Birth of a Galactic Heavyweight

On a clear winter’s night, Orion the hunter twinkles in the evening sky. Just below the three conspicuous stars of his belt, in the middle of his sword, lies a spectacular cloud of gas and stars: the Orion Nebula. Tenuous filaments cradle bright newborn stars at the nebula’s center.

Astronomers have long struggled to understand the origins of the brightest and biggest newborn stars. Now, after decades of uncertainty, they believe they’re finally getting close to an answer.

In the past year, at least two teams have used supercomputers to run three-dimensional models that successfully simulate the formation of those big stars. For Earth-bound astronomers, who see but can’t touch the objects they study, making stars on a computer is one of the only ways to test their star formation theories.

Out in the galaxy, stars form when dense clouds of gas molecules and dust grains collapse together from their mutual gravitational pull. Once a young star turns on, its light pushes away whatever gas and dust lingers nearby. Big stars like those in the Orion Nebula, which are very bright, push away their formation gas with particular vehemence.

These galactic heavyweights — anything bigger than ten times the sun’s mass, or ten “solar masses” in astronomy lingo — created most of the atoms in the universe heavier than hydrogen and helium. “They make the rocks we stand on, the air we breathe, and the atoms in our bodies,” said Nathan Smith, an astronomer at the University of California, Berkeley who has studied Eta Carinae, possibly the most massive star in the galaxy.

“They’re very rare,” said Phil Massey, an astronomer at the Lowell Observatory in Flagstaff, Ariz. “For every 20-solar-mass star in the Milky Way, there are a hundred thousand solar-type stars.” But despite being vastly outnumbered by smaller stars, these galactic titans produce most of a galaxy’s light.

Another rare beast, it turns out, is a working computer simulation of such massive stars.

Early computer models of massive star formation failed for the same reason the Orion Nebula is so spectacular: radiation. Harold Yorke, an astronomer at the Jet Propulsion Laboratory in Pasadena, Calif., said that in models, it is hard to form stars of even a relatively paltry seven solar masses, because the star’s radiation will push away the gas and dust before it can fall onto the star. Astronomers refer to the light’s push as “radiation pressure.”

The radiation pressure is the central problem for astronomers’ star-formation models: a young star becomes bright enough to push away its own formation material before it even reaches the 10-solar-mass mark, never mind 100.

The solution involves a pit stop for the gas and dust in a disk of material surrounding the star, which is so dense and heavy that the radiation pressure can’t blow it away.

In a recent simulation, published in the journal Science in February 2009, a group of California astronomers successfully created a pair of stars with 33 and 47 solar masses out of a 100-solar-mass cloud. The gas in their model first falls into a disk around each star. Material then funnels from the disks onto the stars.

Disks have been known to be key since as early as 2002, when Yorke and a collaborator published two-dimensional simulations — which looked at a single slice of the star and assumed it was the same all the way around — that used a disk to create a 40-solar-mass star.

The recent simulation by the California team was more realistic because it was modeled in a full three dimensions. The three-dimensional model was feasible because of advances in both computing power and in the computer programs themselves, according to Mark Krumholz of the University of California, Santa Cruz, who was the lead author on the Science paper.

Another recent model, which has yet to be published, comes from a different team of theorists in Heidelberg, Germany and New York City. They began with a 1,000-solar-mass cloud of gas and dust and ended with a 30-solar-mass star. Their simulated cloud also created 30 new stars, rather than just two in the California model.

Since big stars are usually seen in groups — like in the Orion Nebula — forming many stars at once matches what’s really out there in the sky.

In general, star formation is very hard to see, which is part of the reason simulations are still a work in progress. The dense stellar nursery clouds block nearly all of the light from the forming stars. Astronomers can observe them only at particular radio wavelengths that sneak out, and even then, they can’t see the whole star forming region at once, said Debra Shepherd, an observer at the National Radio Astronomy Observatory in Socorro, N.M.

Still, it’s important for astronomers to try to observe big stars as they form, in order to support — or refute — the models. Yorke pointed out that no stars above 25 solar masses have been observed with disks around them, so simulations should also explain how and when the disk disappears.

Despite the observational challenges, some star formation theorists think the process is close to being understood. Mordecai-Mark Mac Low, an astronomer at the American Museum of Natural History in New York who was part of the Heidelberg team, said: “I think that our work, in combination with several other recent efforts, is actually pinning down the basic picture.”

Astronomers seem to be well on their way to unraveling the mysteries that the Orion Nebula shrouds.