Hubble’s Variable Nebula

One of several clusters and nebulae that fill the northern regions of the constellation Monoceros, eastward of bright Betelgeuse in Orion. Hubble's Variable Nebula (named for astronomer Edwin Hubble, who studied the nebula, though it was actually discovered by William Herschel) is a reflection nebula lying some 2,500 light years from the Solar System.

Physically the nebula appears as a cone of bright material extending outwards for several light years from a bright point at its southern end. The overall visual effect is rather like a comet with a wide spreading tail, and indeed nebulae of this kind are occasionally referred to as 'cometary nebulae'. The nature of this bright point is not absolutely certain, but it is generally considered to be a T Tauri variable, an active star surrounded by a dense halo of matter, designated R Monocerotis.

The activity of R Monocerotis causes its brightness to vary over time, and because the nebula shines by light reflected from the star, the nebula's magnitude varies too. The source of this variability appears to lie in the dynamics of R Monocerotis and its dust belt, with matter falling into the star from its encircling disc of matter, and then being expelled from its poles. Shadows from this material occlude the nebula's light, changing its brightness over time and creating a variable nebula.