NGC 2261
A variable nebula in Monoceros, illuminated by an unseen star.
NGC 2261, called Hubble's Variable Nebula or Caldwell 46, is a variable nebula in Monoceros. The star R Monocerotis lights it up, though that star itself remains hidden from direct view. The nebula's shape and brightness shift slowly, noticeable in small telescopes over weeks or months, and it appears like a small comet.
William Herschel first recorded the nebula on 26 December 1783, describing it as fairly bright and fan-shaped. It long carried the designation H IV 2, the second entry in Herschel's class 4 category for nebulae and star clusters.
On 26 January 1949, Edwin Hubble captured NGC 2261 as the first-light image for Palomar Observatory's Hale Telescope, roughly twenty years after the observatory project began in 1928. Hubble had already studied the nebula at Yerkes and Mount Wilson. He took photographic plates with the Yerkes 24-inch reflecting telescope in 1916. Earlier plates from the same telescope, taken in 1908 by F.C. Jordan, let Hubble use a blink comparator to search for changes over time.
The Hubble Space Telescope imaged NGC 2261, releasing a picture in 1999. From October 2021 to April 2022, over twenty amateur astronomers at the Big Amateur Telescope created a timelapse over six months. The project resumed in August 2022 after the nebula emerged from behind the Sun. That timelapse shows light ripples propagating at light speed from the central star as its intensity varies, illuminating the surrounding nebula.
One explanation for the variability is that dense dust clouds near R Mon periodically block the star's light, casting a temporary shadow on nearby clouds.
Quick Facts
- Epoch
- J2000.0
- Ra
- 6 · 39 · 10
- Dec
- +8 · 45
- Dist Ly
- 2,500
- Appmag V
- 9.0
- Constellation
- Monoceros
- Radius Ly
- ~0.5
- Names
- Hubble's Variable Nebula, Caldwell 46
Facts from the source article.
Lore & Background
The nebula was first recorded by William Herschel on 26 December 1783, who described it as considerably bright and 'fan-shaped'. It was later designated H IV 2, the second entry in Herschel's class 4 category for nebulae and star clusters. Edwin Hubble studied the nebula extensively, taking photographic plates with the Yerkes 24-inch reflecting telescope in 1916. Earlier plates taken by F.C. Jordan in 1908 with the same telescope allowed Hubble to use a blink comparator to search for changes over time.
NGC 2261 was the first light target for Palomar Observatory's Hale Telescope on 26 January 1949, some 20 years after the Palomar Observatory project began in 1928. The Hubble Space Telescope imaged the nebula, releasing an image in 1999. A timelapse of NGC 2261 was taken over six months by over 20 amateur astronomers at the Big Amateur Telescope from October 2021 to April 2022, and resumed in August 2022 after the nebula emerged from behind the Sun. The timelapse shows light 'ripples' propagating at light speed from the central star as it varies in intensity and illuminates the surrounding nebula.
The variability of the nebula is thought to be caused by dense clouds of dust near R Mon periodically blocking the star's illumination, casting temporary shadows on the nearby clouds.
Reader's Guide
NGC 2261 holds significance as a variable nebula whose brightness and shape change observably over weeks and months, making it a target for both professional and amateur astronomers. Its study by Edwin Hubble, beginning with photographic plates from 1916 and earlier plates from 1908, allowed early detection of variability through blink comparison. The nebula's selection as the first light object for the Hale Telescope in 1949 marks a milestone in observational astronomy, linking the Palomar Observatory's long development to a specific celestial target. The Hubble Space Telescope's 1999 image continued this legacy of high-resolution observation. More recently, a coordinated effort by over 20 amateur astronomers using the Big Amateur Telescope produced a six-month timelapse from October 2021 to April 2022, capturing light ripples propagating at light speed from the central star. This project demonstrates the ongoing role of amateur contributions in monitoring variable phenomena. The proposed explanation—that dust clouds near R Mon periodically block illumination, casting temporary shadows—provides a physical mechanism for the observed changes. The nebula's resemblance to a small comet and its fan-shaped appearance, noted since Herschel's 1783 discovery, have made it a persistent object of interest.
Did You Know?
- The nebula's illuminating star, R Monocerotis, is not directly visible.
- A timelapse of NGC 2261 over six months was created by over 20 amateur astronomers using the Big Amateur Telescope.
- Edwin Hubble used a blink comparator to compare plates from 1908 and 1916 to detect changes in the nebula.
More in NGC Objects, Part 3 1-24
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