1. Luminosity: This is the intrinsic brightness of the star, meaning how much light it actually emits. It depends on factors like the star's size, temperature, and age.

2. Distance: The farther away a star is, the dimmer it appears to us. Light spreads out as it travels, so the intensity decreases with distance.

3. Interstellar Extinction: This refers to the absorption and scattering of starlight by interstellar dust and gas between the star and Earth. This can significantly reduce the apparent brightness of a star, especially for stars that are very far away.

In summary, a star's apparent brightness (how bright it appears to us) is a combination of its inherent brightness, its distance from Earth, and the amount of dust and gas blocking its light.