A new ultraviolet mosaic from NASA's Galaxy Evolution
Explorer shows a speeding star that is leaving an enormous
trail of "seeds" for new solar systems. The star, named
Mira (pronounced my-rah) after the latin word for
"wonderful," is shedding material that will be recycled
into new stars, planets and possibly even life as it hurls
through our galaxy.
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.
The Galaxy Evolution Explorer discovered Mira's strange
comet-like tail during part of its routine survey of the
entire sky at ultraviolet wavelengths. When astronomers
first saw the picture, they were shocked because Mira has
been studied for over 400 years yet nothing like this has
ever been documented before.
Mira's comet-like tail stretches a startling 13
light-years across the sky. For comparison, the nearest
star to our sun, Proxima Centauri, is only about 4
light-years away. Mira's tail also tells a tale of its
history à the material making it up has been slowly blown
off over time, with the oldest material at the end of the
tail having been released about 30,000 years ago.
Mira is a highly evolved, "red giant" star near the end
of its life. Technically, it is called an asymptotic giant
branch star. It is red in color and bloated; for example,
if a red giant were to replace our sun, it would engulf
everything out to the orbit of Mars. Our sun will mature
into a red giant in about 5 billion years.
Like other red giants, Mira will lose a large fraction
of its mass in the form of gas and dust. In fact, Mira
ejects the equivalent of the Earth's mass every 10
years. It has released enough material over the past
30,000 years to seed at least 3,000 Earth-sized planets or
9 Jupiter-sized ones.
While most stars travel along together around the disk
of our Milky Way, Mira is charging through it. Because
Mira is not moving with the "pack," it is moving much
faster relative to the ambient gas in our section of the
Milky Way. It is zipping along at 130 kilometers per
second, or 291,000 miles per hour, relative to this gas.
Mira's breakneck speed together with its outflow of
material are responsible for its unique glowing
tail. Images from the Galaxy Evolution Explorer show a
large build-up of gas, or bow shock, in front of the star,
similar to water piling up in front of a speeding
boat. Scientists now know that hot gas in this bow shock
mixes with the cooler, hydrogen gas being shed from Mira,
causing it to heat up as it swirls back into a turbulent
wake. As the hydrogen gas loses energy, it fluoresces with
ultraviolet light, which the Galaxy Evolution Explorer can
detect.
Mira, also known as Mira A, is not alone in its travels
through space. It has a distant companion star called Mira
B that is thought to be the burnt-out, dead core of a
star, called a white dwarf. Mira A and B circle around
each other slowly, making one orbit about every 500
years. Astronomers believe that Mira B has no effect on
Mira's tail.
Mira is also what's called a pulsating variable
star. It dims and brightens by a factor of 1,500 every 332
days, and will become bright enough to see with the naked
eye in mid-November 2007. Because it was the first
variable star with a regular period ever discovered, other
stars of this type are often referred to as "Miras."
Mira is located 350 light-years from Earth in the
constellation Cetus, otherwise known as the
whale. Coincidentally, Mira and its "whale of a tail" can
be found in the tail of the whale constellation.
This mosaic is made up of individual images taken by
the far-ultraviolet detector on the Galaxy Evolution
Explorer between November 18 and December 15, 2006.
Image credit: NASA/JPL-Caltech/C. Martin (Caltech)/M. Seibert(OCIW)
Text credit: W. Clavin (JPL)
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