Star measurements

[JohnDenim]

can someone please tell me how star diameters, and galaxies for that matter, are measured?

[FraserClarke]

Galaxies are basic trigonometry; measure the angular size (i.e. from an image) and given a known distance, you get a physical size. Of course the tricky part here is knowing the distance! That you can get by measuring certain types of variable stars.

Stars are a bit harder, because they are so small. Only the very largest telescopes (>4m) can resolve only the very largest stars. That does of course give you useful upper limit on the size of stars though. To get the actual size though, there are basically three techniques (though I may have forgotten others?);

1) Interferometry -- combine the light from two (or more) telescopes ~10-100m or so apart. The details of how the light combines gives you information on the angular size of the thing (star) you are looking at. Then apply trig as with galaxies.

2) Spectroscopic eclipsing binaries -- in these systems, you have enough information to completely solve the geometry of the system, including the radius of the stars.

3) Theory -- you can model what you think a star is made of, basic equations of hydrodyanmics/enery transfer, and get the size of the star out. (not really a measure of course, and needs to be tested by observation)

[JohnDenim]

So, easy peasy then eh?

Thanks for taking the time to reply.