Atoms of the same elements differing in the number of neutrons in their nuclei are known as isotopes. Thus, isotopes of an element have the same atomic number but different atomic mass number. Isotopes of an element have similar chemical properties but different physical properties.
I would say the answer is A - a stars mass
The majority of stars in the galaxy, including our Sun, Sirius and Alpha Centauri A and B are all main sequence stars.
Mass is the key factor in determining the lifespan of a main sequence star, its size and its luminosity ( brightness)
Two characteristics define brightness: luminosity and magnitude. Luminosity is the amount of light that a star radiates. The size of the star and its surface temperature determine its luminosity. Apparent magnitude of a star is its perceived brightness, factoring in size and distance, while absolute magnitude is its true brightness irrespective of its distance from earth.
