Hairy Stars

11/12/07 – 11/19/07
by C. Zaitz

We have a comet in the sky! By now you’ve heard that Comet Holmes has exploded into a naked eye comet and has been haunting the northeastern sky near the constellation Perseus for weeks now. Even in ancient times, comets have appeared in the sky, only to disappear weeks later, leaving folks wondering what they were. Labeled “hairy stars” due to their fuzzy appearance in the sky, the Latin word “coma” literally means “hair” and this comet has quite a head of it! Now said to be over 7 times the diameter of Jupiter, the coma of comet Holmes is an amazingly giant, diffuse ball of fluorescent gas and dust, visible to the naked eye.

Comets are made of ice, gases, and dust. The nucleus, which is the main part of a comet, can be a mile or several miles in diameter, but it is the coma, or the cloud of gas and dust surrounding the nucleus, that becomes huge as the ice melts. The melting is more like dry ice turning straight into a gas rather than an ice cube melting into a puddle. As the gas is released, dust grains are also released, and depending on their size, they will travel a certain distance from the nucleus, puffing out into the coma and tail. Comet Holmes has had more of an explosion than a mere melting, it seems, as it has grown an enormous coma. It doesn’t have much of a tail, though. Usually comets grow two tails, one made of gases and the other made of heavier dust grains. The comet drags the tails around the sun, but they are always driven away from the comet by the pressure of the sun’s own radiation, so no matter where the comet is going, the tails always point away from the sun. Comet Holmes is too far from the sun to grow much of a tail.

Many comets come from a distant place in our solar system, beyond any of the known planets. Far beyond the orbit of Pluto, the Oort cloud is thought to extend halfway to the closest star to us and is home to trillions of comets. Some comets live closer to us. Comet Holmes orbits every 7 years, and was discovered in 1892. Orbiting between Mars and Jupiter, it usually passes unnoticed. This time around, some of the ice must have broken off like our melting glaciers of earth, releasing an enormous amount of dust and growing its huge coma. We get to enjoy its brilliant hairy appearance for as long as it continues to glow.

People are often surprised that comets can be seen night after night, because they associate them with shooting stars, or meteors. It’s a common mistake, most likely because of their streamlined, zippy appearance. With the tail blasting out, looking like a rocket to Mars, comets definitely are the speedsters of the solar system. They do move swiftly, clocking over 100,000 mph, but are so distant that you cannot detect their motion over the course of one night. They seem to hang in the sky for weeks, giving us a view of the mysterious, changing nature of the universe, or at least the solar system. Soon comet Holmes will die down and once again become an icy, dirty snowball, one of many silent, small members of our solar system.

Until next week, my friends, enjoy the view.