Where does the proof come from about red giants, brown dwarves, etc.?

Question

Humans have not even existed for "millions" of years in order to record this, where is the proof that it is going to happen? I know that we "see" these things out there, but I've never read anyone that has had who-knows-how-long(-and-how-many-people-... of a journal that says "Today the star got big, and the other one got tiny" whatever in it. Where is the proof?

Answer 1

I think the only transitions that have actually been observed are when a star goes supernova. This is quite easy to observe. I think your looking for observational evidence of the stages in the life of a star?

Its just one of those things where theory says this should be here and in order for that to be true you have to accept certain other aspects of the theory. For instance, we wouldn't expect to find many extremely large stars in an older galaxy because the theory says they would have burned up. We also expect that extremely dense stars like neutron stars would be spinning very fast due to the conservation of angular momentum and the star contracts. We see what we expect and when we don't, we modify our theories. The understanding of the life of a star has become such an accurate description of the observed data, that we pretty much accept the theories to be right.

About your comment that humans haven't been around long enough to record enough data: we have the ability to observe the cosmos in many different stages of its development. The further away we look, the older what we're seeing actually is.  So we get around not having very long studies by this ability. This is also true for nearby stars since stages of the life of a star are ending and beginning pretty much anywhere we look.

Answer 2

There's always at least one star in the known universe that is on a certain phase in its life, thus observation is the answer.

Implication: Every type of star/life phase is proven by seeing it.

Answer 3

Hubble telescope shows various stars in their age.

studying the motion of bodies revolving around them. one can know their gravity effect, either decaying or building up.

Actually many of stars we see are gone ages ago. their light is reaching us now, covering a huge distance.

Answer 4

We can directly observe millions of stars that are in various stages of their existence  - depending on their mass, matter, temperature brightness and radiation.  We understand the physics of gravity, and nuclear fusion and can work out what must happen for a given star of a given size and makeup over its lifetime.  And guess what - the stars we observe match what physics tells us must be happening, allowing us to fairly accurately classify the stars by type and age and to reliably predict their ultimate fate.  

Look around you at all the people the next time you are in a crowded place. You know the approximate age of anyone, not because you have observed them their whole life or even your whole life, but because you understand the pattern of human aging from experience and can form a good guess of someones age by observing.  It is no different, but far more precise for stars.