Main sequence

Main sequence is a term, from Astronomy, used to refer to stars in the early hydrogen fusion part of their history. The term comes from the very widely used Hertzprung-Russel diagram. These diagrams show the brightness and color of stars on the X and Y axes.

The stages of a star's life are:

Protostar	A cloud of interstellar gas and dust massive enough to start gathering a concentrated mass of gas and dust at its center. Gas and dust falling into the centre of the cloud can heats in a process called gravitational collapse. A protostar heats up enough to glow, but it is still dim, compared with a hydrogen fusing star.
Hydrogen-fusing star	If the cloud of gas and dust is massive enough, as mass falls into the centre of the cloud, its temperature and pressure will rise to the point where Hydrogen fuses into Helium. Hydrogen fusion releases an incredible amount of energy. These stars start fusing Hydrogen at their centre. Stars fuse just enough Hydrogen to balance the weight of their outer layers, so they stop their gravitational collapse. This means that very tiny stars, the smallest red dwarf stars, fuse their Hydrogen so frugally it will take them 3 trillion years to use it all up. Sol, our sun, will fuse all its available Hydrogen in slightly more than 10 billion years. The most massive possible stars -- approximately 150 Solar masses -- will consume all their available Hydrogen in just millions of years. The period when a star is fusing Hydrogen is its main sequence period
Helium-fusing star	If a star is massive enough, when it starts to exhausts its available Hydrogen, its Helium core will start to collapse enough to raise its temperature and pressure to the level where Helium atoms fuse together to form Oxygen or Carbon atoms. Sol, our sun, is massive enough to enter a Helium-fusing period. Red dwarf stars are not massive enough, and they should go directly to the white dwarf period. In practice however, since it will take them trillions of years to consume their Hydrogen, there are no white dwarfs that started as red dwarfs -- yet. Hydrogen fusion releases much more energy than Helium-fusion, meaning this phase of a star's life is relatively brief. When stars start to fuse Helium, as the central Helium core grows more dense the outer Hydrogen layers balloon out, and the star moves from the main sequence portion of the Hertzprung-Russel diagram to the Giant portion of the diagram.
Oxygen-fusing star	Stars more than 8 times as massive as Sol are massive enough to fuse the Oxygen in their core to Silicon and Sulphur. This will keep their cores from collapsing, until the Oxygen is exhausted. However, because this releases much less energy per atom, this phase lasts about one day. When stars this massive exhaust their Oxygen their already very dense cores experience a incredibly rapid gravitational contraction that, suddenly, results in a Supernova.
Supernova	Once the very rapid core collapse that triggers a supernova begins it releases so much energy it can outshine its entire galaxy. Supernovas leave either a Neutron Star, or a black hole.
white dwarf	Stars that aren't massive enough to fuse Oxygen, also have their cores collapse, when they run out of fuel to fuse, but, since they are not massive enough to go supernova, this leaves small, dense hot cinder. Because they have shrunk down to the size of Planet, white dwarfs have surface areas so small it takes an incredibly long time to cool. It can take 5 billion years to cool to the current temperature of Sol. It is believed that they will crystalize, as they further cool, which will slow their cooling so much it can take a quadrillion years to cool to room temperature. This means no white dwarf in the Universe has had enough time to cool.