Final answer:
Apparent brightness is the term for how bright a star appears from Earth, influenced by the star's luminosity and distance from Earth. Magnitudes are used to measure apparent brightness, and astronomers must ascertain a star's intrinsic luminosity to accurately gauge distance from its brightness.
Step-by-step explanation:
The term often used to describe the brightness of a star as seen from Earth is apparent brightness. This is a measure of the amount of light received by Earth from a star, which is different from its luminosity, the total energy output of a star. The apparent brightness depends not only on a star's actual luminosity but also on its distance from Earth. A star's brightness is observed in the night sky can vary due to these factors, with most stars requiring a telescope to be seen due to their dimness.
To quantify apparent brightness, astronomers use a system known as magnitudes, which is based on how the human eye perceives light intensity. The challenge in measuring a star's distance arises from the fact that stars emit light at different luminosities, meaning brightness does not translate directly to distance, unlike if all stars had the same luminosity. Astronomers must therefore find additional ways to determine the intrinsic luminosity of a star to effectively measure its distance based on the observed apparent brightness.