One Universe: At Home in the Cosmos

Motion | Pages 26-27 | (back to unlinked version)

the friction to a single point of contact between you and the ball. As a result, it spins much longer and your friends walk away impressed. Jugglers also reduce friction in their routines by perching spinning objects at the ends of pointed sticks, or even on the tips of their noses.

Finally, imagine that the rotating thing touches only air. A tossed disk of pizza dough doesn't stop twirling in midair because the friction between the dough and molecules in the air is very small. In the same way, crisply struck golf balls or smartly thrown boomerangs maintain most of their spins until they fall back to Earth. If we go one step further and subtract the air itself, there's no limit to how long an object can spin. Physicists can do this in the laboratory by levitating objects with magnetic fields in vacuum chambers where almost all of the air is pumped out. In such settings, objects could rotate for years if anyone cared to watch.

But the best vacuum of all is in space. Once a body starts to spin there, it won't stop unless it collides with something or feels some source of drag, such as tidal pulls from nearby objects . The physical principle that governs such long-lasting motion is called the conservation of angular momentum . According to Newton's laws of motion, an object's straight-line momentum stays the same unless some outside force acts upon it. Angular momentum behaves the same way, with an added twist. The amount of momentum carried by a spinning object depends not only on its rotation speed but also on how its mass is distributed around its axis of rotation. That quantity, known as the "moment of inertia ," dictates how much force it takes to spin up an object of a certain shape--and how much the object resists slowing down once it is spinning.

It's easy to see this concept in action. A familiar example is the dramatic spin used by figure skaters to end their performances. Watch as the skater spins slowly at first, then draws her arms and legs toward her body. That brings her distribution of mass closer to her axis of rotation, which lowers her moment of inertia . But because her total angular momentum must stay the same (ignoring the minor friction of the skates against the ice), her rotation speed must increase drastically to make up for it.

Conserving Momentum in the Cosmos

The principle of the conservation of angular momentum explains in part why some galaxies are elliptical and others have the familiar disk shape that characterizes spiral galaxies like our own Milky Way. Just as a skater spins faster when she pulls her arms and legs in toward her body (below), the gas and stars that coalesce to form galaxies begin to swirl faster as they fall inward toward greater concentrations of matter.

A galaxy's final shape is determined primarily by how rapidly its gas condenses to form stars. An initially lumpy distribution of gas may lead to more rapid star formation, using up the available gas and producing an elliptical shape, with stars smoothly distributed outward from a dense core. Disk galaxies, as shown here, may have more diffuse gas to start with but then end up with leftover swirling gas that tends to converge in a disk.

The initial collapse of randomly moving gas and stars forms a bright nucleus of large star clusters with a defined orbital spin (1). The upward and downward motion of the remaining diffuse gas clouds cancels out; conserving angular momentum , the clouds retain only their orbital motion around the nucleus (2).

As the central bulge grows, gas clouds gradually converge into an orbit along the equatorial plane of the galaxy (3). The resulting disk is the most stable compromise between gravity's pull and the energy of the original motion of the gas clouds around the nucleus (4).

a skater spins faster when she pulls her arms and legs in toward her body