Astronomers believe there could be up to one septillion stars in the universe, which is a number represented by a one followed by 24 zeros. The Milky Way galaxy alone holds over 100 billion stars, among them our well-known Sun.
Stars are enormous spheres of hot gases, primarily composed of hydrogen, with helium and traces of other elements. Each star has a unique life cycle, lasting from a few million years to several trillion years, and they evolve as they age.
Stars originate in vast concentrations of gas and dust known as molecular clouds. These clouds are between 1,000 to 10 million times the mass of the Sun and can extend over hundreds of light-years. The cold temperature in these clouds causes the gas to clump together, forming dense regions. Some clumps may collide or gather more material, increasing their gravitational pull as they gain mass. Eventually, gravity causes these clumps to collapse, leading to friction and heating, which results in the formation of a protostar, or a young star. Groups of stars that recently formed from molecular clouds are referred to as stellar clusters, and a molecular cloud brimming with these clusters is called a stellar nursery.
Initially, most of a protostar's energy comes from the heat generated by its collapse. Over millions of years, the extreme pressures and temperatures in the core of the star force the nuclei of hydrogen atoms to merge into helium, in a process called nuclear fusion. This fusion releases energy, heating the star and preventing it from collapsing further due to gravity.
Stars undergoing stable nuclear fusion of hydrogen into helium are called main sequence stars, and this phase is the longest in a star's life cycle. During this time, the star's brightness, size, and temperature change slowly over the course of millions or billions of years. Our Sun is currently about halfway through its main sequence phase.
The mass of a star determines how quickly it consumes its fuel, with lighter stars burning longer, cooler, and dimmer compared to much heavier stars. Larger stars have to burn through their fuel more rapidly to generate the energy needed to counteract the gravitational collapse. Some stars with lower mass can shine for trillions of years, exceeding the current age of the universe, while more massive stars may only last a few million years.
Towards the end of a star's life, its core depletes its hydrogen reserves for fusion into helium, causing the balance between fusion and gravitational forces to shift, and the core starts to collapse. As the core is squeezed, its temperature and pressure increase, causing the star to expand. However, the exact processes of a star's final stages vary significantly depending on its mass.
For stars with low mass, their outer layers will expand, and they will become subgiant or giant stars as the core fuses helium into carbon. This will be the eventual fate of our Sun, albeit in several billion years. Some of these giants become unstable and pulsate, sometimes expanding and shedding part of their outer layers. Eventually, they shed all the outer layers, forming an expanding cloud of gas and dust known as a planetary nebula.
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