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The protosun represents the very early stage in the life of a star, specifically like our own sun. It forms from a vast cloud of gas and dust, known as a nebula, which exists in space. Over time, due to gravitational forces, this nebula begins to collapse upon itself. As it collapses, the materials get drawn towards the center, leading to the formation of a core.
An interesting fact about a protosun is that, at this point, it hasn't yet started the process that will eventually make it a full-fledged star. This is because the conditions required for nuclear fusion have not yet been met. Eventually, the protosun will keep gathering more material from the nebula until it reaches the necessary temperature and pressure to trigger nuclear fusion, setting the stage for its next evolutionary phase.

Nuclear fusion is the process that powers the stars, including our sun. This transformative process occurs when hydrogen nuclei come so close together that they fuse, forming helium and releasing a tremendous amount of energy in the form of light and heat. This is what ultimately causes stars to shine brightly.
For nuclear fusion to occur, extremely high temperatures and pressures are essential. These extreme conditions allow the hydrogen nuclei, or protons, which naturally repel each other due to their positive charges, to overcome their repulsion and merge. Once fusion begins in a protosun, it signifies the birth of a new star.

• It is only after nuclear fusion starts that the protosun becomes a main-sequence star, fully capable of supporting a solar system.
• Nuclear fusion is incredibly efficient, enabling stars like our sun to burn and spread energy for billions of years.

A main-sequence star is a fully developed star that has successfully begun nuclear fusion in its core. At this stage, the conditions for sustained hydrogen burning are met, and the star enters the most stable and longest-lasting phase of its life cycle.
The main-sequence phase represents a star's balance between the outward pressure created by nuclear fusion and the inward pull of gravity. This equilibrium allows the star to maintain its structure and temperature over incredibly long periods, sometimes billions of years.
In our solar system, the sun is a prime example of a main-sequence star. It's in this phase that it generates the energy needed to warm our planet and is the key to supporting life. Over time, as the star exhausts its hydrogen fuel, it will eventually leave the main-sequence phase, evolving into subsequent stages, such as a red giant and ultimately ending its life cycle.