The Main Sequence refers to the H-R diagram of stellar evolution (the Hertzsprung-Russell diagram).
If a star is on the Main Sequence, then it is burning with the rate and temperature expected of its spectral class.
Stars are divided into the following spectral classes on the H-R diagram:
O, B, A, F, G, K, M
O, B and A stars burn very hot because they are very large, and they are usually white/blue-white in color
F and G stars (our sun is a G star) burn moderately, are moderate in size and give off yellow light
K and M stars are red dwarfs. They burn slowly with red color, representing the coolest, least energetic spectrum of light.
Stars which are off the Main Sequence but which are on the H-R diagram include Red Giants (like Betelgueuse and Antares), and white dwarfs, stages at the end of a star's life cycle which our sun will eventually pass through. More massive stars have different endings, including supernovae and black holes or neutron stars. These are not on the H-R diagram.
The sun is a 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.
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.
a main sequence star
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
The sun is a main sequence star, so 1 AU.
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.