the brightness of a star
The brightness as seen from Earth is called the "apparent magnitude".The real brightness (defined as the apparent brightness, as seen from a standard distance) is called the "absolute magnitude".
Absolute Brightness .
Absolute Brightness: How bright a star appears at a certain distance. Apparent Brightness: The brightness of a star as seen from Earth.
Normally you would observe the star's brightness, not its apparent diameter.The star's apparent brightness ("apparent magnitude") depends on its real brightness ("absolute magnitude"), and on the distance. Similarly, the star's apparent angular diameter (which is VERY hard to measure) would depend on its actual diameter, and on the distance.
Energy output, as absolute brightness (magnitude) is taken at a standard distance of 10 parsecs.
The absolute magnitude of the main star in the Polaris system is -3.6
Absolute
The brightness of a star to an observer on Earth is called it's Apparent Magnitude. The intrinsic brightness of a star is known as it's Absolute Magnitude.
You can find the luminosity of a main sequence star by measuring its apparent brightness and distance from Earth. Knowing the distance allows you to calculate the star's absolute brightness. Luminosity is then determined by comparing the absolute brightness of the star to that of the Sun, which has a known luminosity.
Brightness of stars (apparent and absolute magnitude) is measured by convention, taking an another star as a standard.
That means how bright the star really is. The "absolute magnitude" is defined as how bright the star would look if it were at a standard distance from us.
A star's brightness at a standard distance is referred to as its apparent magnitude. This standard distance is 10 parsecs (32.6 light-years) from Earth. Apparent magnitude allows astronomers to compare the brightness of stars as seen from Earth, regardless of their actual distance from us.