A Last Look at Comet Holmes (11/27/2007)

On November 4th, Comet 17P/Holmes was about 2nd magnitude and spanned approximately 20 arc-minutes on the sky, about 2/3 the size of the Full Moon. Separate 20-second exposures through r, i and z filters were combined to make this monster 16-megapixel full-color view. The Megacam instrument at the MMT Observatory was used for this image. Credit: M. Ashby & N. Caldwell (CfA)

Comet 17P/Holmes, which dazzled sky watchers with a dramatic outburst that made it visible to the unaided eye, now is fading from sight. However, before it returns to the obscurity from which it came, astronomers at the MMT Observatory took a final look.

In late October, Holmes brightened by a factor of about one million times when it ejected a vast cloud of dust and gas. That cloud expanded over time and now spans more than 870,000 miles, making it larger than the Sun (which is 865,000 miles in diameter).

On November 4, Smithsonian scientists snapped this photo of Comet Holmes using an instrument called Megacam, which is one of the largest CCD cameras in existence. Megacam holds 36 9-megapixel CCD chips, for a total of more than 300 megapixels.

Separate exposures through three color filters were combined to make this final, full-color image. Individual stars appear as a line of colored dots because the photos were centered on the comet, which moved slightly across the sky.

Currently, Comet Holmes has a total brightness of 3rd magnitude, however its large size on the sky (and resulting low surface brightness) makes it difficult to see without binoculars or a telescope. However, when it was discovered in 1892 it underwent a second bright outburst five months after the first. If Holmes repeats its historical performance, then this comet may offer viewers one more chance to see the show before it bows from the stage.

Note: This story has been adapted from a news release issued by Harvard University

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