carnegie Observatories in Pasadena, California, US. “We never would have predicted a turbulent wake behind a star that glows only with ultraviolet light.” John Monnier of the University of Michigan, who is not a member of the team, says the tail could shed light on why some stars turn into white dwarfs and others explode as supernovae. 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. Martin’s team says the trail could provide a “fossil history” of how Mira shed mass over thousands of years. Monnier agrees: “This is an unexpected way to keep track of this material that Mira gave off. It’s pretty cool.” Journal reference: Nature (DOI: