Approximately how many stars are there in the Milky Way Galaxy?

• A. About a trillion
• B. About ten million
• C. About one hundred thousand
• D. About a hundred billion

Answer: (D)

Estimating how many stars a galaxy contains is about using the total amount of stellar mass and the typical mass of a star. The Milky Way’s stellar mass is on the order of a few times 10^10 solar masses, and a typical star is roughly one solar mass. Dividing gives something around 10^11 stars, i.e., a hundred billion. This fits with detailed star counts in various parts of the disk and the galaxy’s overall light output, since many of those stars are faint, low-mass ones that still contribute to the total. A trillion stars would imply far more stellar mass than the galaxy has, while ten million or a hundred thousand are far too few to account for the Milky Way’s brightness and gravitational effects. So, about a hundred billion is the best approximation.