This topic accounts for approximately 5% of your exam marks.
stable
Rare
Stable5%
Life cycle of stars and the Hertzsprung-Russell diagram appear as descriptive multi-mark questions.
Luminosity: how bright a star actually is
The 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 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:
Object
Apparent magnitude
Sun
−26.7 (brightest object in the sky)
Full Moon
−12.6
Venus at brightest
−4.6
Sirius (brightest star at night)
−1.5
Polaris
+2.0
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. 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