There are three factors, actually.
The star's size and temperature determine the absolute magnitude, or how bright the star really is.
Those two factors can be considered as one - the star's absolute magnitude.
The absolute magnitude combined with our distance from the star determines its apparent magnitude, or how bright the star appears to be from Earth.
So, a big, hot, super bright star very far away may have the same apparent magnitude as a small, cool star that's fairly close to the Earth.
a stars brightness as seen from Earth
Apparent magnitude and absolute magnitude.
Negative The apparent magnitude of our sun is -26.73 whereas Vega is +0.03
Brightness is a function of a stars luminosity as observed from Earth. So our Sun is the brightest. Vega has an apparent magnitude of: 0.03 Betelgeuse has an apparent magnitude of 0.42 So, in apparent magnitude, Vega is brighter.
Apparent magnitude.
The question is: Why is the apparent magnitude of some stars less than their absolute magnitude. Or: Why do some stars not look as bright as they really are ? The answer is: Because they're so far away from us.
a stars brightness as seen from Earth
Apparent magnitude and absolute magnitude.
The star's real magnitude (brightness), its distance from us, and anything in between (usually dust or gas) which might absorb part of the light.
The apparent brightness of stars is called "apparent magnitude", and it is written with a lowercase "m" after the number.
It is actually absolute magnitude, opposed to apparent magnitude which is how much light stars appear to give off.
The apparent brightness of stars is called "apparent magnitude", and it is written with a lowercase "m" after the number.
a star with apparent magnitude of 6 or less, the lesser the magnitude the brighter the star
Negative The apparent magnitude of our sun is -26.73 whereas Vega is +0.03
Brightness is a function of a stars luminosity as observed from Earth. So our Sun is the brightest. Vega has an apparent magnitude of: 0.03 Betelgeuse has an apparent magnitude of 0.42 So, in apparent magnitude, Vega is brighter.
Here is some information on three of Leo's stars - The three brightest stars in Leo the Lion are : Alpha Leonis or Regulus (the Lion's heart) with an apparent magnitude of 1.35, Beta Leonis or Denebola (the Lion's tail) with an apparent magnitude of 2.14 and Gamma Leonis or Algieba (where the Lion's mane meets his body) with an apparent magnitude of 1.98
The apparent magnitude is what we see, and this can be measured directly. The absolute magnitude must be calculated, mainly on the basis of (1) the apparent magnitude, and (2) the star's distance. So, to calculate the absolute magnitude, you must first know the star's distance.