The "main sequence" is the region (on the HR diagram) for stars which burn hydrogen-1. Once stars use up most of their hydrogen-1 (and have significant amounts of helium-4), they leave the main sequence.
red supergiant
Basically, the stars on the "Main Sequence" are the ones that fuse hydrogen-1 into helium-4. Once a star burns up its hydrogen-1 fuel, or doesn't have much left, it will start burning helium-4 into heavier elements. It is then that the star leaves the "Main Sequence".
Our Sun is currently a main sequence star. It is not a supernova, as supernovae are massive explosions that occur at the end of a star's life cycle, and it is not a white dwarf, which is a type of star that has exhausted its nuclear fuel and collapsed to a very dense state.
Before a star becomes a red giant, it goes through the stage of being a main sequence star, where nuclear fusion in its core converts hydrogen into helium, releasing energy in the form of light and heat.
In astronomy the term main sequence is understood to apply to stellar evolution; since black holes are not themselves considered stars so much as "stellar remnants" they would not fall on this sequence. It would be appropriate to say they are most commonly created at the end of life (once the fuel is exhausted) of a larger star and thus would be more likely to pertain to the most massive stars of the upper main sequence.
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.
Yes Star spend most of their life span as a main sequence star. A star end will depend on its size in life the end of a start can be a red giant to supernova, a white dwarf, pulsar, or black hole.
red supergiant
Basically, the stars on the "Main Sequence" are the ones that fuse hydrogen-1 into helium-4. Once a star burns up its hydrogen-1 fuel, or doesn't have much left, it will start burning helium-4 into heavier elements. It is then that the star leaves the "Main Sequence".
The birth line in the Hertzsprung-Russell Diagram represents the path that stars follow as they evolve from protostars to the main sequence. It shows how a star changes in luminosity and temperature as it progresses towards becoming a main sequence star. Stars begin their life on the birth line before settling into the main sequence phase.
Depending on how "low" on the chart it will either be a red dwarf or a PMS (Pre-main sequence) star.
A "main-sequence star" is one that fuses hydrogen into helium. Eventually, the star will run out of this specific type of fuel - in other words, it won't have enough hydrogen (at least, near its core) to continue this process.
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.
the main sequence has a limit at the lower end because as a star's mass decreases, its core temperature and pressure decrease too. This eventually causes the nuclear fusion reactions in the core to stop, leading the star to move off the main sequence.
In Magic: The Gathering, the turn structure consists of the following sequence of actions: untap, upkeep, draw, main phase 1, combat phase (which includes beginning of combat, declare attackers, declare blockers, combat damage, and end of combat), main phase 2, end step, and cleanup step.
Alioth is a white star nearing the end of it's main sequence. Hope this helps! (:
Spica is a dual star system, with the two members too close for resolution by the best telescopes. The larger, primary star is a blue-white giant, and does have the potential to end it's life as a supernova. The primary is not a main sequence star. The secondary, about 70% the size of the primary, is a main sequence star, also blue white.