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How to measuring the energy output of a star over time

Written By on in Solar Physics 1


283 words, estimated reading time 2 minutes.

Flux is a term used to describe the brightness of a star and is a measure of the total energy from an object per unit area over time. Flux calculations are used to calculate luminosity, a more meaningful representation of a star's brightness.
Introduction to Astronomy Series
  1. Introduction to Astronomy
  2. The Celestial Sphere - Right Ascension and Declination
  3. What is Angular Size?
  4. What is the Milky Way?
  5. The Magnitude Scale
  6. Sidereal Time, Civil Time and Solar Time
  7. Equinoxes and Solstices
  8. Parallax, Distance and Parsecs
  9. Flux
  10. Luminosity of Stars
  11. Apparent Magnitude, Absolute Magnitude and Distance
  12. Variable Stars
  13. Spectroscopy and Spectrometry
  14. Redshift and Blueshift
  15. Spectral Classification of Stars
  16. Hertzsprung-Russell Diagram
  17. Kepler's Laws of Planetary Motion
  18. The Lagrange Points
  19. What is an Exoplanet?
  20. Glossary of Astronomy & Photographic Terms
The Inverse Square Law

Flux (F) is scientifically defined as "the total flow of light energy perpendicularly crossing a unit area per unit time". Flux has units of J s-1 m-2 or W -2 (that's Joules per second per metre squared, or simply Watts per second)

In the article about the magnitude scale we saw that Pogson devised a scale whereby a 1st magnitude star is 100 times brighter than a 5th magnitude star. This logarithmic scale states that a 1st magnitude star is 2.512 times brighter than a 2nd, which is 2.512 times brighter than a 3rd and so on...

We can use this constant ratio per magnitude to obtain a formula for the ratio of fluxes. Consider two stars that have apparent magnitudes m and n and measured fluxes of Fm and Fn, the ratio of the fluxes is given by:

Equation 18 - Ratio of Fluxes

If one star is 6th magnitude (n = 6) and another star is 1st magnitude (m = 1) then the magnitude difference is given by (n-m) = 6-1 = 5 We can use Equation 18 to calculate the ratio of these fluxes.

Equation 19 - Ratio of Fluxes

We can see that this equation has shown that a difference in 5 magnitudes affects magnitude by a factor of 100 as per Pogson's rule. We can further reinforce this relationship between flux and magnitude by showing that the magnitude difference between two objects can be expressed in terms of the logarithm of the flux ratio. This form is known as Pogson's relation and one form or another one of the most useful equations in the astronomer's toolbox.

Equation 20 - Pogsons Relation

We will see this equation again when we look in more detail at Apparent Magnitude, Absolute Magnitude and Distance.

Last updated on: Friday 21st July 2017

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  1. Axel

    This really helps. Thanks :)

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