How are stars organized on the HR diagram

It's a graph showing absolute brightness vertically, and temperature horizontally. Temperature is directly related to colour (google black-body radiation to understand why, or just consider that when metal is heated up in a forge, first it glows red hot, then it progresses through the rainbow to be white hot).

So a big bright red giant will be at the top right, and a red dwarf at the bottom right. The hotter the star, the more it will be towards the left.

The interesting part is that the HR diagram doesn't have stars scattered all over it. That gives us clues on the life cycles of stars - most are on a diagonal line from top left to bottom right, called the Main Sequence. Our Sun is bang in the middle.

What answer were you expecting then?

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