Sky & Telescope - February 2020 - 22

Shifting Shadows

them. Photographically it is well deﬁned and has almost the
form of an equilateral triangle with a sharp stellar nucleus at
the extreme southern point. There are faint extensions from the
northern portion of it. One long streamer which projects from
the northern edge extends almost due north."

7h 30m

7h 00m

6h 30m

6h 00m

GEMINI

ξ
During my 20 years of observations, I've seen obvious
variations throughout the body of NGC 2261, but the most
dramatic changes in form are associated with the northcurving streamer on the eastern edge of the nebula, which
has brightened, dimmed, thickened, thinned, lengthened,
shortened, and shown knots that come and go. It has also
completely disappeared at times!
Because this streamer becomes increasingly fainter the
farther it gets from R Mon, the quality of the sky affects
how long it looks. An apparent change in its length may be
partially due to observing conditions, and I'm sure that's
been true at least a little in every one of my observations. The
shortest I've seen it was on October 2, 2019, and the longest
were on February 17, 2010, and March 3 and 4, 2019.
Another dramatic change is the constantly morphing
shape of a dark area that takes a bite out of the western edge
of the nebula immediately north of R Mon. I once saw that
bite extend all the way to the eastern edge, seemingly cutting
off R Mon from the rest of NGC 2261's nebulosity - take a
close look at my sketch from February 17, 2010.
One of the more fascinating changes to look for is the
variability of R Mon itself. When the star is near maximum
brightness it looks like a star to me, but at its minimum it
gives an impression of being a bright and very condensed
nebulous patch. That happens to be exactly what we're
seeing, because research has shown that R Mon is heavily
obscured and truly encased within its nebulosity. We don't
see R Mon itself, just the nebula that the star illuminates
from within. This nonstellar appearance shows well in the
image on page 20. Even though I've glimpsed the fuzzy look
of this part of NGC 2261 during moments of steady seeing, I
have yet to see it as the triangular patch that's so obvious in
the Hubble Space Telescope image.

December 2, 2018
28-inch f/4 at 408×

FEBRUARY 2 02 0 * SK Y & TELESCOPE

2264

2261

+10°

Betelgeuse

α
8

CMi

ORION
18

0°

MONOCEROS

p FINDER CHART Hubble's Variable Nebula, also known as NGC 2261,
lies in the northern reaches of Monoceros, just below Gemini and between Canis Minor to the east and Orion to the west.

Two Decades of Sketches
The series of sketches presented at the bottom of each page
are my complete collection of Hubble's Variable Nebula drawings to date and clearly show the changes I've seen. Each caption lists the observation date, scope used, and magniﬁcation.
The sketches represent what I saw without the use of
nebula ﬁlters. The unﬁltered view was always the most
detailed: Because NGC 2261 shines only by reﬂected starlight
from R Mon, it doesn't respond to nebula ﬁlters. Most of
my observations were made under skies that measured 19.4
to 20.4 with a Sky Quality Meter, with the darkest sky - on
March 4, 2019 - measuring 21.49. That translates to limiting
magnitudes of about 5.0 to 6.4.
All but four of my observations were with my 28-inch
scope. The very ﬁrst observation on March 10, 1999, was
with my old 20-inch Dob. I also observed NGC 2261 once in
January 2019 with my wife's 8-inch f/3.3 Dob to see if I could
detect enough detail to notice changes through a wide-ﬁeld

December 4, 2018
28-inch f/4 at 408×

December 5, 2018
28-inch f/4 at 408×

Sky & Telescope - February 2020

Table of Contents for the Digital Edition of Sky & Telescope - February 2020