The Caldwell Objects

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Helena. How, then, did Halley's observation
become public?
In the 150 years following Halley's observation,
Eta Carinae's brightness was to fluctuate
erratically, achieving a maximum, at 1st magnitude,
in 1827. The star then dipped back to 2nd
magnitude, where it remained for about five
years. Writing in his 1847 "Results of Astronomical
Observations," John Herschel picks up
the story from there:
When first observed by myself in 1834, [Eta
Carinae] appeared as a very large star of the
second magnitude, or a very small one of the first,
and so it remained without apparent increase or
change up to nearly the end of 1837 . . . It was
on the 16th December 1837 that. . . my
astonishment was excited by the appearance of a
new candidate for distinction among the very
brightest stars of the first magnitude . . . After a
momentary hesitation, the natural consequence of
a phenomenon so utterly unexpected . . . I
became satisfied of its identity with my old
acquaintance Eta Argus [Eta Carinae]. Its light
was however nearly tripled. While yet low it
equalled Rigel, and when it had attained some
altitude was decidedly greater. It was far superior
to Achernar. Fomalhaut and Alpha Gruis were at
the time not quite so high, and Alpha Crucis
much lower, but all were fine and clear, and Eta
Argus would not bear to be lowered to their
standard. It very decidedly surpassed Pro-cyon,
which was about the same altitude,
and was far superior to Aldebaran _______ From
this time its light continued to increase. On the 28th
December it was far superior to Rigel, and could only
be compared with Alpha Centauri which it equalled,
having the advantage of altitude, but fell somewhat
short of it as the altitudes approached equality. The
maximum of brightness seems to have been obtained
about the 2nd January 1838 . . . it was judged to be
very
nearly matched indeed with Alpha Centauri . . .
After this its light began to fade. . . On the 20th, it
was visibly diminished — now much less than Alpha
Centauri, and not much greater than Rigel. The
change is palpable.
An even more spectacular event occurred in
1843, when the star reached about -0.7
magnitude, outshining every star in the sky with
the exception of Sirius. Eta Carinae then slowly
faded from naked-eye view, hitting a rockbottom
magnitude of 7.6 in 1968. It then
rebounded to 6th magnitude in 1978. The star
brightened to 5th magnitude between December
1997 and February 1999. On June 12, 1999, I
estimated Eta Carinae's apparent magnitude to
be 4.2 — about as bright as it was when Halley
saw it in 1677. Kris Davidson (University of
Minnesota) believes this latest brightening
episode may be Eta Carinae preparing itself for
another major outburst.
Today we know that the great 1843 outburst
(which became known as Nova Carinae 1843)
was the biggest explosion that any star is known
to have survived. According to Davidson, several
solar masses of matter were ejected at speeds
around 1,000 km per second. The explosion shot
out two opposing lobes of expanding gas, which
squeezed out of a dense, 15-solar-mass torus of
cold dust and gas perhaps 0.1 light-year in
diameter. The gaseous shell expanded like a
balloon inflating in a napkin ring. Also called the
Homunculus, that balloonlike portion of the
nebula has been expanding ever since; presently
it spans about 0.8 light-year. Meanwhile, Eta
Carinae and its eponymous nebula together have
become the brightest object beyond the solar
system at infrared wavelengths.
The Hubble Space Telescope has imaged the
star's expanding bipolar lobes in unprecedented
detail. Two polyps of gas pulse with veins of
dust and strange radial streaks that
Deep-Sky Companions: The Caldwell Objects