Shifting Shadows
24 FEBRUARY 2020 • SKY & TELESCOPE
tions with my 28-inch revealed obvious changes in the nebula
and R Mon on the nights of December 5 and 6, 2018, March
3 and 4, 2019, and October 7, 2019, with respect to the pre-
ceding night’s observations.
Observing conditions were different each night, of
course — darkness, transparency, steadiness of the seeing all
fl uctuated — but the nightly variations of Hubble’s Variable
Nebula were obvious enough to transcend the ebb and fl ow
of our turbulent atmosphere.
I now wonder if changes in NGC 2261 can be detected
over the course of a single night by observing it as soon as the
sky gets dark, and then again just before dawn. This wouldhave to be attempted during the Northern Hemisphere win-
ter, and if I ever get the chance I’ll give it a shot.How Big?
Although faintly visible in my 80-mm fi nderscope, the two
8-inch Dobs were the smallest instruments I used to observe
NGC 2261. Because the nebula is rather small, I needed to
boost magnifi cation beyond what I’d normally use on nebulae
to get enough image scale to see details well. Due to its high
surface brightness, I think it’s possible to track variations in
NGC 2261 with a smaller scope under more pristine skies
than I had, but the only way to fi nd out is to give it a go. Be
prepared to boost the magnifi cation to around 300× to get a
large enough image to more easily follow what’s going on.Why Does Its Appearance Change?
The driver of these apparent changes is a dusty accretion
disc around the young (less than 300,000 years old), mas-
sive (approximately 10 solar masses), and still-forming star, R
Mon. It’s classifi ed as a Herbig Ae/Be star with active accre-
tion and has a small companion, R Mon B, which is probably
a T Tauri star. Both stars are likely the same age and have a
way to go before joining the main sequence.
A current working model for NGC 2261 invokes a dusty
envelope surrounding R Mon and its circumstellar accre-
tion disk. As the star and accretion disk rotate, they entrain
matter from the dusty envelope, which is expelled along with
disk material in the polar directions by a strong and fast
(around 300 km/s) stellar wind. The ejected material takes
the form of bipolar parabolic cones. Interestingly, what we
see as NGC 2261 is only one half of the bipolar nebula: the
outfl ow that is pointed in our direction (the accretion disk
has an inclination angle of some 20°). The other component
is pointed away from us and is hidden by the dust and gas of
the envelope and the cone pointed toward us.
Some of the ejected material is thought to be in the form
of long fi laments that cast shadows inside the cones. These
fi laments appear to form some 2.2 astronomical units from
R Mon and may rotate with the help of magnetic fi elds. It’sHow NGC 2261 Became
Hubble’s Variable Nebula
It may come as a surprise that Edwin Hubble didn’t
discover the variability of NGC 2261. Instead, John
Mellish, an unpaid observer at Yerkes, originally
mistook NGC 2261 for a comet. Based on Mellish’s
observations, Edwin Frost, the second director of
the Yerkes Observatory, sent a telegram to Harvard
College Observatory, as was customary, announcing
the “comet” discovery. In its turn, Harvard promptly
dispatched the usual telegram to observatories
around the world informing them of this latest dis-
covery, before it became apparent that Mellish had
observed NGC 2261 instead of discovering a new
comet. Oops. Mellish subsequently lost credibility
with Frost. Hence, Frost assigned Hubble the task of
substantiating Mellish’s original 1915 visual observa-
tions of NGC 2261.
Shortly thereafter, Hubble confi rmed the varia-
tions that Mellish had noted in NGC 2261 and
wrote his fi rst research paper on the subject in 1916
(https://is.gd/HubbleNGC2261). The rest is history.February 6, 2019
28-inch f/4 at 408×March 2, 2019
28-inch f/4 at 408×March 3, 2019
28-inch f/4 at 408×March 4, 2019
28-inch f/4 at 547×