Final answer:
A star's apparent brightness is primarily affected by its luminosity and distance from Earth, with atmospheric conditions also playing a significant role.
Step-by-step explanation:
The two main factors that affect how bright a star appears in the night sky are its luminosity and the distance from Earth. Luminosity refers to the total energy a star emits per second. A star that is closer to Earth will appear brighter than one that is farther away, assuming both have the same luminosity. This is because the intensity of light diminishes with the square of the distance from the source, known as the inverse-square law.
Furthermore, atmospheric conditions can also influence apparent brightness. The Earth's atmosphere can absorb and scatter some of the starlight, with effects such as light pollution from cities and atmospheric turbulence termed 'bad seeing' reducing the apparent brightness of stars, especially those that are lower in the sky.