As it approaches, ices in the comet vaporize, creating a bright envelope and gas and dust, called a coma, around the nucleus. The heating also generates two tails, each of which can span more than 1 million miles in length. One is made of dust and the other comprised of molecules that have been ionized by sunlight.
Pan-STARRS, which is due to make its closest approach to the sun on Sunday, is the first of two comets expected to pass by Earth this year.
In November, Comet ISON, discovered just this past September, should fly four times closer to the sun than Pan-STARRS. If Comet ISON survives the encounter, it could be as bright as the full moon and possibly even visible in the daytime skies.
"Comet ISON is a much bigger unknown. It gets much closer to the sun than Comet Pan-STARRS so it will get a lot more heating and thus a lot more activity -- and more chance of breaking apart," Gal said.
"If it survives it's close passage by the sun, ISON could, for a short time, be visible in daytime. That would make it a ‘comet of the century.' We usually see about one comet every 100 years that gets so bright," he added.