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 

Consider an atom of gold in which the nucleus contains 79 protons and 118 neutrons. What is its atomic number and atomic weight? 

What are the dark blobs (indicated by the red arrows) in this photograph from the Hubble Space Telescope?

According to this diagram, how much more abundant is hydrogen in the universe than nitrogen?

The following questions refer to the sketch below of an H-R diagram for a star cluster. Based on its main-sequence turnoff point, the age of this cluster is

The most abundant molecule in molecular clouds is

Study this H-R diagram. Which of the following stars is the most massive? (The red arrows help you locate these stars on the diagram.)