ABSTRACT
Star formation is the process of transformation of diffuse interstellar gas into collapsed objects. Stars can be defined as objects that eventually reach a state of balance between their inward self-gravitational force and the outward force due to thermal pressure. Stars are expected to form through direct collapse, but many stars come in binary or multiple systems. A protostar is defined as an object that is accumulating matter from its surroundings. Protostars are usually obscured from view by a layer of dust in the infalling envelope that surrounds them, so that much of what is observed is the infrared emission from the surrounding dust. A pre-main-sequence star is an object that has ended its main mass accumulation phase, although residual mass accretion from a surrounding disk may still be active. One of the most striking aspects of star formation is the formation of a protostellar disk, sometimes also known as a circumstellar disk.
