GALEX — the Galaxy Evolution Explorer — was an interesting mission to begin with, a space-based observatory conducting an all-sky survey of distant galaxies at ultraviolet wavelengths. Now it’s come up with a real newsmaker, a star moving at an unusually fast 130 kilometers a second and sporting a comet-like tail. The material blowing off the red giant Mira is, in fact, forming a wake some thirteen light years long. No such phenomenon has ever been seen around a star before.
Image: Mira appears as a small white dot in the bulb-shaped structure at right, and is moving from left to right in this view. The shed material can be seen in light blue. The dots in the picture are stars and distant galaxies. The large blue dot at left is a star that is closer to us than Mira. Credit: NASA/JPL-Caltech.
From what GALEX is telling us, the elements Mira is leaving behind, including carbon, oxygen and other building blocks for future star and planet formation, have been shed over a period of approximately 30,000 years. Although similar to our own Sun billions of years ago, the star has now swollen to variable red giant status, periodically growing bright enough to become visible to the naked eye. And as will happen to the Sun, its distant future involves its transformation into a white dwarf.
Nor does Mira travel alone. Mira B is itself a white dwarf [but see comments below] that orbits Mira A as the duo move through the constellation Cetus, some 350 light years from Earth. Interestingly, a bow shock has formed in which hot gases build up in front of the onrushing star, and astronomers have noted two streams of material that emerge from the star itself. Evidently the hot gas in the bow shock heats up the gas blowing off the star, causing it to fluoresce with ultraviolet light as it forms the wake.
Researchers admit to a sense of surprise. Here’s Mark Seibert (Carnegie Observatories, Pasadena), a co-author of the paper on this work:
“This is an utterly new phenomenon to us, and we are still in the process of understanding the physics involved. We hope to be able to read Mira’s tail like a ticker tape to learn about the star’s life.”
And from the discovery paper, this comment about putting the Mira findings to work:
The discovery of a two-degree-long wind wake emitting only in the far ultraviolet provides an unprecedented fossil record of post-main-sequence stellar evolution and mass loss, a laboratory for the study of astrophysical turbulence and the complex physics of a multiphase hydrodynamical flow, and suggests a new cooling process for hot gas that entrains a cool molecular phase. After 400 years of study, Mira continues to astound.
Indeed. A 30,000 year passage through the cosmos is now on display, discovered through a mission conceived to study much different things. If surprise is in the air, it’s understandable. A 13-light year long tail is not exactly standard issue, and who would have predicted that a star as well studied as Mira would turn out to have a wake that glowed only with ultraviolet light? How energizing it is to reflect that the pace of discovery is only accelerating, capable of blindsiding us at almost every turn.
The paper is Martin et al., “A turbulent wake as a tracer of 30,000 years of Mira’s mass loss history,” Nature Vol 448 (16 August 2007), pp. 780-783.