A comet that until recently was invisible to the naked eye has become a shining star in the night sky, easily seen even through the bright lights of the Pittsburgh region.

Comet Holmes last week was orbiting the sun between Mars and Jupiter when it inexplicably brightened about a million times. It went from being a nondescript object difficult to see through a telescope to a luminous, fuzzy blob -- without a discernible cometary tail -- that's visible virtually all night in the constellation Perseus, moonlight and light pollution notwithstanding.

"We certainly have seen comets that have had a brightening period, a burst of some level, sometimes quite dramatic, but nothing a million fold," said John Radzilowicz, the Carnegie Science Center astronomer. "So that's got everybody's attention."

A previously recorded outburst of Comet Holmes led to its discovery in November 1892. British amateur astronomer Edwin Holmes was the first to see the comet, which reached the brightness of a less-than-average star.

The current outburst, which far outshines the 1892 appearance, is a welcome surprise and adds to the starscape of Perseus, in the northeast at nightfall.

"It looks like a fuzzy star ... almost as bright as the North Star, Polaris," said Tom Reiland, of Shaler, director of Nicholas E. Wagman Observatory in Deer Lakes Park, Frazer. "It doesn't look like what a comet a little fainter than second-magnitude normally would look like. In binoculars, you'll see a disk, sort of like a slightly compact cloud-like disk."

The veteran astronomical observer has seen about 140 comets and said this one is in a class by itself, at least in terms of being peculiar.

"I was amazed to see something that bright that ... it just looked like a star," he said. "This one is one of the most unusual if not the most unusual I've ever seen. It's changed so much in the last five days or so."

Mr. Reiland said the comet may be visible for months. And though it's bright, Comet Holmes appears to be sailing without its signature tail.

"You have to remember this object is pretty far out" in the solar system, Mr. Radzilowicz said. "So it's out in a region where we don't often see the tail start to develop -- but we also don't see this kind of activity."

To locate the comet, find the bright star Capella in the north-northeast. The star Mirfak, slightly dimmer, is above and to the right of Capella. The comet is just below and to the left of Mirfak.

It's an unmistakable interloper to the autumnal sky.

The comet's sudden brightening is no need for concern. It's not on a collision course with Earth.

"It won't get beyond, or get past Mars," Mr. Reiland said. "Its closest point to the sun is twice the distance of the Earth to the sun."