Brightness and luminosity have two very different meanings when it comes to objects in the universe. To make it a bit easier to explain I am going to use the following analogy.
If you were standing in a pitch black room and there were two torches shining at you it would look a little bit like this:
This looks like two identical torches are shining towards you from the same distance away.
When actually you could have this scenario: Torch B has a larger diameter and a stronger beam than torch A but torch B is further away than torch A.
In the first example, torch A and torch B have the same brightness and luminosity. Whereas in the second example, torch B has a larger luminosity but both torches have the same brightness.
- Luminosity is the objects true brightness measured from its place in the universe (or the torch in this example).
- Brightness is the objects apparent brightness measured from Earth (or the eye in this example).
When you look into the sky and you see stars next to each other it is easy to get caught up in the thought that they are physically next to each other in the sky when from a different perspective these stars may be no where near each other.