4PH1

Stellar Evolution

Astrophysics · 1 question type

Practise

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

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:
ObjectApparent 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
Faintest stars visible to the naked eye+6
Faintest objects seen with Hubble Space Telescope+31
  • 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
A vertical apparent-magnitude scale against a starry background, running from about −27 at the top down past 0 to +20 at the bottom. The Sun sits highest at roughly −27, then the full Moon, then Venus at its brightest, then Polaris near 0, the naked-eye limit around +6, and Pluto at its brightest lowest down, illustrating that brighter objects have smaller (more negative) magnitudes
Source: Absolute magnitude by Save My Exams

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