if u observed a red star, what basic information could u tell about it?

Answer 1

the most obvious thing is the temperature of the photosphere. (red = comparatively cool - say 3000 - 4000 K)

If you knew the distance you could determine the intrinsic brightness which could lead you to knowledge of how highly evolved the star is. (most stars swell, brighten and turn red in old age, but some low mass stars stay dim and red their entire lives.)

Lastly, if you have a detailed spectrum of the stars light, you can accurately determine the temperature and its chemical composition.

Answer 2

Its temperature would be between 5-thousand to less than 3.5-thousand Kelvin.

It would be a huge, super-massive star with a radius of up to 300-million kilometers (..Antares..)

Compared to less massive, hotter stars, its lifetime would be very short.

It will almost certainly end its life in a supernova explosion

It will almost certainly end up as either a neutron star or a black hole.

Answer 3

Its surface temperature.
(There are both large and small "red" stars.)