NGC 2261 (also known as Hubble’s Variable Nebula or Caldwell 46) is a variable nebula located in the constellationMonoceros. It is illuminated by the star R Monocerotis (R Mon), which is not directly visible itself. NGC 2261 was imaged as Palomar Observatory’s Hale Telescope’s first light by Edwin Hubble on January 26, 1949, some 20 years after the Palomar Observatory project began in 1928. One explanation proposed for the variability is that dense clouds of dust near R Mon periodically block the illumination from the star.