Since the cloud was somewhat lumpy to begin with, some of the denser clumps collapse faster and fragment from the rest of the cloud.
Out of these dense knots more than one star may form: if several massive stars form, we see an O-B association
- As the collapse occurs, the protostars are large but cool and very red.
- They will appear perhaps as infrared sources embedded in dust clouds.
- The radiated energy comes from gravitational collapse.
When the core becomes hot enough, thermonuclear fusion begins, converting hydrogen to helium.
If O+B stars are present they will ionize the hydrogen in the molecular cloud with in the stromgren sphere and disassociate the molecular H in the neutral H region.
