The Sun, like other stars, formed from a giant cloud of gas and dust in space, primarily composed of hydrogen and helium. This process, known as stellar formation, can be broken down into several key stages:
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Molecular Cloud: The Sun's birth began in a molecular cloud, a dense region of cold gas and dust. These clouds are often referred to as stellar nurseries because they are the sites where new stars are born.
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Gravitational Collapse: Over time, regions within the molecular cloud can become gravitationally unstable. When a disturbance, such as shock waves from nearby supernovae or collisions with other clouds, occurs, certain areas can collapse under their own gravity. As a result, denser clumps of material form.
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Formation of a Protostar: As the clump collapses, it begins to spin and flatten into a disk-like shape. The center of this clump becomes increasingly hot and dense, forming a protostar. During this stage, material from the surrounding disk continues to fall onto the protostar, increasing its mass and temperature.
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Nuclear Fusion Initiation: Once the temperature and pressure at the core of the protostar become sufficiently high (around 10 million degrees Celsius), nuclear fusion begins. In this process, hydrogen nuclei combine to form helium, releasing a huge amount of energy in the form of light and heat.
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Main Sequence Star: After nuclear fusion starts, the protostar evolves into a main sequence star. The Sun entered this phase approximately 4.6 billion years ago. During this stable period, the forces of gravity pulling inward are balanced by the outward pressure from the nuclear fusion reactions in the core. The Sun has remained in this stable state for about half of its lifespan and will continue to do so for several billion more years.
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Future Evolution: Eventually, the Sun will exhaust its hydrogen fuel and undergo changes that will lead it to evolve into a red giant and finally shed its outer layers, leaving behind a white dwarf surrounded by a planetary nebula.
This process of stellar formation is a fundamental part of the life cycle of stars in the universe and explains how the Sun and other stars have come to exist.