An ultraviolet view of Mira, showing its long, faint tail.
Photo: NASA/JPL-Caltech
The Galaxy Evolution Explorer (GALEX), a space-based observatory surveying distant galaxies in ultraviolet light, has made an important discovery in our own Milky Way galaxy. , a red giant star about 400 light years away from Earth in the constellation , has a tail of gas and dust that stretches 13 light years through space (this means that light emitted by Mira now won't reach the other end of its tail until the year 2020). A tail like this has never been discovered around a star before.
Thousands of millions of years ago, Mira was a star like our own Sun, a main sequence star. When main sequence stars grow old, they expand to enormous size and cool. Mira has reached this stage, becoming a red giant which is gradually blowing its outer layers of gas away into space. Over time, it will become a planetary nebula and eventually die as a white dwarf - the hot, glowing remnant of a dead star's core.
Mira is unusual, among stars in the Milky Way, in that it is moving very quickly with respect to the stars around it. It moves through space at 130 km/s, or 291,000 miles/hour. It is also a binary star – it has a white dwarf companion star (Mira B) and the pair of stars orbit slowly around each other as they move through space.
Mira's tail is made of gas and dust which has been ejected by the star over the last 30,000 years, forming a wake behind it as it moves through the interstellar gas and dust of the Milky Way. Studies of the chemical composition of the tail, at different distances from the star, will offer a fascinating opportunity to learn about the processes of stellar evolution and mass-loss in red giant stars.