Questions on Stars

Here a few questions to get you started. We will have many more questions available within a few weeks so please check back soon. Click on the blue answer tags for answers or guidance on questions.

1. Star A has peak emission at a wavelength of 550 nm whilst star B has peak emission at 480 nm. What can you deduce about the stars based on this information?

   
   

   Star B's peak emission occurs at a shorter wavelength (higher frequency) than star A so it is likely to have a higher effective temperature, that is, it is hotter. Star B would also be somewhat whiter than star A, which would appear similar to our Sun in colour.
2. Two stars with the same effective temperature and at the same distance from Earth appear to differ significantly in brightness to an observer. Account for this.
   
   

   
   

   If two stars have the same effective temperature then they emit the same amount of energy per second per unit surface area. Total luminosity, ie power output, also depends on the surface area of the star. Given that the two stars are at the same distance from us and assuming there is no intervening matter to obscure one of them then the star that appears brighter must have a larger surface area. The brighter star is physically larger than the dimmer star.
3. Explain how a red star can be more luminous than a bluish-white star.
   
   

   
   

   A star's luminosity depends on its effective temperature and on its size. Although the bluish-white star is hotter than the red star and thus emits more energy per unit surface area, if it is much smaller than the red star it may still emit less total energy in a given time.
4. Rank the following in order of increasing luminosity: blue supergiant, the Sun, red giant, white dwarf.
   
   

   
   

   In order of increasing luminosity, from dimmest to most luminous would be: white dwarf, Sun, red giant, blue supergiant.
5. The Milky Way is about 35,000 parsecs (or 35 kpc) across. How far is this in a) light years and b) metres?
   
   

   
   

   a) One parsec is equal to about 3.26 light years. So 35,000 pc = 35,000 × 3.26 = 114,100 light years. The Milky Way is therefore over 110,000 light years across.
   b) One parsec is about 3.09 × 10¹⁶m, so 35,000 pc = 35,000 &times 3.09 × 10¹⁶ = 1.0815 × 10²¹m.
6. Rank the following in order of increasing luminosity: blue supergiant, the Sun, red giant, white dwarf.
   
   

   
   

   In order of increasing luminosity, from dimmest to most luminous would be: white dwarf, Sun, red giant, blue supergiant.