Stars are classified according to two main criteria: mass and luminosity. The more a star is massive, the more warmer it is.
This classification shows that stars are not distributed at random and that the Sun occupies a pretty poor position.
The life expectation of a star depends on its mass.
The bigger the mass, the shorter the life expectancy.
• a big star (~ 10 times the mass of the Sun) “lives”
⇒ some million years
• an average star ( the Sun ) “lives”
⇒ approximately 10 billion years
• a small star (~ a tenth of the mass of the Sun) “lives”
⇒ approximately 100 billion years
The visible luminosity of a star is the power of the emitted electromagnetic radiance which arrives at the observer.