Why do stars change very LITTLE while they are in the main sequence stage?

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(3) High-mass main sequence stars have shorter lifetimes than low-mass stars. ... For one thing, the Sun was not made of pure hydrogen when it formed 4.6 billion years ago. For another, only the hydrogen in the Sun's core is able to be fused; the hydrogen in the Sun's outer layers is at too cool a temperature to fuse. Hope this helps !!!

Where’s the graph 0.0