NGC 2261 - Hubble's Variable Nebula

NGC 2261 is known as Hubble's Variable Nebula. It is a peculiar emission and reflection nebula enveloping the variable star R Monocerotis.

The nebula was first observed by Sir William Herschel in 1783; this curious fan-shaped reflection nebula was originally mistaken as a comet. The variability of its associated star was discovered in 1861, and the variability of the nebula itself was discovered by Edwin Hubble in 1916. Hubble's Variable Nebula was the first object photographed by the 200-inch Hale Telescope at Mount Palomar in 1949.

The nebula appears only 2 x 4 arc minutes in size. The nebula is known to vary in brightness by up to two full magnitudes on a timescale of months to years.

At an estimated distance of 2,500 light years, NGC 2261 is about 3 light years long and 1.5 light years wide. Hubble's Variable Nebula is believed to be an outlying part of the large NGC 2264 nebula complex, appearing about a degree away.

NGC 2261 changes in brightness along with its illuminating star, and has also shown changes in its structure over the last century. The nebula's rapid changes in brightness are in fact shadows cast by dusty matter in orbit around R Monocerotis. The star, R Mon, is a young 10-solar mass-class B star with a small companion; both are in the pre-main-sequence stage.