Hubble's Variable Nebula (NGC 2261) is a beautiful mystery. Discovered by William Herschel in 1783, this was the first object photographed by the 200-inch Hale telescope at Palomar.  What makes this nebula so interesting is its variability. We are used to the unchanging faces of nebulae, yet this one apparently changes in brightness and structure over weeks or months. It was Edwin Hubble in 1916 who first noticed the changes in photographs.

The star at the end of the fan-shaped nebulosity is R Mon, an Orion type variable that varies in brightness by as much as 2 magnitudes. Orion variables are irregular, eruptive variables connected with bright or dark diffuse nebulae or observed in the regions of these nebulae. They have only recently formed and are still in the process of becoming a main sequence star. Strong stellar winds blow material away from the star.

R Mon appears to be surrounded by a disk of material, which is typical for such stars. It is thought that the planets in our solar system formed from a similar disk around our sun billions of years ago. As material from this disk falls into the star much of it gets ejected back outward. The thick disk that lies at the star's equator blocks the outflow of ejected matter through it, but the way out at the poles of the star is relatively clear. The result is a directed flow of matter in two opposing jets.

In the case of R Mon, one of these jets flows into the nebula. As the material flows outward dust forms, making dark streamers. These streamer cast shadows onto the walls of the nebula, which is visible to us via the reflected light of the star.

The nebula itself is a dusty, hollow shell. When we look at it we primarily see the closer side of this shell, but parts of the other side also show through. Some of the structure we see is due to the shadows of relatively immobile clouds of dust which make more or less permanent features on the nebula. The shadows of the fast moving streamers are what cause the apparent changes in brightness as they flow into the hollow interior. For more information regarding this fascinating object see the pages from the Glenlea Astronomical Observatory.

The field in an 8" f/8 at 50x. North is down and east is to the right.

The nebula is quite bright and obvious in my 18-inch truss Dob. It looks very much like the photographs. The surface brightness is high enough to allow for high magnification, so don't be shy about putting in a high power eyepiece. Things to look for are light and dark knots in the nebula, and the faint nebulosity that lies beyond the wider end. R Mon itself can be difficult to distinguish from the bright nebulosity that surrounds it.

Have a look at Mark Birkmann's Hubble's Variable Nebula page for an excellent drawing made with a 40".

NGC 2261 makes an interesting target for CCD imaging. Several amateurs have made animations showing the changes in the nebula. See the Astronomy Pages by Tom Polakis for excellent examples.
 

Photo (c) Anglo Australian Observatory