Re: How big is the Milky Way Galaxy REALLY?

Hi Isaac,

Astrophysicists, like all scientists, always round their numbers. It's a natural thing to do when your measurements have limited accuracy; you need to use an appropriate number of "significant figures". This is an extremely important concept when working with experimental data in science.

Here's an example of the use of significant figures: suppose that you wanted to measure the distance between two buildings, in feet, by "pacing it off". You might start by measuring that one stride = 1 meter. Then you walk between the buildings and note that it takes 51 strides. OK, so you'd calculate "1 meter = 3.28084 feet" So 51 strides * 3.28084 feet per stride = 167.32284 feet is the distance between the buildings. Now, while that is a correct calculation, it's ridiculous to report that the distance is 167.32284 feet! Your measurement was not accurate enough to really notice small fractions of a foot, so all of those decimal places are utterly meaningless. In this situation, a scientist would report measuring "about 170 feet", or "about 167 feet", or perhaps a specific range like "167 plus or minus 1", whichever is appropriate to the accuracy of the measurement,

If your measurement really is more accurate, you can report as many digits of precision as you are confident of. This is why you can write down the speed of light, or the mass of the electron, or the diameter of the Earth, to eight or nine decimal places. But in astronomy, it is rare to measure anything (especially distances!) with more than a few percent accuracy.

That's exactly why you will never hear a figure for the exact size of the galaxy as "119,456.99 light years" or anything like that. Our measurements are not that good. This size of the galaxy is probably more than 90,000; it's probably less than 160,000. Also, the galaxy does not have a sharp edge - it gets wispier as you get further from the center, so there's no clear definition of where to stop measuring.

Most of the galaxy's mass seems to lie within 50,000 light years of the center. We Earthlings are about 25,000 (plus or minus 1500) light years from the center, and we have observed clouds of gas which seem to be 30,000 light years further out - that suggests that the largest radius is less than about 55,000 light years. (and thus the diameter is probably less than 110,000). I actually do not know (and was unable to find out) what the current "preferred" value is or what the errors are - all of my sources say "about 100,000!". And that seems to be as good an estimate as we have.

Check out this lecture for information and pictures of the Milky Way. A Web search, or an introductory physics textbook, can teach you how to use "significant figures."

-Ben