system
Given the following post, respond as if you're commenting on Reddit:
human
Why don't we see any main sequence Red Giants or Blue Dwarfs?

Pretty simple question, probably with a pretty simple answer. Blue Dwarfs are a theoretical stage of life for Red Dwarfs, that much I know, but why don't we see any small main sequence Blue stars? Is it just not possible to get that hot and that small? And the same goes for Red Giants - all the big stars seem to be Blue. Why are none of them Red? Why is that only the dying stage of a larger star?
gpt
A star's evolution is mostly driven by just two numbers: its initial mass, and its age. Almost all stars of the same mass will go through the same evolutionary stages at the same ages. We see a diversity of stars because stars were formed at different times and with different masses. There are other effects, but they are secondary effects, and you can understand most of what's going on by just looking at mass and age.

In the "main sequence", a star burns hydrogen in the core through nuclear fusion. This process is *extremely* sensitive to temperature: doubling the temperature can produce 100x as much power (and consume Hydrogen 100x faster). A more massive star has a higher pressure and temperature in the centre, and produces a *lot* more heat than a less massive star. This sets the colour of the star. A cool star is red. A warmer star is orange or yellow. The hottest stars are white, then blue. This also sets the age of the star. A massive star will burn its fuel *much* faster than a low mass star - so fast that it completely overwhelms the fact that it has more fuel to start with. Our Sun has a life-time of maybe 10 billion years. A star 8x the mass of our Sun will only live for *millions* of years - thousands of times shorter. A star 10% of the mass of our Sun will live for *trillions* of years - no red dwarf star has died "of old age" in the history of the universe.

This is a tight and very strong relation. You have long-lived low-mass cool red stars, short-lived high-mass hot blue stars, and everything in the middle. You don't get small bright blue stars (on the main sequence) because small stars don't have enough gravity to produce enough pressure to get that hot.

Late in life, things get more complicated, and this tight relation breaks down. Stars will run out of hydrogen fuel in the core, and go through stages of burning helium and heavier elements, and/or burning fuel in a shell *around* the core. These tend to puff up the star through increased pressure, and you end up with bright burning core or shell, surrounded by very large but low density "atmosphere", which is somewhat cool just because it's so spread out that each chunk of gas isn't getting that much heat going through it. Because they are large in size (although note they haven't gained any mass), and are cool, they are "red giants".

Once a star completely runs out of fuel, it will run out of pressure and collapse. This can be accompanied by a dramatic explosion as new reactions occur at the extreme densities you get from stuff collapsing without support. That's where you get supernovae and so on. The compression also generates a lot of heat, so you're left with an extremely hot and dense remnant. These are neutron stars and white dwarfs and so on.


Just to sum up: during the "main sequence", stars have a very simple evolution that depends almost entirely on the starting mass, which sets the pressure and temperature, and therefore reaction rate, at the core of the star. Massive stars are hot and blue and short-lived, low mass stars are cool and red and long-lived. But once the hydrogen in the core is used up, the simple relationship is broken, and all sorts of interesting things happen.