Astronomers try to solve mystery of bulging stars

— -- Obesity has apparently reached galactic proportions, apparently — even stars have a problem with over-eating, a stellar mystery that has astronomers asking how some stars grow to be so large. A supercomputer scientist team may have the answer, in a demonstration of how nature can take something simple and make it complex.

About 7,500 light years away resides the star Eta Carinae, first noted in a star catalogue by the venerable astronomer Edmond Halley (best known for Halley's comet) in 1677. Since then, the star has bedazzled astronomers by brightening every century or so, most notably in an 1843 outburst that briefly made it the brightest star in the sky, despite its distance (one light year is about 5.9 trillion miles.) To add to the mystery, the star is a porker, about 120 times heavier than the sun, embedded in a star-forming gas cloud called the Carina Nebula.

University of California, Berkeley, astronomer Nathan Smith proposed last year in the journal Nature that 1843 outburst arose from an explosion deep in the star that spat out a blob of star-stuff perhaps ten times as heavy as the sun. The report counts the explosion as a newly-discovered mechanism for how gigantic stars, which only live a million years or so, start to break down prior to their final implosion (the process that forms black holes.)

But that left the bigger mystery, which is how do stars like Eta Carinae get so big in the first place? "We see stars at least 120 times that of the Sun throughout space, but the mechanism by which the most massive stars form is a longstanding mystery," says astronomer Mark Krumholz of the University of California, Santa Cruz. Astronomers know that stars form by the clumping together of gas clouds. These "proto-stars" ignite through nuclear fusion once they become about 80 times heavier than the planet Jupiter.

Stars are thereafter balancing acts, Krumholz explains, in which the inward pull of gravity from the center of a star fights against the outward "radiation pressure" from light and other electromagnetic radiation emitted by its nuclear furnace. Where the push and pull of the two forces balance out, you have the surface of the star.