Once we know the distance to a star, we can infer its luminosity.

Luminosity is the total energy emitted per second, doesn't depend on distance.

It's an intrinsic property of a star.

Think about it like a light bulb.

You have a 100 watt light bulb.

That will emit 100 watts, that's a measurement of energy.

And that's a light bulbs luminosity.

A light bulb that's far away seems dim

to us but its intrinsic luminosity is the same.

What does vary with distance is flux.

That distant light bulb is dim.

That means the flux that reaches us is small.

We could quantify flux and its

relationship between luminosity and distance as follows.

That the flux we measure from a light bulb depends

on its luminosity, 100 watts, divided by 4 pi D squared.

Let's say the light bulb's a kilometer away.

Well, it's pretty faint.

And the flux we see from a light bulb a kilometer away is quite small.