A massive
red star in the constellation Orion has shrunk in the past 15 years and astronomers
don't know why.
Called
Betelgeuse, the star is considered a red supergiant. Such massive stars are
nearing the ends of their lives and can swell to 100 times their original size
before exploding
as supernovae, or possibly just collapsing
to form black holes without violent explosions (as one study suggested).
Betelgeuse,
one of the top
10 brightest stars in our sky, is a popular target among backyard
skywatchers and was the first star ever to have
its size measured, and even today is one of only a handful of stars that
appears through the Hubble Space Telescope as a disk rather than a point of
light. It was the first star (besides our sun) to have its surface
photographed (by Hubble).
The new
finding, presented today at a meeting of the American Astronomical Society
in Pasadena, Calif., was based on data collected by UC Berkeley's Infrared Spatial
Interferometer (ISI) on the top of Mt. Wilson in Southern California.
In 1993,
measurements put Betelgeuse's radius at about 5.5 astronomical units (AU),
where one AU equals the average Earth-sun distance of 93 million miles, or
about 150 million km. Since then it has shrunk in size by 15 percent. That
means the star's radius has contracted by a distance equal to the orbit of
Venus.
"To
see this change is very striking," said Charles Townes, a UC Berkeley
professor emeritus of physics. "We will be watching it carefully over the
next few years to see if it will keep contracting or will go back up in size."
(Townes won the 1964 Nobel Prize in physics for inventing the laser and the
maser, a microwave laser.)
Though the
star is shrinking, its visible brightness has not dimmed significantly over the
past 15 years, the researchers say.
"But
we do not know why the star is shrinking," said Edward Wishnow, a research
physicist at UC Berkeley's Space Sciences Laboratory. "Considering all
that we know about galaxies and the distant universe, there are still lots of
things we don't know about stars, including what happens as red giants near the
ends of their lives."
Townes, who
turns 94 in July, plans to continue monitoring Betelgeuse in hopes of finding a
pattern in the changing diameter, and to improve the ISI's capabilities by
adding a spectrometer to the interferometer.
"Whenever
you look at things with more precision, you are going to find some surprises
and uncover very fundamental and important new things," he said.
The finding
was also published June 1 in the Astrophysical Journal Letters.
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