•Virial Theorem:
A relationship between gravitational potential energy and velocities
for a dynamically relaxed and bound system.
•Ellipticals not necessarily
rotating.
•T = ½ |U|, where
T is the total kinetic energy and U is the potential
energy.
•So, for a cluster of
stars or a cluster of galaxies, measuring T (by measuring velocities)
can give U and therefore M.