All the stars in this photo are at about the same distance from Earth (some 25,000 light-years away). Which stars in this picture are the largest in size (radius)?

This photo shows an object located in the halo of our Milky Way galaxy. What kind of object is it?

Look again at the loop of hot gas in this X-ray image. Suppose we took another photo looking at the same place 1 hour later. What would we se

In the formula E = mc 2, what does E represent? 

Professional astronomical telescopes generally have a much greater magnification than the telescopes you can buy in stores. 

Absolute zero is 

Which is the strongest of the fundamental forces in the universe? 

One light-hour is the distance that light travels in an hour. How far is this, in kilometers? (Recall that the speed of light is 300,000 km/s.) 

How would the apparent brightness of Alpha Centauri change if it were three times farther away?

The wavelength of a wave is