Hubble's Variable Nebula - NGC 2261

NGC 2261 is a variable reflection nebula located in the constellation Monoceros, at about 2.500 light-years away. The nebula is illuminated by the star R Monocerotis and it manifests changes in the overall brightness over short periods of time. It is popularly called Hubble’s Variable Nebula since Edwin Hubble was the first one to notice the brightness changes over more than 25 years.
The first recorded observation of the nebula was by William Herschel on 26 December 1783, being described as considerably bright and “fan-shaped”. It had long been designated as H IV 2, after being the second entry of Herschel's class 4 category for nebulae and star clusters, in his catalogues of nebulae.
R Monocerotis is a young stellar object, a pre-main sequence star that has not yet started to fuse hydrogen in its core. It is still accreting material from its birth cloud. This is a massive Herbig Ae/Be star with up to 10 times the Sun’s mass and an age of just 100.000 years. The disk around the star has a mass of 0,007 solar masses and it extends to 150 AU from the star. There is also a T-Tauri companion in the vicinity.
R Monocerotis has lit up the nearby cloud of gas and dust, but the shape and brightness slowly change visibly even in small telescopes over weeks and months, and the nebula looks like a small comet. One explanation proposed for the variability is that dense clouds of dust near R Monocerotis periodically block the illumination from the star. This casts a temporary shadow on the nearby clouds. Another reason might be that R Monocerotis is a variable star, with ranges up to 4 magnitudes.