These are the stages in the life of the star which has not yet become a main sequence, one which is fusing hydrogen into helium. There are three pre-stages and generally all stars will go through these stages.
A Molecular Cloud is a large region of gas and dust that could stretch for light years. It is roughly peaceful and nothing going on. It is the first stage of a star that will go on to become a Main Sequence star. There is no length of time a cloud can exist for. Something will cause the cloud to collapse and begin forming into a star. This area is also known as a star-forming nebula.
What causes a cloud to begin to collapse varies from cloud to cloud. Generally, a collapse could be caused by but limited to :-
The first stage on its way to being a star is the Protostar phase, this is when the cloud is collapsing and is trying to build up sufficient mass in order to start nuclear fusion. Only when it has created enough mass can nuclear fusion begin in earnest.
The Protostar will be surrounding by cloud and dust and will therefore be hard to spot. Not all Protostars will become fully fledge stars like the Sun or Regulus for example. Those that fail nuclear fusion are destined to become a Brown Dwarf which are cooler starts.
There is another stage of Pre-Main sequence that some stars go though, that is referred to as T Tauri which is named after T Tauri, a young star in the Taurus star forming region. This stage is when the protostar has cleared the surrounding dust and clouds away but is not yet a main sequence. During the stage, the star is variable star and is larger and cooler than the main sequence. It will eventually collapse in and become smaller when the nuclear fusion has started. A T-Tauri stage can last for 100 million years. Universe Today
Only stars that are less than three solar masses go through the T-Tauri stage, for all others, they go straight to Main Sequence stage.
Size of Main Sequence stars can be anything from minnows such as Proxima Centauri, a Red Dwarf star all the way up to Spica, a large blue star in the constellation of Virgo. Our Sun is somewhere in the middle.
A star will spend 90% of its life in the main sequence. When a star leaves the main sequence stage, it will can go supernova and become a black hole or a neutron star. At this stage, existence will not be over just yet as it will continue to glow/exist for a lot longer than it was in the main sequence. It will be known as a dead star though at this stage. Eventually, the remnants will dissipate.
The above picture, courtesy of Nova Celestia shows the main sequence stars as the curve in the middle. I should point out, there are no green stars, the green is just for illustration purposes. Although there is a belief that Zubeneschamali may actually be a uniquely green star. Our Sun in is the middle of the chart in the yellow zone.
During the main sequence star, the outward pressure of heat is counter-acted by the inwardly pressue of gravity which keeps the star at a fairly equialibrium. What keeps it at the equalibirium is the mass and the fuel that it has to burn. When the mass and energy fuel depeletes, the star begins to grow. Our Sun will one day run out of hydrogen and therefore begin to grow. This will not happen for a billion years or so no need to pack and move to Mars just yet.
Earth is in the Goldilocks Zone for life, its not too hot and not too cold for life to exist. When the Sun moves on from the Main Sequence stage, it will expand and the Earth will become too hot to support life.
Main Sequence stars can be referred to as Dwarf stars as they are smaller than the giant stars out there. UY Scuti, currently the largest known star in the universe is over 1,7800 times the size of the Sun and if that doesn't make it a dwarf star, nothing will.
A star the size of the Sun can expect to live in the Main Sequence for about 10 billion years and then it will evolve further. It is at this stage in its life that planets will have formed and any life to exist. The Sun is half-way through its life, middle-aged so to speak. However, although it is half-way through its life, life on Earth will not be around for that long. Life on Earth has another 1 to 1.75 Billion years left. Hopefully, by that time, we'd have worked out how to get to another star system such as Proxima Centauri to carry on the human race.
The larger a star is, the shorter time it is in the Main Sequence because it uses its fuel up quicker. A smaller sized star such as a Red Dwarf will fuse hydrogen at a much slower rate than our star so will be around for a considerably longer time.
After the star has finished fusing all the hydrogen into helium, it will move on to the next stage. When it has run out of hydrogen, it will then start fusing helium to oxygen and carbon, all the way up to Iron. The star is unable to fuse any higher than iron on the periodic table. It will only be main sequence when it fusing to helium. Different sized stars will fuse up to different types of element, small stars will only fuse to helium in the core. Enchanted Learning.
The Red Giant Betelgeuse is one star that is believed to have left the Main Sequence and will one day soon explode in a supernova. Soon in astronomical terms, not in what we could soon as in tomorrow or next year.
What stage and path it will take depends on the mass and size of the star.