Gravity's growing role in 'The Starry Night'
Jaime Pineda and Jonathan Foster, Harvard University
Talk about astronomy imitating art; Vincent van Gogh would have approved.
This 21st-century "live" version of 'The Starry Night' covers a patch of sky that has become a proving ground for a new way to study cloudy, molecule-rich nurseries where stars and planets are born.
In essence, astronomers have taken a page from modern medical imaging to develop 3D graphics of the cloud. This allows researchers to detect more structure in the cloud than they can see using 2D representations.
One intriguing result: Gravity appears to play a bigger role earlier in the star-forming process than some current models suggest, says Alyssa Goodman. She's an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and the leader of the research team.
Getting a better handle on star formation is important in reconstructing the bigger picture of how galaxies have grown and evolved over the past 13.7 billion years. Researchers are now adopting this approach to studying star-forming regions in other galaxies -- albeit with far less detail than they enjoy looking at clouds in our home galaxy, the Milky Way.
Their target: A molecular cloud dubbed L1448 in the constellation Perseus. It's about 100 light-years across. The patch they worked with is about 1 light-year across. What they see could prompt astrophysicists to ratchet back the number of stars they estimate these clouds can produce.
Hitchhicker's Map to the Molecular Cloud
Typically, astronomers have built up maps of the distribution of mass in these clouds. They look a lot like a hiker's topographical trail map -- oddly shaped loops of lines nested inside one another. Each "loop" in a molecular cloud's map represents a different value for the total mass present along a telescope's line of sight at different places in the cloud.
But contour lines on a hiker's map say nothing about the how different rock types are distributed under the surface of a mountain summit. Likewise, the contours on the molecular clouds' mass maps say nothing about whether or how mass found deeper in a section of the cloud is clumped. Those clumps are important: They represent the seeds of new stars and solar systems.
Goodman and her team took data on the cloud and derived rough distance estimates to various features along the line of sight. Then they used special software they've developed to, in effect, slice the cloud along the line of sight like so much cosmic salami.
By recombining the slices, which give a sense of depth to the graphical representation of the cloud, they could tease out information about how matter in the cloud was clumped in three dimensions.
The approach is yielding insights into a particular patch of the star-formation time line, when a slowly compressing hunk of a molecular cloud is somewhere between 3.3 and 0.33 light years across. The question: Is the impetus for a buildup of clumps into self-gravitating objects, well, gravity? Or do the self-gravitating clumps form initially because of turbulence within the molecular cloud?
Everybody knows that gravity is present on all size scales, Goodman explains. But models -- include some she acknowledges using -- don't take gravity into account. It's thought to be too weak on those distance scales given the distribution of dense clumps seen in two dimensions. So models only consider turbulence as a clumping agent.
But she adds, unclumped gas itself exerts gravity. So do stars that already have formed in the cloud, but may still be hidden from view. Add the third dimension, however, and more clumps, including those masking stars, appear than meets the 2D eye. This boosts the likelihood that gravity is playing a significant role as well.
And inside those cocoons?
Two scenarios are competing for favor. Both assume the collapse continues.
But one holds that as a clump collapses, its gravity can suck up additional gas from its surroundings. In effect, when the star finally ignites, its mass will combine the mass of the original clump, plus any additional gas it's been able to snag. Estimates suggest this growth can result in a star with 10 to 100 times more mass than its original clump had.
The other holds that the clump has all the gas it's ever going to get. It may break into smaller clumps and form binary or other multi-star systems. Tatooine's dual suns come to mind. But gravity is insufficient to play any further role in the stars' final heft.
Neither is fully supported by available observations, Dr. Pudritz notes in an email. "That is why I am personally optimistic" about the Goodman team's approach.
As for Goodman, she says her results suggest that gravity could well be at work on and among these clumps.
"Our paper says that if some of those blobs are gravitationally bound to each other, then over their lifetimes they will gravitationally interact. They won't interact a lot in the particular region we studied," she says. But the results suggest that, as often happens, the truth will embrace elements of each camp's ideas.