Pre-Main Sequence Stars
A Real HR Diagram of a Young Cluster
Young
stars still lie in a region cooler and more luminous than Main sequence stars.
Newly formed proto-stars are still heating up and contracting and are shown as 'x's in the diagram below. Pre-main sequence stars are shown as circles (some filled some not).
ZAMS means Zero Age Main Sequence. Once stars make it to this 'line', they begin fusion reactions in their core.
What fuels pre-main sequence stars and proto-stars?
Pre-main sequence means, BEFORE the main sequence, and BEFORE
nuclear fussion has begun. These stars have not yet contracted
enough in their cores to have achieved the temperature and density
required for nuclear fussion reactions. However, a lot of energy
is being released (and being driven out from the interior) from
these stars due to their Gravitational Contraction.