HST IMAGE: NASA / ESA / THE HUBBLE HERITAGE TEAM / AURA / STSCI; HALE IMAGE: COURTESY OF THE ARCHIVES, CALIFORNIA INSTITUTE OF TECHNOLOGY; ALL SKETCHES BY THE AUTHOR
pTHE HALE CAPTURES HUB-
BLE’S NEBULA When Edwin
Hubble pointed the 200-inch Hale
Telescope skyward for the fi rst time,
he snapped a shot of the nebula that
bears his name.A
s you might expect, Hubble’s Variable Nebula
is named after Edwin Hubble, who, as a graduate
student at Yerkes Observatory, fi rst described the
nebula’s variability in a paper he wrote in 1916. Hubble went
on to bigger things, of course: He established that spiral
“nebulae” were in fact external galaxies; his
name is attached to the law that describes
the expanding universe and its associated
constant; and the Hubble Space Telescope
is named in his honor. But it’s his work on
NGC 2261, the variable nebula that would
also carry his name, that came fi rst.
William Herschel discovered the nebula
in 1783, and Julius Schmidt noted in 1861
that R Monocerotis (R Mon), the star
that illuminates NGC 2261, was variable.
But Hubble was the fi rst to document the
nebula’s changes in form (see the sidebar
on page 24, though, for a curious historical
twist). As it turns out, NGC 2261 is also the
correct answer to “what was the fi rst object
photographed by the 5-meter Hale telescope
at Palomar Observatory?” As a fi tting tribute
to his 1916 discovery, Hubble himself took
that photo on January 26, 1949.
Although Hubble’s Variable Nebula is
not the only one that varies in apparent
brightness and shape, it’s by far the bright-
est and most detailed member of the small class of variable
nebulae. But are its variations obvious enough to be visible
through an amateur-sized telescope? If so, how long does it
take to see changes, and how big a scope does one need to see
them? What causes the changes in the fi rst place? The quick
answers are yes, 24 hours or less, 8 inches or so, and shadows
— but let’s examine each in more detail.Can You See the Changes Visually?
I’ve been tracking the apparent changes in Hubble’s Vari-
able Nebula on and off since 1999 and have accumulated21 observations to date. NGC 2261’s small and remarkable
comet-like shape is quite striking, but the best part is that it
has looked different every time I’ve observed it.
That’s right, every time. My experience shows it’s not
only possible to see variations in NGC 2261 with your own
eyes through a telescope, it’s inevitable if
you observe it more than once.
My fi rst fi ve observations were spread
over several years (15, in fact), and even
though they’re fascinating snapshots
showing dramatic changes, I couldn’t help
wondering just how short a time interval
would show a change. During the winter
of 2018–2019 I decided to fi nd out and
observed NGC 2261 every clear night I
could — that meant most of my observa-
tions were from my home instead of a dark-
sky site. These make up the remaining 16 of
the 21 observations.
It quickly became evident that despite
NGC 2261’s small apparent size, its high
surface brightness means changes can be
seen under less-than-pristine skies. Some-
times the overall shape of the nebula looked
different, and sometimes it was the details
within the nebula that changed, which was
when steady seeing and higher magnifi ca-
tions — 400× to 600× — were the most
helpful. At other times it was the apparent brightness of
R Mon that was the most striking difference, because it var-
ies irregularly between 10th and 13th magnitude. In the end,
every element of NGC 2261 looked different to some extent
for each observation. However, it retained its basic form,
which Hubble described well in his 1916 paper, even when
the details of its appearance changed dramatically:“A striking instance of actual change in form has been found
in the case of the nebula N.G.C. 2261... one of the few real
examples of cometary form in the sky and easily the fi nest ofskyandtelescope.com • FEBRUARY 2020 21
January 11, 2007
28-inch f/4 at 570×February 17, 2010
28-inch f/4 at 408×January 5, 2014
28-inch f/4 at 408×November 19, 2018
28-inch f/4 at 408×