Hubble's Variable Nebula (NGC 2261) -- Reflection Nebula in Monoceros
NGC 2261 is a small tight fan-shaped reflection nebula that has a 10th magnitude star (R
Monocerotis) at its base. It is remarkably easy to see in a telescope. I have no problem
seeing the nebula in my vintage, early 1960s, 60mm/f15 achromatic refractor at 76x
magnification from my suburban backyard. Through the 155mm refractor the tiny fan is
obvious even at the low power I use to star hop to it. It is no wonder why this deep sky
object is the one that is most often mistaken for a comet – it is astonishingly comet like.
This reflection nebula is famous for changing brightness and details within its fan-like
shape. Changes within the fan of one-arcsecond can be detected in as little as four days.
The changes in brightness of the nebula do not coincide with a change in brightness of R
Monocerotis. The nebula is mostly being lit up by another star near R Monocerotis that is
hidden from our sight by the nebula’s dust.
This comet-shaped reflection nebula is a favorite of mine, easy to observe from suburbia
and its changes are within reach of my 155mm (6.1-inch) refractor. In fact, the variability of
the nebula was discovered with a 6-inch refractor in 1861 by J.F. Julius a famous variable
star observer. NGC 2261 is not a Messier Object or included in the Herschel 400 list. If you
have not yet observed Hubble’s Variable Nebula, you are in for a real treat when you do.