About Protostars And Stages of Star Formation...

 Protostar is a celestial object in a late stage of star formation in which the central condensation resulting from the collapse of a dense, interstellar cloud core has become a sphere that has a luminous surface like a star, but yet has to sustain nuclear fusion of hydrogen in its core.

(Star formation stage- Protostar)

A protostar is a very young star that is still gathering mass from its parent molecular cloud. The protostar phase is the earliest one in the process of star evolution. For our Sun or a low mass star, it lasts about 500,000 years. 

STAGES OF STAR FORMATION:

1. Giant Gas Cloud: A star originates from a large cloud of gas. The temperature in the cloud is low enough for the synthesis of molecules.
2. Protostar: When the gas particle in the molecular cloud run into each other, heat energy is produced. This results in the formation of a warm clump of molecules referred to as the Protostar. Protostars are warmer than other materials in the molecular cloud.
3. T-Tauri Phase: A T-Tauri star begins when materials stop falling into the protostar and release tremendous amount of energy. The mean temperature of the Tauri star is not enough to support nuclear fusion at its core.
4. Main Sequence Phase: The main sequence phase is the stage in development where the core temperature reaches the point for the fusion to commence. In this process, the protons of hydrogen are converted into atoms of helium. This reaction is exothermic; it gives off more heat than it requires and so the core of a main sequence star releases a tremendous amount of energy.
5. Red giant: A star converts hydrogen atoms into helium over its course of life at its core. Eventually, the hydrogen fuel runs out, and the internal reaction stops. Without reactions occuring at the core, a star contracts inward through gravity causing it to expand. As it expands, the star first becomes a subgiant star and then a red giant. Red giants have a cooler surfaces than the main-sequence star and because of this, they appear red than yellow.
6. The Fusion of Heavier Elements: Helium molecules fuse at the core, as the star expands. The energy of this reaction prevents the core from collapsing. The core shrinks and begins fusing carbon, once the helium fusion ends. This process repeats until iron appears at the core. The iron fusion reaction absorbs energy, which causes the core to collapse. This implosion transforms massive stars into supernova while smaller stars like the sun contract into white dwarfs.
7. Supernovae and Planetary Nebulae: When most of the star material is blasted away into space, the core implodes into a neutron star or a singularity known as the black hole. Less massive stars don't explode, their cores contract instead into a tiny, hot star known as the white dwarf while the outer material drifts away.

By- Kumar Amityush