A zero-age main-sequence star is one that can __________.

When we see a meteor shower, it means that ________.

The axes on a hertzsprung-russell (h-r) diagram represent _________.

In addition to earth, the planet ________ shows clear evidence of water erosion.

The debris from a supernova explosion is called a supernova _________.

The habitable zone refers to ________.

The schwarzschild radius of a black hole depends on ________.

Io experiences tidal heating primarily because __________

A comet entering the inner solar system from afar will __________

The total number of stars in the observable universe is about _________.

We estimate the mass of the galaxy by observations of its ________ curve.