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Images trace history of Halley's outburst.

Comet Halley, normally a quiescent body of ice and dust when it's not whizzing past the sun, took astronomers by surprise last February when they discovered it had suddenly brightened some 2 million kilometers away from the sun. Scientists had never observed such an outburst in a comet so far from the solar spotlight (SN: 3/2/91, p.133).

Astronomers have now released four false-color images that depict Halley before and after the event. The photos, made with the Danish 1.5-meter telescope at the European Southern Observatory (ESO) in La Serena, Chile, focus on the comet's coma, the cloud of dust and gas that shrouds its icy nucleus.

Near the sun, ice from a comet's nucleus steadily vaporizes, replenishing the ever-dispersing coma. Dust in the coma reflects sunlight, making Halley readily visible from Earth. But when a comet moves away from the sun, as Halley has since 1986, its temperature drops and most of its core material can no longer vaporize. Thus, as the coma's gas and dust disperse into space, relatively little new material emerges to replenish it, leaving only a bare, dim nucleus.

Photos of Halley a few years before the outburst (top images) illustrate this scenario, showing the decrease in the brightness and size of the cloud. But a photo taken last February (bottom left) shows a brighter, denser coma. A month later (bottom right), the brightness declined and the cloud became larger and more diffuse. White and green denote the highest intensities, red and blue the lowest.

Using these images, ESO astronomers calculate that the outburst occurred during the third week of last December. It remains unclear whether an internal eruption or a collision triggered the event. Moreover, it's possible that the coma received its enhanced supply of dust and gas -- about one-millionth the mass of its nucleus -- in two or more separate events.
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Title Annotation:why the comet suddenly brightened
Author:Cowen, Ron
Publication:Science News
Date:Jun 15, 1991
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