A protostar is a star in the making. It is a large, rotating ball of gas and dust that is collapsing under its own gravity. A protostar has not yet reached nuclear fusion, the process by which atoms are combined to form larger atoms. This makes protostars very different from stars like our Sun.
Protostars are usually found in molecular clouds, huge masses of cold gas and dust that drift through space. As a protostar collapses, it heats up. The heat causes the infalling gas and dust to bounce off the central object and creates a disk of material around it. This disk eventually becomes a planetary system. Our own Solar System may have begun as just such a disk around a young protostar.
As a protostar continues to collapse, it gets hotter and denser at its center. Finally, when the temperature reaches about 10 million degrees Celsius (18 million degrees Fahrenheit), nuclear fusion begins and the star “turns on.” At this point, the star is no longer considered a protostar; it is now an ordinary star like our Sun