Nuclear fusion produces heat, and heat creates the pressure which keeps the star from collapsing under its own gravity. The relationship between heat and pressure in a gas is described by the Ideal Gas Laws. It also applies to plasma (which can be described as a super heated gas).
hydrostatic
Dynamic equilibrium.
The primary force that prevents a main sequence star from collapsing under its own gravity is the pressure generated by nuclear fusion in its core. As hydrogen atoms fuse into helium, this fusion process releases an immense amount of energy, creating an outward pressure that counteracts the inward pull of gravity. This balance between gravitational force and the energy produced by fusion is known as hydrostatic equilibrium, allowing the star to maintain its stability throughout the main sequence phase of its lifecycle.
Sirius is not a single star but a binary star system consisting of a white main sequence star and a white dwarf.
The correct order of these stellar evolutionary stages is main sequence, red giant, white dwarf. A star begins its life on the main sequence where it fuses hydrogen into helium. As it runs out of fuel, it expands into a red giant before shedding its outer layers and collapsing into a white dwarf.
hydrostatic
Hydrogen fusion to make helium. When a star runs out of hydrogen in its core to fuse, it begins collapsing, leaves the main sequence, then ignites helium fusion to make carbon, becoming a red giant.
Dynamic equilibrium.
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.
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