Comets
For thousands of years people have viewed comets in the night sky. Ancients Greeks called comets hairy stars. The most famous comet is Comet Halley which returns every 76 years. It was visible in 1910, 1986 and will return in 2061.
At the centre of a comet is the nucleus, a jagged chunk of frozen ice and dust less than ten kilometres in diametrer. Surrounding the nucleus is the coma, a large cloud of gas shaped like a ball. Extending out behind the coma is a long tail of gas and dust.
Scale model of a comet showing the size of the coma and tail
Astronomers think the nucleus of a comet originates from the outer solar system beyond Neptune where the temperature is colder than -200 degrees Celsius. There are two possible sources of comets, the Kuiper belt just beyond Neptune and the Oort cloud. The Oort cloud is thought to be located about 100,000 times further from the Sun than the Earth. It surrounds the Sun, allowing comets to fall toward the Sun from any direction. When near the Sun, heat melts the outside of a comet, producing a long tail that points away from the Sun.
The International Astronomical Union is responsible for assigning names to a comet. Normally when a comet is observed in the night sky for the first time it is given the surname of the astronomer who discovered it. For example, Alan Hale a professional astronomer and Thomas Bopp an amateur astronomer discovered a new comet in July 1995. Named Hale-Bopp, it become bright enough in early 1997 to be visible from suburban areas. Other famous comets are Halley and Shoemaker-Levy 9 that crashed into Jupiter in 1994.
