Free-fall: With no internal pressure, what is the timescale over which a point source of mass m falls unhindered to the center of a cloud of mass M? Equivalently, with no internal support, how long would it take for the Sun's radius to change significantly?

Kelvin-Helmholz: Stellar evolution is a strong function of the available energy reservoir. Could the binding energy of the Sun be sufficient to have fueled its luminosity?

Einstein: The total gravitational potential energy is insufficient to have powered the Sun over its lifetime, even assuming 100% conversion of potential to radiant energy. We next consider the energy equivalent to the total mass of the Sun.

Hubble: The Hubble constant, H, is a measure of the time over which the Universe has been expanding. Measurements of the Hubble time agree well with globular cluster ages and white dwarf cooling times, the oldest relics known. How do we determine both the distance and the expansion rate for various types of objects?