The cloud of interstellar gas and dust can be seen in telescopic observations changing its appearance dramatically over periods of several weeks. The bright star R Monocerotis is located at the edge of the fan-shaped nebula known as NGC 2261, which was discovered about 200 years ago. This nebula, which is about one light-year across and 2500 light-years away, was studied in detail by Edwin Hubble in the early 20th century and became known as “Hubble's Variable Nebula”. But why is this nebula variable? NGC 2261 is a dusty nebula that reflects the light of R Monocerotis. The most common theory is that dense clumps of dust passing close to R Monocerotis cast moving shadows that fall on the rest of the nebula, causing the variable appearance.

January 31, 2025 Astronomy Photo of the Day (NASA APOD)

Image Source & Copyright: Tommy Lease (Denver Astronomical Society)