NGC 2261, Hubbles variable Nebula

Object Description

NGC 2261
NGC 2261: Animated Hubbles Variable Nebula; Cassegrain 400 mm f/9 + STL11000M; L:90-100min R:20min G:20min B:20min; Observatory Oberes Schlierental, Obwalden; © 2009-2010 Eduard von Bergen

Like many interesting deep-sky objects accessible to amateurs today, NGC 2261 was discovered by William Herschel. He observed it on December 26, 1783 and cataloged it as IV 2 and described it as «considerably bright, fan-shaped, about 2' long from the center.» [463]

At the beginning of the 20th century, the eponymous discovery was made: the famous astronomer Edwin Hubble determined on the basis of photographs that the brightness and shape of the nebula change from time to time. Two different causes were found:

  1. Within the nebula is an irregularly variable star, R Monocerotis, whose brightness varies by up to four magnitudes.
  2. R Monocerotis is temporarily obscured by dust clouds, certain areas of the nebula then no longer receive starlight and are darkened. Changes can be made in larger telescopes for short periods of time such as months, weeks and become visible even days!
NGC 2261
NGC 2261: Hubbles Variable Nebula in Monoceros; 500 mm Cassegrain 5800 mm f/11.4; SBIG STL11K; 75+20+20+20 min LRGB; Bernese Highlands; © 2005 Radek Chromik

This nebula is also called the Santa Claus Nebula because of NGC 2261's peculiar shape, which resembles a long-bearded man wrapped in a long, thick winter coat. In addition, it is particularly easy to see in winter during the Christmas period.

— 1996, Philipp Reza Heck

The star R Monocerotis is about 2500 light-years from Earth. It cannot be seen directly, but can be seen through the light reflected from dust clouds. The mass of the star is estimated at 10 solar masses and its age at only 300'000 years. A symmetrical counterpart to the fan-shaped nebula probably still exists on the southern side of the star, but this is obscured by intervening dust on our line of sight. [220]

«Revised NGC/IC», «Historic NGC/IC», Version 2022-09-01, © Dr. Wolfgang Steinicke [277]
DesignationNGC 2261
TypeEN+RN
Right Ascension06h 39m 09.5s
Declination+08° 44' 40"
Diameter3 × 1 arcmin
Visual magnitude9.0 mag
Metric Distance0.800 kpc
Dreyer DescriptionB, vmE 330°, N com = * 11
Identification, RemarksLBN 920, Hubble's variable nebula

How to find Hubbles Variable Nebula?

Hubble's Variable Nebula lies in the constellation Monoceros about halfway between Alzirr (ξ Geminorum) and the Rosette Nebula (NGC 2237+), whose open star cluster can be seen with the naked eye on dark nights. The Christmas Tree Cluster (NGC 2264) of the 4.7 mag star 15 Monocerotis is about a degree away towards eleven o'clock and can also be seen with the naked eye. The 1° closeup shows the path from the Cone Nebula in NGC 2264 via the 7 mag binary star HD 47888 to the Hubble's Variable Nebula (NGC 2261). The best viewing time is November to March.

Monoceros: NGC 2261
Chart NGC 2261
Chart created using SkySafari 6 Pro and STScI Digitized Sky Survey. [149, 160]

Visual Observation

NGC 2261
NGC 2261: Drawing; 200mm SCT; © 7. 9. 1996 Stefan Meister

200 mm aperture: Observing NGC 2261 is highly dependent on atmospheric conditions! It is a relatively bright and small object, so the calmness of the air (seeing) is particularly important here, whether structure is recognizable or not.

With bad to mediocre seeing I observe with magnifications between 100x and 150x, with mediocre to good conditions between 150x and 200x and with good to very good conditions I also go over 200x. (This applies to a reflecting telescope with an aperture of 20 cm and a focal length of 2 m.)

Looking at Hubble's Variable Nebula, I see three regions: (I) the star R Monocerotis at the apex of the nebula, (II) the bright southern region containing the star, and (III) the fainter region to the north. The border between areas II and III is irregular. With good contrast, this is clearly visible.

Because of the variability of the nebula, it is worth making sketches at the telescope. Hubble's Variable Nebula is one of the few deep sky objects in which the amateur astronomer can detect visible changes within a short period of time - making it one of the most interesting and exciting objects!

— 1996, Philipp Reza Heck

NGC 2261
NGC 2261: Drawing; 200mm SCT; © 8. 12. 1994 Philipp Reza Heck
NGC 2261
NGC 2261: False colors of the areas I through III (see text)

400 mm aperture: The sight of the fan-shaped nebula is pretty impressive. It appears bright and can stand up to higher magnifications. Structures within the nebula were not detected, only an outwards decreasing brightness. As reflection nebula can best be seen without any filter, I did not try it with an O-III filter. — Taurus T400 f/4.5 Dobsonian, Bernd Nies, Glaubenberg Langis, February 28, 2022

References

149SkySafari 6 Pro, Simulation Curriculum; skysafariastronomy.com
160The STScI Digitized Sky Survey; archive.stsci.edu/cgi-bin/dss_form
220Hubble's variable nebula (NGC 2261); esahubble.org/images/opo9935c (2021-01-08)
277«Historische Deep-Sky Kataloge» von Dr. Wolfgang Steinicke; klima-luft.de/steinicke (2021-02-17)
463«Catalogue of one thousand new nebulae and clusters of stars» William Herschel, Philosophical Transactions of the Royal Society of London, 1 January 1786; DOI:10.1098/rstl.1786.0027