What can contract to give birth to a star?

Answer 1

Stars are born when a clump of space debris grows large enough from accretion that it begins to contract and shrink into itself from its own force of gravity. As the debris contracts it begins to heat up from the pressures exerted by the contraction. And at a critical point, the heat becomes so intense that hydrogen begins to fuse into helium and lithium.

When that fusion begins, a new star is born. The fusion energy is like trillions of hydrogen bombs going off each second in our own Sun. That's 5 million tonnes of hydrogen per second.