The Family of Stars

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  • Created by: Victoria
  • Created on: 23-04-12 20:34
  • Nearest star after the sun is Proxima Centauri
  • Proxima is 266,453 times further away than the Sun
  • Fewer than 3,000 stars closer than 80 ly from the Sun
  • The Sun is very ordinary

Intrinsic brightness

Brightness and distance

The brightness of a star depends on its distance
If you know the apparent magnitude of a star and its distance from Earth, you can use the inverse square law to correct for the effect of distance 

Absolute visual magnitude

Astronomers take 10 pc as the standard distance and refer to the intrinsic brightness of the star as its absolute visual magnitude (Mv), the apparent visual magnitude the star would have it if were 10 pc away

Calculating absolute visual magnitude

The magnitude-distance formula relates the apparent magnitude mv, the absolute magnitude Mv, and the distance d in parces: 
mv - Mv = -5 + 5 log(d)

m2 - m1 = 2.5 log(b1/b2)

Where m2 = magnitude of star 2, m1 = magnitude of star 1, b2 = brightness of star 2, and b1 = brightness of star 1

Luminosity, radius and temperature

Luminosity - the total amount of energy the star radiates in 1 second - not just visible light, but all wavelengths

  • Must make adjustments for the radiation emitted at wavelengths humans cannot see. 
  • The absolute visual magnitudes of hot stars and cool stars will underestimate their total luminosities because those stars radiate significant amounts of radiation in the UV or infrared parts of the spectrum.
  • Gives an idea of its energy output or its absolute brightness.
  • Apparent brightness depends on how far away it is.
  • Decreases 1/r^2.
  • Star may be bright because it is intrinsically bright or close.
  • Allows us to see things which are dim and bright.
  • Differences in brightness are hard to see.
  • Faintest we can see with the naked eye is magnitude +6.

Absolute bolometric magnitude - the absolute magnitude you would observe if you could detect all wavelengths
You can find a star's luminosity by comparing its absolute bolometric magnitude with the sun's. The absolute bolometric magnitude of the sun is 4.7. For every magnitude a star is brighter than 4.7, it is 2.512 times more luminous than the sun.

The luminosity and temperature of a star can tell you…


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