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Representative lifetimes of stars as a function of their masses The change in size with time of a Sun-like star Artist's depiction of the life cycle of a Sun-like star, starting as a main-sequence star at lower left then expanding through the subgiant and giant phases, until its outer envelope is expelled to form a planetary nebula at upper right Chart of stellar evolution
A protostar is a very young star that is still gathering mass from its parent molecular cloud. It is the earliest phase in the process of stellar evolution . [ 1 ] For a low-mass star (i.e. that of the Sun or lower), it lasts about 500,000 years. [ 2 ]
The gas is instead forced to spread outwards near its equatorial plane, forming a disk, which in turn accretes onto the core. [2] [39] [40] The core gradually grows in mass until it becomes a young hot protostar. [38] At this stage, the protostar and its disk are heavily obscured by the infalling envelope and are not directly observable. [16]
The W51 nebula in Aquila - one of the largest star factories in the Milky Way (August 25, 2020). Star formation is the process by which dense regions within molecular clouds in interstellar space, sometimes referred to as "stellar nurseries" or "star-forming regions", collapse and form stars. [1]
The collapse releases gravitational energy, which heats up the protostar. This process occurs on the free fall timescale, which is roughly 100,000 years for solar-mass protostars, and ends when the protostar reaches approximately 4000 K. This is known as the Hayashi boundary, and at this point, the protostar is on the Hayashi track.
The protostar at first only has about 1% of its final mass. But the envelope of the star continues to grow as infalling material is accreted. After 10,000–100,000 years, [1] thermonuclear fusion begins in its core, then a strong stellar wind is produced which stops the infall of new mass. The protostar is now considered a young star since its ...
A star forms by accumulation of material that falls in to a protostar from a circumstellar disk or envelope. Material in the disk is cooler than the surface of the protostar, so it radiates at longer wavelengths of light producing excess infrared emission. As material in the disk is depleted, the infrared excess decreases.
A protostar may have too little mass to become a star. If so it becomes a brown dwarf. If the protostar has sufficient mass, the density reaches a critical level where the temperature exceeds 10 million kelvin at its center. At this point, a nuclear reaction starts converting hydrogen to helium and releasing large amounts of energy.