How are stars created?

Today, we know the basics: a star begins as a huge, swirling cloud of gas and dust that gradually spins into a disk. The spinning disk, which will eventually become a new solar system, gathers a dense ball of matter at the center, with mass flowing from the disk onto the ball in a process called “accretion.” If the ball becomes massive enough to fuse atoms of hydrogen, then a star is born, powered by the nuclear fusion at its core.

Researchers still don’t know exactly how or why the clouds collapse to form stars, and how it accretes the mass through the disk. But Ian Stephens, a postdoctoral associate at BU’s Institute for Astrophysical Research, has brought science one step closer to understanding the process of star formation.

For the first time, Stephens has directly observed the magnetic field in the disk of a baby star—thought to be critical for a star’s birth—and found its shape to be more complex than anyone expected. The discovery, published on October 30, 2014, in Nature, gives new clues—and raises new questions—about how stars are born. And because the baby star that Stephens examined will be similar to our sun, his work may shed light on the origins of our solar system.

Stephens is investigating what astrophysicists call the “accretion problem.” As clouds of rotating gas and dust collapse, they begin to accrete, or deposit, matter onto a core. But, like a spinning figure skater pulling her arms in tight, the cloud spins faster and faster as it gets smaller and smaller. At some point, the spinning dust cloud seems to violate the laws of physics, leaving astrophysicists scratching their heads.

“The clouds that collapse and form stars are already spinning. But when they collapse into the baby star, the gas would be spinning so fast that, according to the equations, it should actually spin apart,” says Stephens. “But, obviously stars do form, so how does the mass get on the star? That is a big problem.”

A few years ago, a California telescope called CARMA (the Combined Array for Research in Millimeter-wave Astronomy) was outfitted to measure magnetic fields by analyzing the polarized light emitted from the dust. Stephens decided to point the telescope at the disk of a young star called HL Tau, about 450 light years away. And for the first time, he found the predicted magnetic field.

Artist’s conception of a dusty disk orbiting a whirling young star. The green lines represent the magnetic field. Image courtesy of NASA/JPL-Caltech

“I’m not the first person to try this, I’m just the first person to try this successfully,” says Stephens. “Other people have pointed it at the biggest disk or the closest disk, but I pointed it at the brightest. And that worked.”

Stephens found that shape of the magnetic field was more complicated than anyone expected. Some models had predicted it would be circular or “toroidal” and others suggested it would be moving toward the disk’s poles, or “poloidal.” It was neither.

“To me, the finding says that the models we’re using are too simplified,” says Stephens. “At the same time, for the models to really have constraints, we really need more observations.”

Stephens is planning another round of observation for 2015, at the $1.1 billion Atacama Large Millimeter/submillimeter Array telescope in Chile, the best telescope in the world for making the long-wavelength measurements that interest him. He’ll be collecting data for HL Tau again, hoping to add at least one more piece to the puzzle of star formation.