Sky & Telescope - October 2022 - 22

Cosmic Clouds
at the southern end of the dark nebula that John Herschel
named the Keyhole.
Trumpler 16 holds the tiny (22″ across) Homunculus
Nebula, a bipolar and expanding shell of dust and gas that
the unstable binary star Eta Carinae (a hypergiant with a
massive companion) probably ejected during its 1843 outburst,
when it briefly reached magnitude -1 (S&T: Oct. 2016,
p. 26). The Homunculus is one of the very few orange deepsky
objects! One very steady night in March 2001, I used
Australian amateur Andrew Murrell's 20-inch f/5 Dobsonian
at 363× and was able to see the major features visible in
Hubble Space Telescope images (such as the one on page 23).
The Homunculus's northwestern lobe was narrower and had
a tiny dusky inclusion; the southeastern lobe was wider and
had two small, dark inclusions arranged along the nebula's
major axis. At the waist of the lobes wee spikes extended
both northeast and southwest of the hidden central star,
with the northeastern spike being the sharper of the two.
One evening an overcast of stratus formed, and for many
hours the faintest stars that I could see through the thin,
low cloud were of magnitude 3. But the Eta Carinae Nebula
continuously shone through the cloud!
uGEM OF THE SOUTHERN SKIES John Herschel wrote of the Eta
Carinae Nebula: " It is not easy for language to convey a full impression
of the beauty and sublimity of the spectacle which this nebula offers
. . . " Most observers would likely agree. The nebula lies west of the
Southern Cross, surrounded by three eye-catching open clusters (labeled).
Trumpler 16 is one of eight open clusters within the Eta Carinae
Nebula (A) and contains one of the more striking objects in the sky: the
Homunculus Nebula (B).
When I returned to North America and mentioned that
Eta Carinae was completely hidden within the Homunculus,
I was met with skepticism by famous Northern Hemisphere
observers who had easily seen an " astonishingly red " Eta a few
years earlier. Fortunately for me, Australian amateurs confirmed
that Eta was invisible in March 2001. By January 2005,
observing with my extremely generous host Tony Buckley and
using his 14.5-inch Dob at 322×, I wrote: " Eta has brightened
since 2001, and now there is a definite deep orange star and
only the eastern lobe of the now over-powered Homunculus is
easily visible - the western lobe is tough. "
During the late 2000s, former Sky & Telescope contributing
editor Les Dalrymple noted that the evolution of the
Homunculus was occurring over relatively short time scales.
In just five to eight years the fainter of the two lobes appeared
somewhat lower in surface brightness, while the star itself
was somewhat brighter. Recently, in 2019, Gottlieb reported
that Eta was " very small but non-stellar. "
In February this year, Dutch amateur (based in Chile)
Wouter van Reeven observed Eta Carinae from the Atacama
Desert. He noted that the star was naked-eye visible within the
nebula, and with his 20-inch Dob it was a light, golden yellow
in color and bright. South African observer Magda Streicher
confirmed the color change and added it was " much more starlike. "
At last report Eta Carinae shone at magnitude 4.1.
Shifting our gaze northward to familiar territory, let's look
at the Eta Carinae Nebula's only true rival, the Orion Nebula.
French astronomer Nicolas-Claude Fabri de Peiresc discovered
the Great Orion Nebula (M42) in 1610, shortly after the
telescope's invention.
NGC 3532
A
NGC 3114
IC 2602
22 OCTOBER 2022 * SKY & TELESCOPE
WIDE VIEW: AKIRA FUJI I; ETA CARINAE NEBULA: FRANK SACKENHEIM;
HOMUNCULUS: ESA / HUBBLE / NASA
Sky & Telescope - October 2022

Table of Contents for the Digital Edition of Sky & Telescope - October 2022
