Click on the image for QuickTime Movie of
Stars Can't Spin Out of Control
This artist's animation demonstrates how a dusty planet-forming disk can
slow down a whirling young star, essentially saving the star from spinning
itself to death. Evidence for this phenomenon comes from NASA's Spitzer
Space Telescope.
The movie begins by showing a developing star (red ball). The star is
basically a giant ball of gas that is collapsing onto itself. As it
shrinks, it spins faster and faster, like a skater folding in his or her
arms. The green lines represent magnetic fields.
As gravity continues to pull matter inward, the star spins so fast, it
starts to flatten out. The same principle applies to the planet Saturn,
whose spin has caused it to be slightly squashed or oblate.
A forming star can theoretically whip around fast enough to overcome
gravity and flatten itself into a state where it can no longer become a
full-fledged star. But stars don't spin out of control, possibly because
swirling disks of dust slow them down. Such disks can be found orbiting
young stars, and are filled with dust that might ultimately stick together
to form planets.
The second half of the animation demonstrates how a disk is thought to
keep its star's speed in check. A developing star is shown twirling inside
its disk. As it turns, its magnetic fields pass through the disk and get
bogged down like a spoon in molasses. This locks the star's rotation to
the slower-turning disk, so the star, while continuing to shrink, does not
spin faster.
Spitzer found evidence for star-slowing disks in a survey of nearly 500
forming stars in the Orion nebula. It observed that slowly spinning stars
are five times more likely to host disks than rapidly spinning stars.