August 15, 2007
M aterial blowing off Mira is forming a wake 13 light-years long, or thousands of times the length of our solar system. The space-based Galaxy Evolution Explorer (GALEX) was scanning the star during its ongoing sky survey in ultraviolet light when astronomers noticed what looked like a comet with a gargantuan tail.
Mira, Latin for "wonderful," has been a favorite of astronomers for about 400 years. It is a fast-moving red giant, which sheds massive amounts of surface material, but nothing like this has ever been seen before around a star.
"This is an utterly new phenomenon to us, and we are still in the process of understanding the physics involved," said Mark Seibert of Carnegie Observatories in Pasadena, California. "We hope to be able to read Mira's tail like a ticker tape to learn about the star's life."
Astronomers say Mira's tail offers a unique opportunity to study how stars like our sun die and ultimately seed new solar systems. As Mira hurls along, its tail drops off carbon, oxygen and other important elements needed for new stars, planets, and possibly even life to form. This tail material, visible now for the first time, has been shed over the past 30,000 years.
Billions of years ago, Mira was like our Sun. Over time, it began to swell into a variable red giant — a pulsating, puffed-up star that periodically grows bright enough to see with the naked eye. Mira will eventually eject all of its remaining gas into space, forming a colorful shell, or a planetary nebula. The nebula will fade with time, leaving only the burnt-out core of the original star (a white dwarf).
Compared to other red giants, Mira is traveling unusually fast, possibly due to gravitational boosts from other passing stars. It now plows along at 291,000 mph (130 kilometers per second). Racing along with Mira is a small, distant companion thought to be a white dwarf. The pair, also known as Mira A (the red giant) and Mira B, orbit slowly around each other as they travel together in the constellation Cetus, 350 light-years from Earth.
In addition to Mira's tail, GALEX also discovered a bow shock, a buildup of hot gas, in front of the star, and two sinuous streams of material coming out of the star's front and back. Astronomers think hot gas in the bow shock is heating up the gas blowing off the star, causing it to fluoresce with ultraviolet light. This glowing material then swirls around behind the star, creating a turbulent, tail-like wake. The process is similar to a speeding boat leaving a choppy wake, or a steam train producing a trail of smoke.
"GALEX is so exquisitely sensitive to ultraviolet light and it has such a wide field of view that it is uniquely poised to scan the sky for previously-undiscovered ultraviolet activity," said Barry F. Madore, senior research astronomer at the Carnegie Observatories. The fact that Mira's tail only glows with ultraviolet light might explain why other telescopes have missed it.
"We never would have predicted a turbulent wake behind a star that glows only with ultraviolet light," said Seibert. "Survey missions like the Galaxy Evolution Explorer can provide many surprises."