## How many stars are there in our Galaxy (Milky Way)? (Intermediate)

I am confused about the number of stars in our Milky Way galaxy. Some sources say that the Milky Way consists of 100 billion stars. Other say that the MASS of our galaxy is roughly 100 billion times the mass of the Sun. So because most of the galaxy mass is in the interstellar gaseous and dust nebulae there must be less than a 100 billion stars in it or the total mass must be greater. Which of these is correct? And did someone estimate the number of ACTUAL stars in our Galaxy?

Most of the mass in the galaxy is NOT in interstellar gaseous and dust nebulae. Most of the luminous matter is in stars and not nebulae. Now, the mass of the galaxy is mostly dominated by dark matter, which is something that is not detected by any telescope, or anything except through its gravity. But as far as the luminous matter goes, most of it is stars.

About the number of stars: People have studied the mass distribution of stars in the galaxy. Further, one also knows the amount of light put out by each type of star. So, by measuring the total amount of light in the galaxy (called luminosity), and knowing the mass, one can estimate the number of stars that are there in the galaxy. So, even though we cannot actually count the number of stars in the galaxy, we can estimate the number of stars in the galaxy as roughly 100 billion (100,000,000,000). It turns out that there are many more stars with mass less than the mass of the Sun than with mass more than the mass of the Sun. So, it all works out right.

### About the Author

#### Jagadheep D. Pandian

Jagadheep built a new receiver for the Arecibo radio telescope that works between 6 and 8 GHz. He studies 6.7 GHz methanol masers in our Galaxy. These masers occur at sites where massive stars are being born. He got his Ph.D from Cornell in January 2007 and was a postdoctoral fellow at the Max Planck Insitute for Radio Astronomy in Germany. After that, he worked at the Institute for Astronomy at the University of Hawaii as the Submillimeter Postdoctoral Fellow. Jagadheep is currently at the Indian Institute of Space Scence and Technology.

## Our Reddit AMAs

AMA = Ask Me (Us) Anything