A bright photogenic patch of the southern Milky Way holds one of the most enigmatic and exotic stars known. Eta Carinae is the centerpiece and ionizing star of the great HII region, the Eta Carinae Nebula. The nebula itself spans some 260 light years across, about 7 times the size of the Orion Nebula. Massive is an understatement as the great star weighs in at some 100 to 150 solar masses and shines with the light output of 5 million suns. As one of the most massive stars known, Eta Carinae pushes the theoretical limits on energy output of stars and has attracted much interest among astronomers trying to understand the physics of supermassive stars. The young supergiant star ( only 2 to 3 million years old) pumps out as much energy in 6 seconds as our sun does in an entire year. Its prodigious stellar wind blows off the equivalent mass of Jupiter each year, exceeding our suns yearly rate of mass loss a 100 billion fold.
The stars explosive variability has been studied extensively although much still remains speculative. Incredibly during three hundred years of observations the star has ranged in brightness wildly from magnitude -0.7 to magnitude 7.6. The variability has been shown to have a periodicity of 5.5 years. The current belief is that Eta coexists in a binary state with another massive supergiant of some 30 to 60 solar masses. At their closest approach every 5.5 years the two supergiants approach within 2 to 3 astronomical units of each other allowing their powerful stellar winds to collide, with a subsequent enormous outflow of light and radiation. The outflow is apparently responsible for the step up in brightness. In fact astronomers observed a dramatic 10 fold increase in the stars brightness in a 5 year period during the 1840's (when it brightened to mag -0.7, also known as Nova Carinae 1843). Recent HST images reveal a large bilobed nebula which surrounds the star having the mass of about 10 to 15 suns. The nebula stretches about 0.8 light years and is believed to have formed at the time of that outburst. The gases of the nebula are rushing outward at 2000 kilometers per second.
Because of its extraordinary mass the star is certainly expected to end as a great supernova in the near future. An energy outburst of this order could possibly devastate starfields and planets within a few thousand light years radius.