Protostar

The difference between a brown dwarf and a protostar that makes it to “star status” depends largely upon the mass of the protostar. As the star accretes (adds) more gas and dust, this is more material that can help counteract the effect of gravity by helping to maintain gas pressure. As the hydrogen and helium atoms contract into a common “center,” i.e. the core, the temperature, density, and pressure in the core reaches a critical level, somewhere in the neighborhood of 15,000,000 ^oC. The atoms are really close together, really hot, and trying to move around in a very tightly packed space. At this point, a special nuclear reaction called nuclear fusion begins. This occurs when two atoms “fuse” together to form one atom. Nuclear fusion releases thermal energy (heat), photons (light), supports the star, and stops the contraction. Thus, a star is born!
It takes a lot of mass to reach fusion temperatures, and that’s why some protostars never reach “star status.” Becoming a “star” depends upon a protostar’s initial mass. Small protostars (less than 0.05 solar masses) don’t make it.
Protostars are often hard to see, even in great Hubble Telescope pictures, because they are hidden deep inside a big dusty gas cloud. Using an infrared telescope makes them more visible. As the more massive protostars contract to become hot, glowing stars, the light and gas flowing away from the newborn star blows the material that was not accreted away, revealing the new stars. A single nebula can give birth to many stars.
How long does this whole process take? For a medium-sized star, around 1 million years. That may seem like a long time, but it’s rather fast when you consider how long most stars “live.”