Hello Mr. Dorn,

I was forwarded the email you sent to the AAS regarding the number of stars in our galaxy. I have been a member of the Society for a number years and serve on its Astronomy Education Board, so perhaps that is why your email was forwarded to me.

The numbers you mention, 100 billion to 500 billion, are reasonable. They are based upon measuring the mass of the galaxy, which is done using the speed at which its stars and gas orbit. Most of the mass is dark matter, which we cannot see at all because it does not interact with any kind of light. However, most of the luminous matter in the Galaxy is composed of stars. We know how bright stars of various kinds are, and we know how many of each kind of star there is. So by combining what we know about the brightness of stars with what we know about the fraction of mass that is stars (we get this by comparing to other similar galaxies), we can estimate the total stellar mass. From that we can make an estimate of the number of stars.

The number we arrive at depends upon several assumptions and on the details of several things: how many of each kind of star there are, the total mass of the galaxy, how much mass produces how much light, etc. This is not random guesswork, but there are different studies for each of these parts that give slightly different results, and there is no reason a priori to use one over another. Using different combinations will give slightly different results for the total number of stars.

You might think that it would be better to simply count the stars, but we cannot count all the stars because we cannot even see most of them. They lie too far away, and intervening material blocks our view. It would be a bit like trying to count all the trees in a forest when you are standing inside it. However, it is not necessary to count the stars to get a rough estimate of how many there are.

Another point of confusion might be that when astronomers say there are 100 billion stars or 500 billion, they are not intending that the number they quote be taken as exact. They are well aware of the uncertainties I've tried to describe above, and they intend any number they give as a rough estimate, good to a factor of a few. In this context the two numbers you cite are about the same in astronomers' minds: we know that there are many more than ten billion stars, and far fewer than ten trillion, but we don't know precisely how many. All we can say is that it's on the order of a few hundred billion. Oftentimes the public does not have an appreciation that such numbers are not meant to be precise.

I hope this answers your question. Feel free to ask for clarification if any of this doesn't make sense.

K. McLin

Dr. Kevin McLin
GTN Director
Sonoma State University