In the HS diagram, why do low magnitude sequences correspond to giants and high magnitude ones to dwarfs?

Question

Is there a direct relationship between magnitude and mass/size?

Answer 1

Do you mean the HR diagram?

Yes there is a correlation between magnitude and size (radius - not mass).

It works like this - hot things give off more per unit surface area than cool things - so for stars of identical size - the hotter one is brighter.

But a bigger star has a larger surface area - so for stars of the same temperature, the bigger star is brighter than the smaller car.

Now mass is a different issue.

A star like the sun will eventually become a red giant - which is cool, but so big that it is very bright - but it hasn't gained mass, so its still the same mass as the sun.

ADDED - forgot to mention - magnitude scale is backwards - so low number magnitude is bright and high number magnitude is dim! this is for historical reasons...

Hope that helps...