Object of the Week - NGC 2261

This weeks object is NGC 2261 (RA:6h39m10.00s DE:+08°44'11.0") also known as Hubble's Variable Nebula. It is located in the constellation of Monoceros (Mon) the Unicorn. It is about 2500 light-years away and is associated with the star R Mon, a variable star. It is a comet-like reflection nebula that is about 1 light-year across or about 6 trillion miles across.

Image: HST image of NGC 2261

The nebula was discovered in the 19th century but it was not until the early 1900's the the variability was discovered by Edwin Hubble, whom the nebula is named after. Hubble is of course the same one who realized that the redshift observed in distant galaxies was from their recession away from us. This led to the development of the big bang theory. He noted the variation in the nebula over a series of weeks. Taking the size of the nebula into account, this appeared to be a violation of the theory of relativity as these changes would have to be occurring faster than the speed of light!

Image: Changes in the nebula over time by Chris Brown Glenlea Astronomical Observatory, University of Manitoba

The nebula itself is not changing and the changes are not really in sync with changes in R Mon's brightness. The leading hypothesis is mostly what we are observing is simple shadow play. As gas and dust orbit R Mon, it casts shadows against the bright part of the nebula. Giving the impression that the nebula itself is changing. The gas and dust are part of a disk of material that is still accreating onto the young star.

R Mon is a relatively young star at about 300,000 years old and is still in the process of formation as stated above. It is classified as a T Tauri type star and has an estimated mass of about one and a half times that of the sun.

Finder charts below.

Image: 90 Deg Finder Chart for NGC 2261

Image: 30 Deg Finder Chart for NGC 2261

Image: 01 Degree Finder Chart for NGC 2261