| Exploring our own back yard. |
| The comets pictured here would look nothing like these photographs if we could observe them from nearby. They would not be neat round balls and they would be very dark, perhaps nearly as black as soot. The late astronomer Fred Whipple famously described comets as “dirty snowballs” as he hypothesized that they consist of a conglomeration of water ice and very dark, apparently fine-grained rocky material. Whipple was proven correct when we finally got a close-up look at a comet from a spacecraft. In fact, what we are seeing when we “see” a comet is the coma, a cloud of gas and dust that has been boiled off the nucleus—the dirty snowball itself—by sunlight and that surrounds the nucleus before being blown away by the solar wind, the stream of particles emitted by the Sun. If we’re lucky we also see a tail (or two; see bottom photos) formed from the gas and dust. Although the tail seems to be following the comet through space, it will be on the side of the comet opposite the Sun because it is being shaped and moved by the solar wind. Comets are named after their discoverers. Here is a stunning photograph of the spectacular 2007 Comet McNaught made by its discoverer, Dr. Robert H. McNaught, an Australian astrophysicist. Unfortunately for those of us who live north of the Equator, Comet Mcnaught was seen only poorly from northern latitudes. |
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Comet 17P/Holmes and the star Mirfak on November 13, 2007 |
This photo was made through moderately dense ground fog in an otherwise good sky. The fog may be seen around Mirfak. A rotational gradient applied in Photoshop causes stars in the coma to be portrayed unrealistically, but emphasizes the jets coming from the nucleus. The star field outside the coma is accurately portrayed. |
Comet 17P/Holmes was as obscure as a known comet can be until October 24, 2007. Within a few hours on that day Comet Holmes became as much as a million times brighter than its typical 17th magnitude. It brightened to at least magnitude 3, which made it an easy object for the unaided eye, even in many urban areas where little can be seen in the night sky. The presumed cause of the brightening is a sudden release of a large quantity of gas and/or dust that reflects sunlight back to the Earth. No tail was evident in early photographs. |
