How is the number of light years figured between two celestial objects?

Answer 1

To find the number of light years between two celestial objects, we first find the distance from each object to earth. If we connect the dots between Earth and the two objects, we have a triangle. We to sides lengths of that triangle (the distances between Earth and the objects), and we can measure one angle (the angle at the vertex where Earth is. This is enough information to find the distance between the objects using trigonometry (in this case, the law of cosines).

Finding the distance from Earth to an object can be a bit complex. One commonly used method is to look for a pulsating star. We can figure out the absolute brightness (how bright it is without factoring in distance away) of these stars by how often they pulse. Then we can measure the apparent brightness (how bright it looks to us). We can then use both these values to find the distance to the star. (This also works for some supernovae.)

Another method is to use objects that are considered to be 'standard candles'. These objects do not pulse, but we know the relationship between their absolute brightness, apparent brightness, and distance away.