Eta Carinae, a Home-Grown Mystery
|An HST optical
image of Eta Carina, showing two large bubbles of gas expanding in
opposite directions from the bright central region.|
Even as astronomers probe the farthest edges of the Universe billions of
light years away to unlock its wondrous mysteries, there are still
baffling mysteries right here in our own Milky Way Galaxy. A big,
beautiful star named Eta Carinae is one such example. At the American
Astronomical Society meeting in January, scientists presented new data
about this enigmatic star that challenges long-held beliefs.
Among the most fundamental mysteries is the life cycle of really big
stars, those that are over ten times more massive than our Sun. This
includes how such stars are created, how they live their lives (with
important consequences for how matter and energy are distributed
throughout the galaxies), and how they die in spectacular supernova
explosions. Such explosions, equal in power to that of an entire galaxy,
forge the elements that make up Earth and everything on it, including us.
Eta Carinae is perhaps the most famous of the really big stars. It first
gained widespread attention about 160 years ago when it flared
spectacularly from obscurity to become one of the brightest in the sky.
Found in the Southern Hemisphere skies in the constellation Carina about
7,500 light years from Earth, Eta Carinae is one of the most massive and
intrinsically brightest stars known in the Milky Way Galaxy. It has a
mass of at least 100 Suns, a figure that challenges theories concerning
the upper limit of stellar mass.
|A Chandra X-ray
image of Eta Car showing a horseshoe-shaped ring surrounding the hot
As a result of continuous monitoring of its X-ray emissions since 1996,
as well as spottier observations back to the earlier 1990s, some
astronomers think "Eta-Car" may in fact be two stars. These observations have
revealed a distinct five-and-a-half year cycle in Eta-Car's X-ray
emissions. "If Eta Carinae is in fact two stars, the X-ray output should
vary with the orbital period of a smaller star revolving around a larger
star in each cycle, and this is in fact what we see," said Michael F.
Corcoran, a researcher with USRA working at NASA's Goddard Space Flight Center, at the AAS
Corcoran and his team have been carefully tracking the X-ray emissions
from Eta-Car using the data gathered continuously by the Rossi X-ray Timing Explorer satellite since 1996, as well as earlier with the
Roentgen Satellite (ROSAT). They now have seen Eta-Car repeating its
cycle, bolstering their initial interpretation.
As explained by Corcoran, the two-star theory proposes that a smaller or
"companion" star with a mass of about 30 Suns swings around on a highly
elliptical orbit every 5.52 years around a
central or "primary" star. The primary star has a mass of at least 100
Suns. As the companion star swoops in, the streams of charged particles
blown off from each star begin colliding, producing highly energetic
radiation in the form of X-rays.
The X-ray emission intensity rises as the two stars get closer because
the clouds of charged particles flowing out from each star gets denser. The
emissions abruptly drops to zero as the companion star swings -- from our
perspective on Earth -- behind the primary star in such a way that its
stellar winds are pointing away from us. The emissions begin again as
the stellar winds of the companion star begin facing us again.
Surprisingly, though, Corcoran and his colleagues have also found that
Eta Carinae is emitting more X rays during this particular cycle than
last time, even as it emits less of other types of electromagnetic
radiation. "This may be due to differing amounts of stellar material
being blown off of each star during the 5.52-year cycle," Corcoran said.
Not everyone agrees with the two-star idea. Scientists cite the
complicated visible and ultraviolet emissions from Eta-Car, which are not
explained as well with a two-star model as with the "single star" model
periodically belching out big emissions of gas. Such emissions could
result from the combined effects of rotation, gravity, and pressure in an
unusually large star.
With the next X-ray peak expected in early summer, a large observation
campaign is planned that will use an array of ground-based and satellite
instruments, including the Hubble Space Telescope and Chandra X-ray
Observatory. The multiwavelength data it provides could answer
fundamental questions about what the upper mass limit on giant stars is.
These answers, in turn, will help astronomers determine the life cycle of
stars like Eta-Car that, during their lives, shape the Galaxy and,
through their deaths, seed the galaxy with the elements materials from
which ourselves and our world come.