IGCSE Physics / Stellar evolution
Brightness and absolute magnitude revision notes
Edexcel International GCSE Physics (4PH1), chapter 27: Stellar evolution, specification point 8.11P. Free lesson.
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How bright a star looks from Earth depends on both the light it gives out and its distance. Absolute magnitude allows stars to be compared fairly by saying how bright they would look at a standard distance.
What affects how bright a star looks?
A star’s apparent brightness, or how bright it looks from Earth, depends on two things. A star that gives out more light looks brighter, but a star that is further away looks fainter. This means a faint dot in the sky could be a very powerful star that is a long way off. More light given out makes a star look brighter. Greater distance from Earth makes a star look fainter.
Why compare stars at the same distance?
The Sun looks much brighter than a distant supergiant because it is very close to Earth. The supergiant gives out far more light, but its great distance makes it look like a faint dot. If both stars were at the same distance, the supergiant would look much brighter. Fixing the distance makes the comparison fair. The Sun is very close and gives out much less light than the distant supergiant. At the same distance, the supergiant would look much brighter.
What absolute magnitude means
Absolute magnitude is how bright a star would look if it were placed at a standard distance from Earth. That distance is 10 parsecs, about 33 light years. Using the same distance for every star makes it possible to compare how much light they give out fairly. Include the phrase “at a standard distance” when defining absolute magnitude. The standard distance is 10 parsecs, about 33 light years.
How to read the backwards scale
The absolute magnitude scale runs backwards: the more negative the number, the brighter the star. Bigger positive numbers mean dimmer stars. For example, a star with magnitude −2 is brighter than one with magnitude +5, even though +5 is the bigger number. A supergiant can have an absolute magnitude of about −8. Magnitude 0 is still a bright star. The Sun has an absolute magnitude of about +4.8. A faint white dwarf can have an absolute magnitude of about +12.
Comparing stars using absolute magnitude
Because absolute magnitude compares stars at the same standard distance, you can use it to decide which star gives out more light. Choose the star with the more negative magnitude: it would look brighter at that distance.