There are a number of methods that astronomers can use to determine the diameter of a star. The simplest way that this is done is through direct imaging. Astronomers can point a telescope at a star, take a picture, and then make a measurement. They will then take a measurement of the star's angular size and multiply that by its distance from Earth.

This, however, is an imprecise method. Given the vast distance of even the closest stars, it is not possible to get an image of a star that is completely in focus. Diffraction will occur, which will give the star a somewhat blurry outline, making an exact measurement impossible. Consequently, there is a significant margin of error in determining a star's diameter through direct imaging.

Another method that astronomers use is to look at the luminosity and temperature of a star. Since stars radiate energy in a predictable way based on their size, these measurements help astronomers determine the diameter of the star. By looking at the star's color and brightness, we can determine its temperature. This is then compared to the sun, whose diameter we know. From there, astronomers can extrapolate the true diameter of a distant star.

There are other common methods determining a star's diameter, including using the Hertzsprung-Russell diagram, lunar occultations, and interferometry.