Making Massive Stars

Twinkle, twinkle little star,
My, oh my, how large you are.
How’d you get to be so big? When gas pressure and magnetic fields should have prevented you from forming?

Okay, so my poem doesn’t rhyme, but it does communicate two important issues regarding stars. First, astronomers have detected many stars with masses ranging from 20 times the mass of the Sun to around 150 times. Second, astronomers struggled to understand how such massive stars form.

Gravity, radiation pressure, gas pressure, and magnetic fields constitute the dominant processes in star formation. The basic process starts when a gas cloud becomes sufficiently dense that it begins to collapse under its own gravity. As the cloud collapses, the radiation pressure and rotational energy of the gas and the magnetic fields permeating the cloud grow larger. In relatively simple formation models, these three effects halt the collapse of clouds larger than roughly 20 solar masses. The existence of stars more massive than this limit means that some process must overcome this difficulty, or that the model is oversimplified.

It turns out that these older models assumed that star formation occurred in a spherically symmetrical manner. In other words, the only direction in which things changed was the distance from the center of the gas cloud. These models were one-dimensional in that the radius represented the only variable. More sophisticated, two-dimensional models accounted for the fact that the gas would begin rotating about an axis. They revealed that energy from the cloud’s collapse could dissipate though jets emitted along the rotation axis. Nonetheless, rotational and magnetic effects still stunted growth of stars beyond 40 solar masses.

A team of U.S. scientists recently generated a fully three-dimensional model of stellar formation. Their model showed that gravitational instabilities form in the cloud and in the disk that surrounds the cloud. These instabilities serve to channel gas onto the star while the radiation escapes through optically thin parts of the cloud. Furthermore, the instabilities cause the disk to fragment and form companion stars. Not only does this model demonstrate how stars up to 100 solar masses can form, it also explains why massive stars tend to form in binary systems.

Why is this important? First, it is these massive stars that produce the bulk of elements heavier than helium. At the end of their “lives,” the stars undergo catastrophic explosions that distribute the heavy elements through space for future stars to use. They also live very short lives—a few million years for the most massive stars—so they inject these heavy elements into other stars that are forming from the same gas cloud. A growing body of evidence indicates that our solar system formed in such a chaotic environment. It’s good to know that the laws of physics permit these life-essential processes to occur in the universe. Seems like they might even be designed for that purpose.