The activity is -fusing hydrogen into helium.
Normal hydrogen fusion forming helium.
A red main sequence star would be a red dwarf or a branch red giant. To be on the main sequence, you have to have hydrogen nuclear fusion.
Algol is a blue main-sequence star in the constellation Perseus
Unfortunately, it is impossible to tell accurately where a star is on the main sequence.
Polaris - the current North star is a multiple star system., consisting of the main star and smaller companions. The main star Alpha Ursae Minoris is a bright star, a 6 solar mass supergiant and it is a main sequence star. Orbiting very close to this main star is a white dwarf of roughly 1.5 solar masses. This is not a main sequence star. Orbiting further out is the third companion, a 1.39 solar mass star. This is a main sequence star. There are also two more distant components (α UMi C and α UMi D) - Polaris is thought to be part of an open cluster - I do not know if these later two stars are main sequence or not.
It's the size. A dwarf star has reached the end of the road and has collapsed. It's a little old star and people don't notice it, mostly.
No. Red giants are not on the main sequence.
A red main sequence star would be a red dwarf or a branch red giant. To be on the main sequence, you have to have hydrogen nuclear fusion.
The sun is a main sequence star, so 1 AU.
The sun is a main sequence star
Main-Sequence star
Sirius is not a single star but a binary star system consisting of a white main sequence star and a white dwarf.
None of those is a main sequence star.
The defining characteristic of a main sequence star burns hydrogen to helium in its core.
Most stars are on the main sequence; that includes red dwarves. Specifically, in this case, the closest known star - Proxima Centauri - is also the closest main-sequence star.
Algol is a blue main-sequence star in the constellation Perseus
When a star "goes off the main-sequence" it generally means the star has run out of hydrogen fuel and is beginning the post-main-sequence or its end of life phase. The main sequence of a star is the time where it is no longer just a proto-star but is burning hydrogen as a primary source of fuel.
No, a white dwarf is not considered a main sequence star. A main sequence star is a star that is still fusing hydrogen in its core. A white dwarf is the remnant of a low to medium mass star in which fusion has stopped.