The main sequence is where stars sit while they burn hydrogen and produce helium. They sit there for about 90% of their lives because:

1. The hydrogen fuel is extremely abundant.
   
2. The reaction produces more energy per ton of fuel than any other.
   

The key to a long life is to still have gas and dust and be forming new stars. Since all galaxies were born at about the same time, the key is to be inefficient at forming stars so that you have gas and dust left. Irregular galaxies are most inefficient at forming stars so they will die last.

The rate of nuclear reactions in a star's core is dependent on the temperature. O stars have a higher temperature in their core than M stars. Physically, this is caused by the fact that O stars are more massive than M stars. They, in turn, need to generate more pressure to maintain hydrostatic equilibrium. From the ideal gas law, we know that PαρT, which implies that as pressure increases, the quantity (ρT) also increases. The increase in temperature causes the nuclear reactions to proceed faster producing more energy per second or higher luminosity.

O stars have much shorter lifetimes compared to other main-sequence stars because they have high masses. For instance, a 20 M O star has a lifetime of about 6 million years. Since its lifespan is relatively brief, if we observe O stars in a galaxy, we know they must have been born near their present location since 6 million years doesn't give much time for an O star to have moved far from its birthplace. Compare the orbital period of the sun around the galaxy (240 million years) to the lifetime of a 20 M O star (6 million years). It is clear that O stars don't travel very far in their short lifetime.

1. The star has used up the fuel for the current nuclear reaction.
   
2. After using up this fuel, the temperature is not high enough to ignite the subsequent nuclear reaction.
   

1. Gravity is the force between the objects with mass, and it pulls the objects which are expanding. Thus, gravity will slow down the expansion of the universe.
   
2. The current popular model is flat universe, whose expansion is slowing down and will stop in an infinite time.