17P/Holmes

17P/Holmes is a periodic comet in our solar system, discovered by the British amateur astronomer Edwin Holmes on November 6, 1892. In only 42 hours in October 2007, the comet brightened from a magnitude of about 17 to about 2.8. This represents a change of brightness by a factor of about half a million and is the largest known outburst by a comet.

On November 9, 2007 the coma, the thin dissipating dust ball around the comet, was found to be the largest object in the solar system, with a diameter greater than that of the Sun. (Though by Solar System standards, the mass of the comet is minuscule.)

As of February 2008, the comet remains visible, a challenging target at about magnitude +5 in the constellation Perseus. It has expanded to greater than 2 degrees of arc as seen from the Earth, and thus has very little surface brightness.

		Comet P17/Holmes Taken on 10/30/2007 Single 30-second exposure ISO 400 Equipment and Processing: Orion ED80 Unmodified Canon RebelXT 350D Processed with PhotoShop CS Copyright © 2007 Antonio Pascarella
		Taken on 10/30/2007 Re-processed on 04/25/2012 Orion ED80 Unmodified Canon RebelXT 350D 5 30-second exposures ISO 400 Stacked with DeepSkyStacker Processed with Photoshop CS Copyright © 2012 Antonio Pascarella