CHICAGO, Ill., June 9 -- One of the best-known stars in the southern night sky, Eta Carinae, has suddenly doubled in brightness. This action leaves astronomers puzzled; they theorize that the star may be undergoing a massive eruption, much as it did more than 150 years ago. Occasionally something happens in astronomy that is so bewildering that it makes astronomers nervous, said Kris Davidson, an astrophysicist from the University of Minnesota. This is weird. We don't understand. Eta Carinae has attained a certain prominence in astronomy lore due to its size, its history and its expected fate. The star is one of the most massive and energetic in the known universe, with a size equal to that of 100 suns. Astronomers predict that sometime in the next 10,000 years, Eta Carinae will collapse suddenly and then explode. The resulting gamma ray bursts might even be hazardous at 7,500 light years, the distance from the star to Earth, according to Davidson.