Stellar Evolution

Luminosity: how bright a star actually is

• The luminosity of a star is:

the total amount of light energy the star emits per second

• Units: watts (W). Luminosity is a measure of the star's power output
• Luminosity is an intrinsic property and does not depend on where you are looking from. The Sun has a luminosity of about 4 × 10²⁶ W, whether you are standing on Earth or on Pluto
• Astronomers often express luminosity in solar units, where the Sun's luminosity = 1. So a star with luminosity 100 emits 100 times as much energy per second as the Sun

Apparent magnitude: how bright a star looks

• The apparent magnitude of a star is:

a measure of how bright the star looks from Earth

• Apparent magnitude depends on two things:
  • The star's luminosity, because a more luminous star looks brighter
  • The star's distance from Earth, because a closer star looks brighter (light spreads out with distance, so a distant star looks fainter than the same star nearby)
• A bright nearby star and a luminous but very far star can have the same apparent magnitude

The reversed scale: lower number = brighter

• The apparent magnitude scale is back to front from what you might expect:
  • The brighter the star looks, the smaller (or even negative) the magnitude
  • The dimmer the star looks, the larger the magnitude
• Examples of apparent magnitude:

• Each step of 5 magnitudes is exactly a factor of 100 in brightness. So a magnitude 1 star is 100 times brighter than a magnitude 6 star

Absolute magnitude: putting all stars at the same distance

• To compare two stars fairly, you have to remove the distance effect. Absolute magnitude does exactly that:

absolute magnitude = how bright the star would look if it were placed at a standard distance of 10 parsecs (about 32.6 light-years, or 3 × 10¹⁴ km) from Earth

• This puts every star on the same footing. A star with a low absolute magnitude is truly very luminous; a star with a high absolute magnitude is truly dim, regardless of where they actually are
• The Sun's absolute magnitude is +4.8. From 10 parsecs away the Sun would be a dim naked-eye star, confirming that the Sun is a fairly modest star intrinsically; it looks bright only because it is so close

Why both scales are useful

• Apparent magnitude answers "what will I see in the sky?", useful for navigation, for telescopes, for finding objects
• Absolute magnitude (and luminosity) answers "what is this star really like?", useful for classifying stars and understanding their life cycles