If we take the difference of the apparent and absolute magnitudes of a star we get something called the “distance modulus” ( m-M): We also need a standardized measure of the star’s brightness called the “absolute magnitude” ( M), which is a star’s apparent magnitude at a distance of 10 parsecs: Where F is a measure of the amount of light from the star (called the “flux”) and d is the distance to the star. That star brightness scale is called the “magnitude scale”, and on that scale a star’s brightness as it appears to our eyes, which is called the “apparent magnitude” ( m), is defined as: To get an idea as to how far away we could see our own Sun if we ventured out into space, we can use the measured brightness of our Sun using a brightness scale that reflects how our eyes respond to changes in brightness. The light from a star is sent out in all directions, and is largely unimpeded in its path from the star to us, as on average there is very little matter to block the star’s light between the star and us. Even though the stars that we see in the night sky are all very far away, we can see them not by actually being able to resolve their sizes, but by measuring the light that they produce.
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