Transcript: It sounds simple to measure the size of a galaxy, but it is not because galaxies do not have sharp edges. As you can see in any deep image of a galaxy, the brightness of the stars that the galaxy contains just fades away gradually until it disappears into the darkness of the night sky. Galaxies are finite objects bound by gravity, but it requires a convention to measure their total size. Typically astronomers measure their brightness within a large aperture such as to include most of the light of the galaxy. This is not a good measure of the typical size because of the slow fading away of the light into the sky brightness. The more conventional measure of size is an aperture that contains fifty percent of the total light of the galaxy, or some other fixed fraction. As long as astronomers use a fixed convention for defining size, they can easily compare the sizes of different galaxy types.