October 19, 2021

Star is a large, luminous sphere of plasma, held together by gravity and radiation pressure, which at the end of its life may contain a proportion of degenerate matter. Observations suggest that the formation of these stars began around 180 million to 250 million years after the Big Bang. The Sun is the closest star to Earth and its greatest source of energy. Others are visible from Earth at night, when they are not obscured by sunlight or blocked by atmospheric phenomena. Historically, the most important stars in the celestial sphere have been grouped into constellations and asterisms, with the brightest given proper names. Extensive star catalogs were composed by astronomers, which allows for standardized designations. For at least part of its life, a star shines due to the nuclear fusion of hydrogen in its core, releasing energy that passes through its interior and radiates into outer space. Almost all of nature's elements heavier than helium were created by stars, either by stellar nucleosynthesis during their lifetime or by supernova nucleosynthesis when they explode. Astronomers can determine the mass, age, chemical composition and many other properties of a star by observing its spectrum, luminosity and movement in space. Its total mass is the main determinant of its evolution and possible destiny. Other characteristics are delimited by their evolutionary history, including diameter, rotation, movement, and temperature. The Hertzsprung-Russell Diagram (H-R Diagram), a distribution graph that shows the relationship between absolute magnitude or luminosity versus spectral type or stellar classification and its effective temperature, allows you to determine its age and evolutionary state. A star is formed by the collapse of a cloud of material, composed mostly of hydrogen and traces of heavier elements. Once the stellar nucleus is dense enough, some of the hydrogen is gradually converted to helium by the process of nuclear fusion. The rest of the star's interior transports energy from the core by a combination of radiant and convective processes. The internal pressure prevents it from collapsing due to its own gravity. When the core's fuel (hydrogen) is exhausted, stars that are at least 40% of the Sun's mass expand to become red giants, in some cases fusing heavier elements in the core or in layers around the core. The star then evolves into a degenerate form, recycling some of the material into the interstellar environment, where a new generation of stars with a higher proportion of heavy elements will form. Binary and multistellar systems consist of two or more stars that are gravitationally bound, moving around each other in stable orbits. When two of them are in relatively close orbits, their gravitational interaction can significantly impact their evolution. Stars can be part of a much larger gravitational relationship structure, like a cluster or a galaxy.

History of observation

Historically, the stars have been important to civilizations around the world, as part of religious practices, navigational aids, and astronomical guidance. Many ancient astronomers thought they were permanently attached to a celestial sphere and were immutable. By convention, astronomers grouped stars into constellations and used them to track the movements of the planets and the inferred position of the Sun. Solar motion relative to the star background (and horizon) was used to create calendars that could be used to regulate agricultural practices. The Gregorian calendar, currently used almost all over the world, is a solar and basic calendar.

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