In celebration of the 31st anniversary of the launching of NASA's Hubble
Space Telescope, astronomers aimed the renowned observatory at a brilliant
"celebrity star," one of the brightest stars seen in our galaxy, surrounded
by a glowing halo of gas and dust.
The price for the monster star's opulence is "living on the edge." The star,
called AG Carinae, is waging a tug-of-war between gravity and radiation to
avoid self-destruction.
The expanding shell of gas and dust that surrounds the star is about five
light-years wide, which equals the distance from here to the nearest star
beyond the Sun, Proxima Centauri.
The huge structure was created from one or more giant eruptions about 10,000
years ago. The star's outer layers were blown into space - like a boiling
teapot popping off its lid. The expelled material amounts to roughly 10
times our Sun's mass.
These outbursts are the typical life of a rare breed of star called a
luminous blue variable, a brief convulsive phase in the short life of an
ultra-bright, glamorous star that lives fast and dies young. These stars are
among the most massive and brightest stars known. They live for only a few
million years, compared to the roughly 10-billion-year lifetime of our Sun.
AG Carinae is a few million years old and resides 20,000 light-years away
inside our Milky Way galaxy.
Luminous blue variables exhibit a dual personality: They appear to spend
years in quiescent bliss and then they erupt in a petulant outburst. These
behemoths are stars in the extreme, far different from normal stars like our
Sun. In fact, AG Carinae is estimated to be up to 70 times more massive than
our Sun and shines with the blinding brilliance of one million suns.
"I like studying these kinds of stars because I am fascinated by their
instability. They are doing something weird," said Kerstin Weis, a luminous
blue variable expert at Ruhr University in Bochum, Germany.
Major outbursts such as the one that produced the nebula occur once or twice
during a luminous blue variable's lifetime. A luminous blue variable star
only casts off material when it is in danger of self-destruction as a
supernova. Because of their massive forms and super-hot temperatures,
luminous blue variable stars like AG Carinae are in a constant battle to
maintain stability.
It's an arm-wrestling contest between radiation pressure from within the
star pushing outward and gravity pressing inward. This cosmic match results
in the star expanding and contracting. The outward pressure occasionally
wins the battle, and the star expands to such an immense size that it blows
off its outer layers, like a volcano erupting. But this outburst only
happens when the star is on the verge of coming apart. After the star ejects
the material, it contracts to its normal size, settles back down, and
becomes quiescent for a while.
Like many other luminous blue variables, AG Carinae remains unstable. It has
experienced lesser outbursts that have not been as powerful as the one that
created the present nebula.
Although AG Carinae is quiescent now, as a super-hot star it continues
pouring out searing radiation and powerful stellar wind (streams of charged
particles). This outflow continues shaping the ancient nebula, sculpting
intricate structures as outflowing gas slams into the slower-moving outer
nebula. The wind is traveling at up to 670,000 miles per hour (one million
km/hr), about 10 times faster than the expanding nebula. Over time, the hot
wind catches up with the cooler expelled material, plows into it, and pushes
it farther away from the star. This "snowplow" effect has cleared a cavity
around the star.
The red material is glowing hydrogen gas laced with nitrogen gas. The
diffuse red material at upper left pinpoints where the wind has broken
through a tenuous region of material and swept it into space.
The most prominent features, highlighted in blue, are filamentary structures
shaped like tadpoles and lopsided bubbles. These structures are dust clumps
illuminated by the star's reflected light. The tadpole-shaped features, most
prominent at left and bottom, are denser dust clumps that have been sculpted
by the stellar wind. Hubble's sharp vision reveals these delicate-looking
structures in great detail.
The image was taken in visible and ultraviolet light. Ultraviolet light
offers a slightly clearer view of the filamentary dust structures that
extend all the way down toward the star. Hubble is ideally suited for
ultraviolet-light observations because this wavelength range can only be
viewed from space.
Massive stars, like AG Carinae, are important to astronomers because of
their far-reaching effects on their environment. The largest program in
Hubble's history - the Ultraviolet Legacy Library of Young Stars as
Essential Standards - is studying the ultraviolet light of young stars and
the way they shape their surroundings.
Luminous blue variable stars are rare: Less than 50 are known among the
galaxies in our local group of neighboring galaxies. These stars spend tens
of thousands of years in this phase, a blink of an eye in cosmic time. Many
are expected to end their lives in titanic supernova blasts, which enrich
the universe with heavier elements beyond iron.
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Space & Astrophysics