Ever since the 2000 event, the star, Rho Cassiopeiae, has been restless, its atmosphere pulsating wildly, with no cadence. Its outer atmosphere now seems to be collapsing again, an event that looks similar to one that presaged the last outburst. Lobel's international team of researchers said another eruption -- possibly a stronger one -- could be imminent.

Rho Cas, as it is often referred to, is visible to skywatchers in the Northern Hemisphere who have dark sky conditions away from city lights, prompting the researchers to suggest amateur astronomers might spot the next explosion. It would be evident, however, as no more than an increase in brightening and then a decrease. Lobel said it's possible the star could briefly become so dim as to be invisible to the naked eye and exit the night sky altogether, though the chances of that occurring are slim.

An eruption would pose no danger to Earth, and in fact would likely be spotted only by seasoned skywatchers who know how to navigate the sea of stars surrounding the relatively dim Rho Cas.

The star shines at magnitude 4.5 on a scale astronomers use to note brightness. Larger numbers represent dimmer objects, with magnitude 6 being about the dimmest visible to most people. Very bright stars are near or below zero magnitude.

Rho Cas is faraway compared to most stars visible at night, more than 10,000 light-years. It is visible because it is among the most massive stars known, 20 to 40 times more heavy than the Sun, and shines nearly a million times more brightly than the Sun. If it replaced our Sun, its girth would consume Earth and Mars.

Only about a dozen such hypergiants are known in the Milky Way Galaxy.

The star was first seen erupting in 1946, dimming by a factor of six, to sixth magnitude, as it lost material. But researchers learned little at the time about the eruption process. The 2000 eruption was watched by five telescopes and was also seen by some amateur astronomers around the world. The star brightened briefly, then dimmed.

Lobel's team then analyzed the event's spectra, or separated light, to determine temperature changes and learn how much material was shot into space. They also found that the star cooled by several thousand degrees during the eruption.

"We were exceptionally lucky to witness a stunning explosion in Rho Cas and to have acquired the spectra to make a quantitative analysis possible," said Smithsonian astrophysicist Andrea Dupree. The research will be detailed in the Feb. 1 issue of the Astrophysical Journal.

Dupree explained what the event might look like to a viewer on a hypothetical planet surrounding Rho Cas: "The whole sky would be lit by the wrath of an angry star," she said. "I'm sure glad we're 10,000 light years away."

The events observed recently actually took place more than 10,000 years ago, for that is how long it took the light to traverse the space between the star and Earth.

All stars leak mass into space as they age. Our own Sun will begin to expire in a few billion years. The most massive stars shed matter explosively and more quickly, so they don't live long. That's why there are not many of them around. Eventually, Rho Cas will die in a final, colossal supernova explosion that will tear its nearby space apart.

Lobel said because of the star's advanced age, it could go supernova in as little as 50,000 years.

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