Shooting Bullet Star Leaves Vast Ultraviolet Wake

Fluctuating star discovered four centuries ago still holds surprises for astronomers
Mira wake

Like witnessing a bullet's supersonic trail through the air, astronomers have discovered a vast ultraviolet contrail streaming from the red giant variable star Mira A, about 420 light-years away. Spanning a full two degrees of sky as viewed from Earth, the muddied wake may consist of cold hydrogen and heavier elements that have sloughed off from the star over the past 30,000 years and interacted with interstellar dust in the object's path.

The wake stretches 13 light-years, or about three times the distance between the sun and the next closest star.

Researchers say the finding provides an unprecedented record of the twilight years of stars like our own, including the kind of stellar breeze from spent stars that was the source of, among other things, the carbon in our bodies and the oxygen we inhale.

"If Neanderthal man had had ultraviolet eyes and could look above the atmosphere, he could have seen the beginning of this tail forming," says astronomer and team leader Christopher Martin of the California Institute of Technology in Pasadena.

Seventeenth-century astronomers marveled at the star Mira A, or Omicron Ceti, for its dramatic changes in brightness every 332 days. (Mira is Latin for "wonderful.") Researchers now believe this variation occurs because the star has burned all the hydrogen-helium nuclear fuel in its core into the heavier carbon and oxygen and puffed up into a cooler red giant, heated by unsteady nuclear fusion reactions in its remaining fuel shell. Such burned-out stars are thought to seed still-forming stars and planets with medium-weight elements such as carbon, nitrogen and oxygen that blow off of the spent stars' envelopes.

Martin and his colleagues noticed Mira's tail in images captured by NASA's Galaxy Evolution Explorer (GALEX) ultraviolet satellite, launched in 2003. The imagery revealed a bow shock—like the wave that piles beneath the prow of a moving ship—in front of the star and its smaller companion, Mira B, as well as a wake broken into turbulent knots or loops, according to a report in Nature.

The researchers suggest that because the star is moving at a relatively quick pace of 130 kilometers per second [see note below] relative to the galaxy, it has compressed and heated the interstellar gas in front of it. This hot gas excited the cool, five-kilometer-per-second breeze of hydrogen molecules and heavier elements emanating from Mira, causing it to glow in the ultraviolet.

The team estimates that Mira A loses one millionth of the sun's mass a year, out of its total bulk of perhaps 1.5 solar masses.

Martin says ultraviolet tails may be quite common despite going unnoticed until now. The red giant phase awaits many smaller stars, including our own sun in four billion to five billion years.

Note: The text originally stated that Mira A's velocity (130 kilometers per second) is three times that of a bullet; in fact a bullet travels at about one kilometer per second.

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