| Reflection nebula | |
|---|---|
| Variable Nebula | |
 | |
| Observation data: J2000.0epoch | |
| Right ascension | 6h 39m 10s[1] |
| Declination | +8° 45′[1] |
| Distance | 2,500 ly |
| Apparent magnitude (V) | 9.0 |
| Apparent dimensions (V) | 2′ |
| Constellation | Monoceros |
| Physical characteristics | |
| Radius | ~0.5 ly |
| Designations | Hubble's Variable Nebula,[1]Caldwell 46 |
| See also:Lists of nebulae | |
NGC 2261 (also known asHubble's Variable Nebula orCaldwell 46) is avariable nebula located in theconstellationMonoceros. The nebula is illuminated by the starR Monocerotis (R Mon), which is not directly visible itself.
The first recorded observation of the nebula was byWilliam Herschel on 26 December 1783, being described as considerably bright and 'fan-shaped'.[2] It had long been designated as H IV 2, after being the second entry of Herschel's class 4 category for nebulae and star clusters, in his catalogues of nebulae.[3]
NGC 2261 was imaged asPalomar Observatory'sHale Telescope'sfirst light byEdwin Hubble on January 26, 1949,[4] some 20 years after the Palomar Observatory project began in 1928. Hubble had studied the nebula previously atYerkes andMt. Wilson.[4] Hubble had taken photographic plates with the Yerkes 24-inch (60.96 cm) reflecting telescope in 1916.[5] Plates were taken using the same telescope in 1908 by F.C. Jordan, allowing Hubble to use ablink comparator to search for any changes over time in the nebula.[5]
NGC 2261 was imaged by theHubble Space Telescope, and an image of the nebula was released in 1999.[6]
A timelapse of NGC 2261 was taken over a period of 6 months by over 20amateur astronomers at theBig Amateur Telescope from October 2021 – April 2022. In August 2022, the project was resumed as NGC 2261 came out from behind the Sun.[7]
The star R Monocerotis has lit up a nearby cloud of gas and dust, but the shape and brightness slowly changes visibly even in small telescopes over weeks and months, and the nebula looks like a small comet.[8]
One explanation proposed for the variability is that dense clouds of dust near R Mon periodically block the illumination from the star.[9] This casts a temporaryshadow on the nearby clouds.[10]
