Like puffy smoke from a steam locomotive in an old Western, Mira’s tail consists of gas and dust puffed out by the star during its speedy trek through the interstellar gas of the Milky Way. The tail was discovered only now because it emits only far-ultraviolet light. As reported in the August 16th issue of Nature, D. Christopher Martin (Caltech) and his colleagues spotted signs of Mira's tail during routine inspection of images from NASA's Galaxy Evolution Explorer (GALEX) satellite, which is on a mission to map the far-ultraviolet sky. “One of our team members noted a bit of fluff around Mira,” says Martin, “and we decided to take deeper images.”
Mira is an old, pulsating red giant on the verge of becoming a planetary nebula. The star has about 1.5 times the mass of the Sun and is orbited by a much fainter white-dwarf companion (orbital period about 400 years). Like all long-period red variables, Mira is shedding a lot of material into space: about one Earth mass every eight years.
Mira is plunging through our part of the Milky Way with an unusually large space velocity, 130 kilometers per second. This accounts for the length of the tail that it has left behind in the interstellar medium. In addition, some the blown-off gas piles up in a bow shock in front of the star, as is dimly visible in the GALEX images.
According to the team, the far-ultraviolet glow is probably caused by fluorescence of hydrogen molecules hit by energetic electrons.At Mira's distance of 350 light-years, the 2° length of the tail translates into a physical length of 13 light-years the distance the star covers in about 30,000 years. Thus, the tail serves as a fossil record of the star's mass loss over the past 300 centuries. Says Martin, “This will tell us how other stars like our Sun [in the far future] age over time.”
The tail is wider and brighter at positions that correspond to roughly 10,000, 20,000, and 30,000 years ago, suggesting a long-term periodicity in Mira's stellar wind. “It all fits together very nicely,” says astrophysicist Lex Kaper (University of Amsterdam). Theoretical models predict long-term “thermal pulses” for Mira-like stars, Kaper notes; these are probably related to the shell-like structures seen in the outer regions of some planetary nebulae.
However, Martin and his colleagues note in their paper that the thermal-pulse period for lightweight old red giants like Mira should be much longer than 10,000 years. So, large-scale turbulence or density variations in the interstellar medium may be what shaped the tail instead.