A Star is Born

August 24, 2008

"Black hole sun
Won't you come
And wash away the rain...."
     -- Soundgarden

One of the biggest stars to emerge from the Seattle grunge music scene is Soundgarden, and one of their biggest hits was the melancholic "Black Hole Sun." (You can see the freakishly surreal music video here.) Frontman Chris Cornell, who penned the tune, freely admitted that the lyrics made no sense, despite fans' attempts to uncover deep hidden meaning. What the heck is a "black hole sun," anyway, other than the remnants of a cold, long-dead massive star after it's gone supernova?

Surprisigly, it seems that black holes can play a role in creating massive stars, not just forming out of the death throes of expiring ones. Black holes might not strike most people as especially, um, maternal. They have a nasty reputation for ripping apart anything that tumbles into them with powerful gravitational forces, which seems to preclude the possibility that baby stars could form in their vicinity. And yet,there are young massive stars in the center of our own Milky Way galaxy orbiting elliptically around a supermassive black hole. Like Cornell's lyrics, it just didn't make sense.

But now, a new computer simulation by a pair of Scottish scientists is shedding light on the mysterious process by which massive stars can indeed form around massive black holes. And it's prompting astronomers to rethink the finer points of their current understanding of star formation.

It's safe to say that the primary "wombs" for birthing baby stars are the nebulae found inside certain galaxies. Made out of gas and fine particles of dust containing various elements, they look like clouds, often stretching several light years across, despite being thin as a wisp of smoke. While they might seem to just be loitering about in interstellar space, waiting for their close-up (nebulae are very photogenic) and obscuring our view of other distant celestial objects, they are actually undergoing turbulent birthing pangs. The Orion Nebula, for instance, poses prettily for the Hubble Space Telescope in the image below, but also houses any number of massive young stars, at various stages of development, in its pillars of gas.

Nebulae aren't very dense; stars are. It takes a fair amount of gravity to compress all that dusty material and hydrogen and helium gas into a small (relatively speaking) ball of matter. As the ball continues to contract under its own gravity, its core heats up and starts to rotate. Now it's called a protostar.

If the mass of a protostar is too low, it never really "ignites" and becomes a brown dwarf. But the really massive protostars  -- the size of our sun, or larger -- eventually start converting hydrogen into helium via thermonuclear fusion. And thus, a star is born. (The pop group They Might Be Giants has a classic tune about this: "Oh the sun is a mass of incandescent gas/A giant nuclear furnace....")

The problem with stars forming around black holes is that the initial molecular clouds would be ripped apart by the hole's immense gravitational energy, rather than condensing into a dense stellar core. This does, indeed, happen, according to the new simulation involving giant gas clouds 100,000 times more massive than the sun, but the story doesn't end there. There's a twist.

Ian Bonnell of the University of St. Andrews and Ken Rice of the University of Edinburgh found that as the clouds are pulled apart, the gas forms a spiral-patterned accretion disc as it orbits the black hole. Some of the kinetic energy from the gas passing close to the black hole gets transferred to gas on the outer edges of the spiraling cloud remnant. The result: only part of the cloud meets its doom inside the black hole; the rest escapes and fragments to form massive stars that inherit their strange elliptical orbit from that original spiral pattern.

Not only that, but sufficiently massive stars can sometimes give birth to their own baby stars, as a new multi-generational "family portrait" by NASA's Spitzer Space Telescope illustrates (see below). Current theory holds that if a star is massive enough, some of its material slides off in the form of winds. Combined with the scorching-hot radiation, these winds blast away nearby clouds, forming cavities that expand.

As the cavities expand, gas compresses along the "rims," giving birth to generations of new stars. Ultimately you get a kind of "family tree" of stars, with the oldest in the middle of the cavity and younger ones further out -- kinda like tree rings. It's a complex, cosmic cycle of life, and it seems black holes play a role in the evolution of stars all the way from the cradle to the grave. How cool is that?

Photos: (top) Hubble image of the Orion Nebula. Credit: NASA/ESA, via Wikimedia Commons. (bottom) Infrared image of W5 nebula taken by the Spitzer Space Telescope. Source: NASA/JPL/Caltech.