Stellar astronomy

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Stellar astronomy

Stellar astronomy

ClassificationThe current stellar classification system originated in the early 20th century, when stars were classified from A to Q based on the strength of the hydrogen line.[134] It was not known at the time that the major influence on the line strength was temperature; the hydrogen line strength reaches a peak at over 9000 K, and is weaker at both hotter and cooler temperatures. When the classifications were reordered by temperature, it more closely resembled the modern scheme.

The concept of the constellation was known to exist during the Babylonian period. Ancient sky watchers imagined that prominent arrangements of stars formed patterns, and they associated these with particular aspects of nature or their myths. Twelve of these formations lay along the band of the ecliptic and these became the basis of astrology.[39] Many of the more prominent individual stars were also given names, particularly with Arabic or Latin designations.As well as certain constellations and the Sun itself, individual stars have their own myths.[40] To the Ancient Greeks, some "stars", known as planets (Greek πλανήτης (planētēs), meaning "wanderer"), represented various important deities, from which the names of the planets Mercury, Venus, Mars, Jupiter and Saturn were taken.[40] (Uranus and Neptune were also Greek and Roman gods, but neither planet was known in Antiquity because of their low brightness. Their names were assigned by later astronomers.)


The energy produced by stars, as a product of nuclear fusion, radiates into space as both electromagnetic radiation and particle radiation. The particle radiation emitted by a star is manifested as the stellar wind, which streams from the outer layers as free protons, and electrically charged alpha, and beta particles. Although almost massless there also exists a steady stream of neutrinos emanating from the star's core.The production of energy at the core is the reason stars shine so brightly: every time two or more atomic nuclei fuse together to form a single atomic nucleus of a new heavier element, gamma ray photons are released from the nuclear fusion product. This energy is converted to other forms of electromagnetic energy of lower frequency, such as visible light, by the time it reaches the star's outer layers.

Formation and evolutionStars form within extended regions of higher density in the interstellar medium, although the density is still lower than the inside of a vacuum chamber. These regions - known as molecular clouds - consist mostly of hydrogen, with about 23 to 28 percent helium and a few percent heavier elements. One example of such a star-forming region is the Orion Nebula.[55] As massive stars form from molecular clouds, they powerfully illuminate those clouds. They also ionize the hydrogen, creating an H II region.All stars spend the majority of their existence as main sequence stars, fueled primarily by the nuclear fusion of hydrogen into helium within their cores. However, stars of different masses have markedly different properties at various stages of their development. The ultimate fate of more massive stars differs from that of less massive stars, as do their luminosity and the impact they have on their environment. Accordingly, astronomers often group stars by their mass:[56]Very low mass stars with masses below 0.5 M☉ do not enter the asymptotic giant branch (AGB) but evolve directly into white dwarfsLow mass stars (including the Sun) with a mass above about 0.5 and below about 1.8–2.2 M☉ (depending on composition) do enter the AGB, where they develop a degenerate helium coreIntermediate-mass stars undergo helium fusion and develop a degenerate carbon-oxygen core.Massive stars have a minimum mass of 7–10 M☉, but this may be as low as 5–6 M☉. These stars undergo carbon fusion late in their lives, which ends in a core-collapse supernova explosion.



A short film showing the steps of the life cycle of stars.


How stars are formed and born


Stellar evolution of low-mass (left cycle) and high-mass (right cycle) stars, with examples in italics

Star formation is the process by which dense regions within molecular clouds in interstellar space, sometimes referred to as "stellar nurseries" or "star-forming regions", collapse to form stars. As a branch of astronomy, star formation includes the study of the interstellar medium and giant molecular clouds (GMC) as precursors to the star formation process, and the study of protostars and young stellar objects as its immediate products. It is closely related to planet formation, another branch of astronomy. Star formation theory, as well as accounting for the formation of a single star, must also account for the statistics of binary stars and the initial mass function.


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