Pedagoguery

Disks are everywhere in the universe. From tiny systems like the rings of the outer planets to huge spiral galaxies, disks are everywhere you look. In addition, they often exhibit some spectacular effects. Where to they come from and how to they produce these effects? Many of the answers are still being discovered, but some interesting theories exist.

In general, disk formation is a consequence of the conservation of angular momentum. Consider a large cloud of gas. It is likely that it will have some overall rotation, no matter how small. If that cloud starts to contract under its gravitation, then as it gets smaller overall, it rotates faster in the same way that an ice skater spins faster as she pulls in her arms. However, as the roughly spherical cloud contracts, gas that falls in parallel to the rotational axis will fall faster than gas along the “equator” of the sphere, for the simple reason that the rotational velocity of the gas will prevent it from falling. Eventually, the gas will all collect into a single plain, resulting in a disk. This is why, for example, the planets all lie within roughly the same plain – they formed from a disk of gas that was left over from the formation of the sun.

While conservation of angular momentum does explain why disks are so ubiquitous in the universe, it does not explain some of the exotic physics that takes place in accretion disks around compact objects like neutron stars and black holes. In these cases, the disk heats up, often hot enough to emit x-rays, and it will frequently emit jets of matter traveling at relativistic speeds along the axis of rotation. In some cases, these jets can travel thousands of light-years from their origin. How do these affects arise? The answer appears to be turbulence. But what causes the turbulence?

Turbulence comes from friction. But what could cause friction in space? One source of the friction could be heat generated when particles within the disk collide with each other. This is a mechanism that operates within Saturn's rings: as various rocks and pebbles within the rings collide, they exchange energy and angular momentum. Some of the energy is lost as heat, so the angular momentum on average is transferred outwards, causing individual particles to spiral inwards. In an environment around a neutron star or black hole, this effect would be much more intense, because the greater gravity of those objects would mean that collisions would take place at higher speeds, and would therefore be much more energetic. In fact, they would be so energetic, that they would frequently ionize the material. And when the material is ionized, a new major factor comes into play: the electromagnetic force.

Up to this point, we were dealing primarily with gravitational forces. The electromagnetic force is tremendously more powerful, but it only operates on charged particles. When a material becomes ionized, it becomes a collection of charged particles.

Electromagnetic effects would amplify the transfer of angular momentum outwards, because the interactions between particles would no longer be limited to physical collisions – instead they would include electromagnetic interaction. As a result, more heat would be generated and material would spiral inwards much faster. Furthermore, a fast moving charged medium would generate a strong magnetic field. This, plus the intense heat near the center of the disk, could explain the jets that are observed in many disks. The jets would also serve to take away even more angular momentum, allowing the accretion process to proceed even faster. In fact, it is now believed that jets are critical in allowing star formation, since they are frequently observed around very young pre-stellar objects called Herbig-Haro objects. The jets could take away enough angular momentum to allow the central protostar to acquire enough mass to start the central fusion process.

There are still many aspects of disks that are not understood, not the least of which is the exact process by which jets arise, but with better computer modeling, our understanding is growing.

Next issue, I will talk about anomalous x-ray pulsars.