Why does a can collapse when a vacuum pump removes air from the can?

The star betelgeuse is about 600 light-years away. if it explodes tonight, a. we'll know because debris from the explosion will rain down on us from space.b. we'll know it immediately because it will be brighter than the full moon in the sky.c. we won't know about it until 600 years from now.d. none of the above.

Positive ions differ from neutral atoms in that

What is the name of the perceived change in a sound wave’s frequency due to motion between the observer and the sound source?