NGC 2261 is a curious variable reflection nebula in Monoceros that's known as Hubble's Variable Nebula. It was discovered by William Herschel in 1783 and is illuminated by variable star R Monocerotis (R Mon). The nebula is unusual in that it changes shape over just a period of days and can vary by up to 2 magnitudes in brightness. The variations are believed to be due to periodic changes in the amount of dust surrounding R Mon, thus affecting the amount of light that reaches us. With an apparent magnitude of +9.0, it can be spotted with binoculars under dark skies.
The variability of R Mon (between magnitudes +10 and +12) was discovered at the Athens Observatory in 1861 but it wasn't until 1916 that Edwin Hubble realised that the nebula also changes in brightness. The variations are such that even on images taken days apart structural changes can be observed.
NGC 2261 was chosen as the "first light" photograph on January 26, 1949 for the 200-inch (5.1 m) Hale reflecting telescope under the direction of American astronomer Edwin Powell Hubble. At the time, the newly constructed Hale telescope was the largest telescope in the World and remained so until 1976. The nebula is listed as number 46 in the Caldwell catalogue.
It's easy to locate the general area of sky surrounding NGC 2261 as its just 10 degrees east and a little north of brilliant red supergiant Betelgeuse (α Ori - mag. +0.42v) the second brightest star in spectacular Orion. The nebula itself is positioned about a degree north of the midway point of an imaginary line connecting stars 13 Mon (mag. +4.5) and 17 Mon (mag. +4.8). Positioned less than 2 degrees northeast of NCG 2261 is naked eye open cluster NGC 2264 (mag. +3.9). Surrounding this cluster is a great cloud of faint nebulosity.
At 9th magnitude, Hubble's Variable Nebula is visible with binoculars albeit faintly. A plus point for the nebula is that it has a high surface brightness - unusual for refection nebulae - and therefore can be well seen with small/medium telescopes. Through a 100mm (4-inch) refractor the southern tip appears like a slightly out of focus bright star. Here lies R Mon but the star itself is difficult to observe since it tends to be lost in the nebulosity. The nebula fans away from the star looking somewhat like a comet. At magnification 100x it appears mostly smooth but on closer inspection some mottling can be seen. A larger 200mm (8-inch) scope enhances the view revealing more twists and knots with large amateur scopes hinting at a bluish haze.
It total, Hubble's Variable Nebula covers 3 x 1 arc minutes of apparent sky and is estimated to lie 2,500 light-years distant. It's best seen during the months of December, January and February.
NGC 2261 Data Table
|Name||Hubble's Variable Nebula|
|Object Type||Variable Reflection Nebula|
|RA (J2000)||06h 39m 10s|
|DEC (J2000)||08h 44m 39s|
|Apparent Size (arc mins)||3 x 1|