observable properties of stars back to teaching back to the course start of previous section first page of this section previous page next page last page of this section start of next section help on navigating these pages symbols, constants and quantities

There are six fundamental properties of stars which can be readily determined by observation. These are:

  1. mass (Ms)
  2. luminosity (Ls)
    defined as the total energy radiated per second, i.e. the power, from a star. 
  3. radius (rs)
  4. effective temperature (Te)
    defined as the temperature of the black body of the same size as the star that would emit the same total power. The effective temperature is related to the luminosity and the radius of the star by

    Ls = 4rs2 Te4

    where is the Stefan-Boltzmann constant. 
  5. age 
  6. chemical composition (X, Y, Z) 
    where X, Y, Z are the fractional proportions, by mass, of hydrogen, helium and metals.

Table 1 lists how each of these properties are determined and gives the range of their values in terms of the properties of the Sun (denoted by the subscript - click on symbols, constants and quantities for a list of solar values).

Table 1:  Six fundamental properties of stars, how they are determined from observation and the range in their values.


determined from

range of values

mass (Ms)  binary stars 
or g (spectrum) and Rs
10-1 M <  Ms < 50 M 
luminosity (Ls)  apparent magnitude and distance 
or spectrum (luminosity class)
10-4 L <  Ls < 106 L 
radius (rs)  Ls, Te [Ls = 4rs2 Te4]
or interferometry (angular diameter) and distance
or eclipsing binary stars
10-2 r <  rs < 103 r 
effective temperature (Te)  continuous spectrum 
or spectral type
2 × 103 K <  Te < 105 K
age  star clusters and theory  0 - 1010 y
chemical composition (X, Y, Z)  line spectrum  X=0.747 

Note that the very high luminosities of exploding supernovae and the very low luminosities of neutron stars have been omitted from the above limits, as have the properties of brown dwarfs. For detailed definitions of the above properties and the techniques of measurement, see chapter 2 of Tayler.

©Vik Dhillon, 27th September 2010