What causes interstellar dust and gas to start to contract and form a protostar?

Interstellar dust and gas start to contract and form a protostar due to the presence of gravitational force. As a protastar begins to condense from dust and gas clouds, it emits infrared light and these single stars fragment into smaller objects forming many stars at one time.

The gravitational forces are what pull the interstellar dust and gas to form a protostar. The gravity force is strong enough to hold the dusts together to form clouds which later become big enough to be called protostars. However there are some clouds that are too small to become clouds and are called brown dwarfs.