A few astronomy questions?

Question

Will a star of 2 solar masses always have a higher luminosity than a 1 solar mass star ?

Are planetary nebulae the most important source of heavier elements heavier than iron because they represent the end point of evolution of most types of stars ?

As a star evolves it's powered by fusion of more massive nuclei. Does this mean that less energy is released because the energy per nucleon decreases with mass, and therefore the luminosity is less ?

Is a brown dwarf a star of less mass than 0.8 solar masses ?

Any help would be appreciated.

Answer 1

Will a star of 2 solar masses always have a higher luminosity than a 1 solar mass star ?

If you take them at comparable moments in their evolution, yes.

Are planetary nebulae the most important source of heavier elements heavier than iron because they represent the end point of evolution of most types of stars ?

Supernova remnants (planetary nebula is part of the envelope that is tossed well before, so it will not be rich in heavier atoms)

As a star evolves it's powered by fusion of more massive nuclei. Does this mean that less energy is released because the energy per nucleon decreases with mass, and therefore the luminosity is less ?

Less energy per nucleus, but more nuclei per second.  The natural balance is dictated by the fight between gravitational collapse and radiation pressure.  For example, if a helium core attains degeneracy before helium fusion starts, the entire helium core could fuse in a matter of minutes instead of millions of years (yet, the burst of energy will take forever to reach the surface).  

The link with luminosity is far from being so direct.

Is a brown dwarf a star of less mass than 0.8 solar masses ?

Less than 80 Jupiters  (closer to 0.08 solar masses)

Answer 2

I have no answers for you myself but the answer by Raymond is very interesting.  I can't wait for the other answers.