The Caldwell Objects

92
Eta Carinae produces the brightest
known stellar wind, which
might also explain the star's
unique X-ray-emitting sheet of
scorching hot gas (60 million
degrees Kelvin). The X-ray emission
probably arises as stellar
winds flowing away from each
member in the binary pair collide,
or, if Eta Carinae is a solitary
star, by a fast wind colliding with
an older, slower one. A binary
system would also explain why
the star's spectrum changes, with
clocklike precision, every 5½
years. Augusto Damineli
(University of Sao Paulo, Brazil)
and his colleagues observed Eta
Carinae during a predicted 1998
episode of spectral change. From
their data they deduced that the
star is indeed a binary system whose members
travel around a common center of mass in highly
elongated orbits every 5.53 years. However, in a
2000 Astrophysical Journal paper Davidson admits
that HST could not confirm that model with its
high-resolution spectrographs.
As astronomers continue to delve deeper
into these many mysteries, most agree on one
thing: Eta Carinae doesn't have much longer to
live. The star is one of only a few luminous blue
variables in the Milky Way. These extremely
massive stars live their lives in the fast lane of
stellar evolution, and Eta Carinae could demolish
itself in a supernova explosion today, tomorrow,
or in a few hundred thousand years. (In fact,
since it lies thousands of light-years away, it may
have done so already. Doesn't all this excitement
make you want to grab your telescope, and, if
necessary, a seat on a southbound jet?)
366
Back to our historical conundrum. Considering
Eta Carinae's dramatic variability, isn't it possible
that the brightness of the Eta Carinae Nebula
itself changes with time? Of course it is —
especially since erratic Eta Carinae is the most
powerful star in what is now the brightest section
of the nebula. According to Allen, the "most
brilliant portion [of the nebula], as drawn by Sir
John Herschel, seems to have disappeared at
some time between 1837 and 1871." Impossible?
Not really, especially if you consider how other
variable nebulae, such as Hubble's Variable
Nebula (Caldwell 46) and the R Coronae
Australis Nebula (Caldwell 68), behave. (The Eta
Carinae case also tells us something about the
value of astronomical drawings.) I know of no
catalogs that offer brightness estimates for the Eta
Carinae Nebula. In June 2000 I estimated the
nebula's brightness as magnitude 4.5. Given the
nebula's likely variability, it's entirely possible
that
Deep-Sky Companions: The Caldwell Objects