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Hubble's Variable nebula (NGC 2261)
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Name: Hubble's Variable nebula, NGC 2261, R Mon, Caldwell 46
Description: Reflection Nebula
Position (J2000): RA 06hr 39m 10s Dec +08° 45.0
Constellation: Monoceros
Visual Magnitude: 9.0
Apparent Dimensions: 2'
Distance: 2500 light years
Image Credit: NASA/ESA and The Hubble Heritage Team (AURA/STScI)
Release Date: October 7, 1999
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ABOUT THIS IMAGE:

Hubble's variable nebula is named (like the Hubble telescope itself) after the American astronomer Edwin P. Hubble, who carried out some of the early studies of this object. It is a fan-shaped cloud of gas and dust which is illuminated by R Monocerotis (R Mon), the bright star at the bottom end of the nebula. Dense condensations of dust near the star cast shadows out into the nebula, and as they move the illumination changes, giving rise to the variations first noted by Hubble. The star itself, lying about 2,500 light-years from Earth, cannot be seen directly, but only through light scattered off of dust particles in the surrounding nebula. R Mon is believed to have a mass of about 10 times that of the Sun, and to have an age of only 300,000 years. There is probably a symmetrical counterpart of the fan-shaped nebula on the southern side of the star, but it is heavily obscured from view by dust lying between this lobe and our line of sight.

The Hubble Heritage team made this image from observations of R Mon acquired by William Sparks (STScI), Sylvia Baggett (STScI) and collaborators.

From Wikipedia:

NGC 2261 (also known as Hubble's Variable Nebula or Caldwell 46) is a variable nebula located in the constellation Monoceros. It is illuminated by the star R Monocerotis (R Mon), which is not directly visible itself.

NGC 2261 was imaged as Palomar Observatory's Hale Telescope's first light by Edwin Hubble on January 26, 1949, some 20 years after the Palomar Observatory project began in 1928. Hubble had studied the nebula previously at Yerkes and Mt. Wilson.

One explanation proposed for the variability is that dense clouds of dust near R Mon periodically block the illumination from the star.