The most explosive star in our corner of the galaxy has a companion, astronomers announced today.

"Until now, Eta Carinae's partner has evaded direct detection," said Rosina Iping, a research scientist at Catholic University of America in Washington. "This discovery significantly advances our understanding of the enigmatic star."

Eta Carinae is one of the most massive stars in the Milky Way, packing 100 times more material than the Sun. It's visible from the Southern Hemisphere, residing in the constellation Carina.

It has erupted before, will likely erupt again, and it's only 7,500 light-years away.

Scientists have long expected that Eta Carinae was not alone. The huge star's strange behavior can best be described by putting another star into the setup.

Eta Carinae is thought to be too cool to generate X-rays, yet X-rays emanate from the region. Oddly, every five years the X-rays disappear for about three months. Astronomers theorized the X-rays were created when the outflow from Eta Carinae ran into the outflow from an unseen companion. Perhaps every five years, the thinking went, the orbits of the two stars put Eta Carinae in front of the collision site, as seen from Earth.

But direct evidence was lacking.

Now NASA's Far Ultraviolet Spectroscopic Explorer (FUSE) satellite has weighed in.

The companion is thought to be much hotter than Eta Carinae and ought to be brighter in ultraviolet light. FUSE detected far-ultraviolet light from Eta Carinae, right before an expected X-ray eclipse, and then watched the UV light vanish.

Eta Carinae eclipsed the X-rays at the same time it blocked the UV light from the companion star, the astronomers conclude.

"This far ultraviolet light comes directly from Eta Carinae's companion star, the first direct evidence that it exists," said George Sonneborn, FUSE Project Scientist at NASA's Goddard Space Flight Center. "The companion star is much hotter than Eta Carinae, settling a long-standing mystery about this important star."

This discovery is to be published today in the Astrophysical Journal Letters. The data behind the finding did not yield a conventional photograph.

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