Hypergiant Star Erupts
Seattle, WA — An international team of astronomers, led by
astrophysicist Alex Lobel of the Harvard-Smithsonian Center
for Astrophysics, announced today at the 201st meeting of
the American Astronomical Society in Seattle that they have
observed a huge eruption by a hypergiant star that blasted
nearly 10,000 times the mass of the Earth into space.
During this spectacular event in the year 2000, the star
Rho Cassiopeiae (in the constellation of Cassiopeia) lost
more mass than in any other stellar eruption observed by
During the outburst the star brightened briefly, then dimmed
for a period of months. The initial brightening occurs when
gases fall in towards the star and are compressed and
heated. Then, that material blasts outward and the star
dims. It is a dramatic process, comparable to jumping on
a trampoline, where the atmosphere drops downward only to
shoot upward again.
The team advises that another eruption could take place at
any time. Continued observations show that the colossal
atmosphere of Rho Cas has remained unsettled in the
aftermath of the outburst two years ago. Since Rho Cas is
bright and visible all year to astronomers in the northern
hemisphere, it can be seen easily even without a telescope.
It is a promising target for amateur observers, who can
monitor it for the next explosion and report their
findings to the Central Bureau for Astronomical Telegrams.
A Massive Blast
Rho Cas is a member of a rare breed of stars called
hypergiants, of which only about a dozen examples are
known in the Milky Way Galaxy. Larger and more massive
than the Sun, they pump enormous amounts of light into
space. Rho Cas is visible to the unaided eye despite its
great distance of some 10,000 light-years, because it is
over 500,000 times more luminous than our Sun.
Rho Cas interests astronomers not only for its size and
luminosity, but also for its behavior. In 1946,
astronomers were astonished to see the star dim by a
factor of six from fourth to sixth magnitude, and alter
its spectral type from F-class to M-class, indicating a
drop in surface temperature from 12,000 degrees Fahrenheit
(7,000 Kelvin) to 5,000 degrees F (3,000 K). Astronomers
recognized that they had seen an explosion on the star’s
surface, but could not determine the size of the explosion.
The international team caught Rho Cas erupting in the year
2000 and calculated the amount of mass lost by carefully
studying the star’s light. They split that light into a
spectrum of colors, similar to the way that sunlight
passing through a prism creates a rainbow. The scientists
then created a computer model that matched the observed
spectra. The model showed that the star had explosively
ejected enough material to equal 10,000 Earths.
Dark bands appeared in the optical spectrum of Rho Cas,
showing that molecules not normally present were forming
in the star’s atmosphere. “These peculiar spectral bands
were also temporarily observed when Rho Cas dimmed
suddenly in 1946. We know now that the star produced
these molecules because its outer atmosphere cooled
by more than 3,000 degrees during the outburst. A
tremendous amount of gas was ejected — lifted into space
by a blast wave, similar to the loud shock wave or ‘sonic
boom’ you hear when a supersonic aircraft flies past,”
said Lobel. “The shock wave, of course, was much
stronger and wrapped around the entire star, blasting
huge quantities of gas into space.”
Because Rho Cas is bright and required constant monitoring,
the team observed it using five modest-sized telescopes
in the U.S. (at Oak Ridge Observatory, Mass., and Ritter
Observatory, Ohio) and Europe (the William Herschel
Telescope and Nordic Optical Telescope on the Canary
Islands, and the Special Astrophysical Observatory in
Russia). For nearly a decade, they regularly studied
Rho Cas to watch it for any changes.
“We were exceptionally lucky to witness a stunning
explosion in Rho Cas and to have acquired the spectra to
make a quantitative analysis possible,” said Smithsonian
astrophysicist Andrea Dupree, a member of the team and
second author on the paper reporting their results.
The Riddle of Ultraluminous Stars
By monitoring Rho Cas, astronomers are trying to solve one
of the most enduring riddles of stellar physics: Why are
there no stars that are more luminous than about a million
Suns? Based on the observations of Rho Cas, their
calculations show that a shock wave blasting from such a
star can only travel so fast and far before losing its
punch because the star’s atmosphere produces huge amounts
of hydrogen atoms. When these hydrogen atoms are formed,
a small amount of energy is released that helps to push
the shock wave outwards. The inflating atmosphere cools
even more and yields an avalanche of gas expelled by
the star. This explosion on Rho Cas demonstrates how
unstable these hypergiant stars are. Lobel conjectures
that such recurrent mass ejections prevent them from
sustaining such large luminosities.
“This process makes the atmosphere of stars with a surface
temperature like our Sun — but pouring out about a
million times more radiation — very unstable. More
luminous stars don’t make it for long. We don’t get a
chance to find them because they perished in their
galactic cradles long ago, or because they no longer
shine as brightly as they once did,” said Lobel.
Astronomers predict that Rho Cas will perish in a
supernova explosion as it rapidly uses up the nuclear
fuel at its core. When that final catastrophic blow
arrives, it will rip the entire star apart in one last
blaze of glory.
A paper on the team’s research will be published in the
February 1, 2003 issue of The Astrophysical Journal.
This research was supported by NASA and the Smithsonian
Astrophysical Observatory. The science team mentioned in
this press release consists of Drs. Alex Lobel, Andrea
Dupree, Robert Stefanik, and Guillermo Torres (Harvard-
Smithsonian Center for Astrophysics, Mass.), Garik
Israelian (Instituto de Astrofisica de Canarias, Spain),
Nancy Morrison (University of Toledo, Ohio), Cornelis
de Jager and Hans Nieuwenhuijzen (Space Research
Organization of the Netherlands, The Netherlands), Ilya
Ilyin (University of Oulu, Finland), and Faig Musaev
(Special Astrophysical Observatory, Russia).
Headquartered in Cambridge, Massachusetts, the Harvard-
Smithsonian Center for Astrophysics (CfA) is a joint
collaboration between the Smithsonian Astrophysical
Observatory and the Harvard College Observatory. CfA
scientists organized into seven research divisions
study the origin, evolution, and ultimate fate of the
universe.
An image to accompany this release can be found at
http://cfa-www.harvard.edu/press/pr0302_image.html
