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About how many stars can be seen from somewhere on Earth with the naked eye?

What species absorbs photons in a protostar’s outer layers?

This is the H-R diagram for stars in a 12-billion-year-old globular cluster. Based on our understanding of stellar lives, which of the four labeled stars is currently generating energy through hydrogen fusion in a shell around an inert (nonburning) helium core?

You discover a binary star system in which one member is a15M Sun main-sequence star and the other star is a 10M Sun giant. How do we believe that a star system such as this might have come to exist?

Which of the following correctly states the luminosity-distance formula? 

This figure shows frames from a computer simulation of star formation, starting at the left with a large molecular cloud measuring more than a light-year across. What is happening as time passes (from left to right)?

Based on its surface temperature of 5,800 K, what color are most of the photons that leave the Sun’s surface?

Although it is difficult to tell from our vantage point inside the galaxy, astronomers suspect that the Milky Way is a barred spiral.

A binary system that looks like the one shown in this painting should shine brightly in X rays. Which one of the four labeled regions is the source of most of the X rays?

On a cosmic calendar, in which the history of the universe is compressed into one year, how long is the average human life span?