Ordinary momentum is a measure of an object's tendency to move at
constant speed along a straight path.
Momentum depends on speed and mass. A train moving at 20 mph has more momentum
than a bicyclist moving at the same speed. A car colliding at 5 mph does not
cause as much damage as that same car colliding at 60 mph. For things moving in
straight lines momentum is simply mass ×
speed. In astronomy most things move in curved paths so we generalize the idea
of momentum and have *angular momentum*. Angular momentum measures an object's
tendency to continue to spin. An ``object'' can be either a single body or two
or more bodies acting together as a single group.

Very often in astronomy, the object (or group of objects) we're observing has
no outside forces acting on it in a way to produce ``torques'' that would disturb
the angular motion of the object (or group of objects). A ``torque'' is
simply a force acting along a line that is off the object's spin axis. In these cases,
we have **conservation of angular momentum.**

A planet's velocity and distance from the Sun will change but the
**combination** of speed×distance will not change unless another
planet or star passes close by and provides an extra gravity force.

To calculate the orbital angular momentum use *v _{t}*
for the velocity. So, the angular momentum = mass ×

The core spins at 2 - 10 km/sec at the core's equator. If no external forces produce
torques, the angular momentum is constant. During a supernova the outer layers
are blown off and the core shrinks to only 10 kilometers in radius! The core angular
momentum is approximately = 0.4×*M×V×R* and the mass *M*
has stayed approximately the same. When the radius *R*
shrinks by factors of 10,000's, the spin speed
*V* must increase by 10,000's of times.

Sometimes the neutron star suddenly shrinks slightly (by a millimeter or so) and it spins faster. Over time, though, the neutron star has been producing radiation from its strong magnetic field. This radiation is produced at the expense of the rotational energy and the angular momentum is not strictly conserved---it slowly decreases. Therefore, the neutron star spin speed slowly decreases.

All the time as the cloud collapses, the spin speed must increase. Since no outside forces produce torques, the angular momentum is conserved. The rapidly spinning part of gas cloud eventually forms a disk. This is because the cloud can collapse more easily in a direction parallel to the spin axis. The gas that is orbiting perpendicular to the spin axis has enough inertia to resist the inward pull of gravity (the gas feels a ``centrifugal force''). The most dense parts of the disk will form stars.

last updated: 19 August 1997

Nick Strobel -- Email: strobel@lightspeed.net

(805) 395-4526

Bakersfield College

Physical Science Dept.

1801 Panorama Drive

Bakersfield, CA 93305-1219