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Comet with aurora
Comet Hale-Bopp seen over trees silhouetted by an aurora borealis display. Hale-Bopp was one of the brightest comets of the 20th century, and was seen for much of early 1997. A comet is a lump of ice and rock. Its tail is formed as its ice evaporates as it nears the Sun. The tail always points away from the Sun, and may stretch for millions of kilometres. The aurora borealis is an atmospheric phenomenon caused by the interactions of charged particles from the Sun with the Earth's magnetic field. The magnetic field channels particles to the poles, where they excite gas molecules in the upper atmosphere, causing them to glow. Photographed from Finland in spring 1997.
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