What is the star on the main sequence?

Answer 1

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.