What had been a modest comet seen only with binoculars or telescopes flared up this week to become visible to the naked eye.
Comet Swan, as it is called, is in the western sky after sunset from the Northern Hemisphere. It remains faint, likely not easy to find under bright city lights but pretty simple to spot from the countryside.
It is a "fairly easy naked-eye comet," said Pete Lawrence, who photographed the comet from the UK. "The tail is now showing some interesting features too."
Late Thursday, however, Lawrence reported that the comet already may be getting dimmer. It is not clear what skywatchers should expect of this comet.
The comet, also catalogued as C/2006 M4, is about halfway up in the sky in the direction of the constellation Corona Borealis.
As with most comets, this one looks like a fuzzy star. It has an interesting green tint, however, indicating it has a lot of the poisonous gas cyanogen and diatomic carbon, astronomers say.
Sam Storch, a long-time sky watcher from Long Island, NY, said the comet appears "quite a bit deeper than any other green I have seen in any sky object, even planetary nebulae."
"Comet Swan is very easy to find," said Joe Rao, SPACE.com's Skywatching Columnist. "In good binoculars it appears as a bright, symmetrical and surprisingly green blob."
Comets, the stuff of legend and myth, are frozen leftovers of the solar system's formation. Most orbit the Sun out beyond Neptune, but a few wander through the inner solar system now and then. As a comet gets closer to the Sun, solar radiation boils the frozen gases, along with dust, off the comet's surface. Sunlight reflects off this material, creating a head, or coma. Some comets never get very bright. Others brighten dramatically. Some even come unglued as they round the Sun.
Some comets, like Swan, also sport a tail or two. Such detail is best seen with binoculars or a small telescope.
Comet Swan was discovered last year. It makes its closest approach to Earth today. Eventually it will return to the distant reaches of the solar system. Rao said nobody knows how long the comet will grace the night sky.
