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This famous image from the Hubble Space Telescope shows what is sometimes called the “pillars of creation.” Which of the following best describes what it shows?

What is happening inside a star while it expands into a subgiant?

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.