The matter from a nebula that begins to condense under the weight of gravity and begins to form a star is called a protostar.

Here's a breakdown:

* Nebula: A giant cloud of gas and dust in space.

* Gravity: The force that pulls matter together.

* Condensation: The process of gas and dust particles clumping together due to gravity.

* Protostar: A hot, dense, and luminous object that is formed by the condensation of matter from a nebula. It's essentially a baby star still forming.

As the protostar continues to condense and heat up, nuclear fusion begins in its core, marking the birth of a star.