Comet Holmes, October 27 and 30, 2007. Certainly one of the most unusual celestial objects I've seen. A week or two before these photographs were taken, Comet Holmes was quietly receding in obscurity from the inner solar system, and it was so dim (thousands of times dimmer than the stars in the photographs) that it could only be glimpsed through a very large telescope. For some unknown reason it suddenly brightened almost a million fold like a candle turning into a searchlight!
A reflective cloud of dust and gas started to expand rapidly around the comet core, and within a few days it had a volume comparable to the planet Jupiter, even though the core of the comet is probably only three or four kilometers across. The comet at this point was about 240 million kilometers from Earth. To get a sense of scale, at that distance the Earth would appear about the size of the small "o" in the word October labelled on the photographs.
These two comparison photographs were taken with exactly the same photographic equipment. They show how the dust cloud became dimmer as it expanded, and a comparison of the star patterns show how much the comet travelled along its orbit in three days. No one knows for sure why this normally inconspicuous comet had this sudden, dramatic change in its appearance.
Three weeks later the comet looked like this.
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