Star formation is a complex process that involves the gravitational collapse of a region within a large molecular cloud, leading to the formation of a protostar and, eventually, a main-sequence star. The stages of star formation can be broadly categorized as follows:

Molecular Clouds:
- Formation: Star formation begins in giant molecular clouds composed mostly of molecular hydrogen, helium, and trace amounts of other molecules. These clouds are often cold and dense.
- Triggering Mechanisms: Various processes, such as shockwaves from nearby supernovae, compression due to the collision of different molecular clouds, or the influence of external factors, can trigger the collapse of a region within the molecular cloud.
Protostellar Stage:
- Collapse: A dense region within the molecular cloud undergoes gravitational collapse due to its self-gravity, leading to the formation of a dense core.
- Protostar Formation: As the core contracts, it heats up, and a protostar is formed at the center. This phase is characterized by the accumulation of material onto the protostar and the presence of a surrounding protostellar disk.
Pre-Main Sequence (T Tauri) Stage:
- Accretion Disk: The protostar is surrounded by an accretion disk, where material from the surrounding disk falls onto the protostar. This process may last for tens of thousands to a few million years.
- Outflows and Jets: The protostar may launch powerful outflows or jets along its rotational axis. These outflows play a crucial role in removing angular momentum from the system.
Main Sequence Stage:
- Nuclear Fusion: Once the temperature and pressure at the core of the protostar become high enough, nuclear fusion reactions (mainly hydrogen to helium) begin, and the protostar becomes a main-sequence star.
- Stellar Winds: The star settles into a stable phase on the main sequence, and stellar winds may occur, influencing the surrounding environment.

The time scales for these stages can vary, with the collapse of a molecular cloud taking millions of years, the protostellar phase lasting a few hundred thousand to a few million years, and the main sequence stage lasting billions of years.

Throughout these stages, various factors such as mass, angular momentum, and the surrounding environment influence the characteristics of the forming star. The study of star formation provides valuable insights into the formation and evolution of stellar systems, including our own Sun.

Molecular clouds, where stars form, are extremely cold. Stars can be formed in a molecular cloud if its mass exceeds Jeans mass limit and its radius is less than Jeans length limit. If the cloud's temperature and density are known, the denim mass relation and length relation can be estimated as

Here, we talk about the trajectory of the collapsing cloud in the HR diagram. The path of the HR diagram of the collapsing cloud to the main sequence evolve on the thermal time scale.

The cloud collapses due to Jean's instability. Here we discuss the paths of SM collapsing clumps of a molecular cloud in the star forming region.

Here AB is the contraction of molecular cloud released gravitational potential energy and transformed into thermal radiation, Because of low density and small opacity, this thermal radiation can propagate throughout the collapsing material. Therefore most of the liberated energy radiated away. Because of this free propagation of thermal radiation luminosity of the cloud increases. Thus the temperature of the cloud doesn't increase. In BC as the cloud contracts, the density and opacity of the cloud increases. Because of this, photons are trapped in the collapsing cloud and hence luminosity doesn't increase. Hence, the released gravitational potential energy is formed into thermal energy and temperature of the cloud begins to rise. This increase in temperature increases the non degenerate gas pressure. This increase in pressure resists the in fall in the central part, however outer parts fall freely.

Density, opacity and pressure increases as contraction continues.

This note is a part of the Physics Repository.

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