Credit: NASA/JPL-Caltech/POSS-II/DSS/C. Martin (Caltech)/M. Seibert(OCIW)

NASA's Galaxy Evolution Explorer (GALEX) spacecraft recently spotted a fantastically long comet-like tail emanating from a giant star. The star, known as Mira (Latin for "wonderful"), belongs to a category of old stars, termed red giants. These stars blow massive amounts of their material into space. Mira is dashing through space at a supersonic speed. It has intrigued astronomers for about 400 years.

GALEX observed Mira during a current survey of the entire sky in ultraviolet. Astronomers then noticed what resembled a comet with a gigantic tail. In fact, material hurling off Mira is forming a tail 13 light-years long, or approximately 900,000 times the average distance of Earth from the Sun (approximately 130 × 1012 km)! No phenomenon such as this has ever been recorded before in the realm of stars.

"I was shocked when I first saw this completely unexpected, humongous tail trailing behind a well-known star," said Christopher Martin of the California Institute of Technology (Caltech). "It was amazing how Mira's tail echoed on vast, interstellar scales the familiar phenomena of a jet's contrail or a speedboat's turbulent wake." Martin is the Principal Investigator for GALEX, and lead author of a recent article about the discovery.

Astronomers concluded that Mira's tail offers an unprecedented opportunity to study how stars like our Sun, seed space with important chemical elements. As Mira rushes along, its tail sheds carbon, oxygen and other elements, essential for the formation of new stars, planets and possibly even extraterrestrial life. The tail material has been ejected over the past 30,000 years.

Billions of years ago, Mira was a Sun-like star. Over time, it reached the end-throes of its long evolution and swelled into a luminous red giant star whose brightness is variable. This variation in brightness is believed to be due to the periodic expansion and contraction of the star, of a period of about 330 days. Mira is therefore classified as a pulsating star.

Mira will eventually eject all of its remaining gas into space, forming a colorful shell known as a planetary nebula. The nebula will expand and dissolve with time, and only the burnt-out core of the original star will remain. The exposed, fading cores of red giant stars are termed white dwarfs.

Compared to other red giants, Mira's velocity is unusually large, probably due to gravitational interactions with other stars. Its speed is about 130 kilometers per second, or 470,000 km/hr.

Mira is gravitationally bound to a small, distant companion, believed to be a white dwarf. The two stars are also known as Mira A (the larger star) and Mira B (the white dwarf). They are located at a distance of approximately of 350 light years from Earth, and orbit slowly around each other. They are visible among the stars of the constellation Cetus, the legendary Sea Monster.

The fact that Mira's tail only glows in ultraviolet might explain why optical telescopes have not detected it. GALEX is very sensitive to ultraviolet emission and also has an extremely wide field of view, allowing it to scan the sky for various ultraviolet phenomena.

 
Further Reading
GALEX
http://www.galex.caltech.edu/

Aymen Mohamed Ibrahem
Senior Astronomy Specialist