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)?

The most abundant molecule in molecular clouds is

Which of the following persons reorganized the spectral classification scheme into the one we use today and personally classified over 400,000 stars? 

Which of the following best describes what we mean by the focal plane of a telescope? 

Newton’s second law of motion tells us that the net force applied to an object equals its 

Absolute zero is 

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

Study this H-R diagram. Which of the following stars has (or had) the longest hydrogen burning lifetime? (The red arrows help you locate these stars on the diagram.)

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

What is the main idea captured by this graph?