Inconstant Nebulae
DEEP SKY OBSERVERS have a seemingly endless reservoir of nebulous objects to observe. Galaxies, unresolved star clusters, planetary nebulae and diffuse nebulae of many types abound. The vast majority are static and unchanging on human timescales, but a few defy constancy. Variable nebulae constitute a small but intriguing class of reflection nebulae illuminated by variable stars that make interesting and fun targets. Changes visible on timescales as short as weeks or months endow these mysterious clouds with the ability to excite our imagination and pique our curiosity. Our summer sky contains some notable examples.
The most famous and best understood of these enigmatic jewels is (NGC 2261), which is the peculiar nebula associated with R Monocerotis (R Mon). The brightest and largest of the variable nebula genre, it stands out among its brethren and is a well-known showpiece of the summer sky. It has long been a personal favourite of mine, and my observing notes emphasise how very easy it is to witness the changes in its shape and brightness distribution from year to year, and even month to month. One might imagine an
