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 ________.

A main-sequence star twice as massive as the sun would last __________.

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.

Venus's atmosphere has much more carbon dioxide than earth's because ________.