Important theoretical

Important theoretical work on the physical structure of stars occurred during the first decades of the twentieth century. In 1913, the Hertzsprung-Russell diagram was developed, propelling the astrophysical study of stars. Successful models were developed to explain the interiors of stars and stellar evolution. Cecilia Payne-Gaposchkin first proposed that stars were made primarily of hydrogen and helium in her 1925 PhD thesis.[31] The spectra of stars were further understood through advances in quantum physics. This allowed the chemical composition of the stellar atmosphere to be determined.[32]

Infrared image from NASA’s Spitzer Space Telescope showing hundreds of thousands of stars in the Milky Way galaxy
With the exception of rare events such as supernovae and supernova imposters, individual stars have primarily been observed in the Local Group,[33] and especially in the visible part of the Milky Way (as demonstrated by the detailed star catalogues available for our galaxy) and its satellites.[34] Individual stars such as Cepheid variables have been observed in the M87[35] and M100 galaxies of the Virgo Cluster,[36] as well as luminous stars in some other relatively nearby galaxies.[37] With the aid of gravitational lensing, a single star (named Icarus) has been observed at 9 billion light-years away.[38][39]

Designations
Main articles: Stellar designation, Astronomical naming conventions, and Star catalogue
The concept of a 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.[40] Many of the more prominent individual stars were given names, particularly with Arabic or Latin designations.

As well as certain constellations and the Sun itself, individual stars have their own myths.[41] 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.[41] (Uranus and Neptune were Greek and Roman gods, but neither planet was known in Antiquity because of their low brightness. Their names were assigned by later astronomers.)

Circa 1600, the names of the constellations were used to name the stars in the corresponding regions of the sky. The German astronomer Johann Bayer created a series of star maps and applied Greek letters as designations to the stars in each constellation. Later a numbering system based on the star’s right ascension was invented and added to John Flamsteed’s star catalogue in his book “Historia coelestis Britannica” (the 1712 edition), whereby this numbering system came to be called Flamsteed designation or Flamsteed numbering.[42][43]

The internationally recognized authority for naming celestial bodies is the International Astronomical Union (IAU).[44] The International Astronomical Union maintains the Working Group on Star Names (WGSN)[45] which catalogs and standardizes proper names for stars.[46] A number of private companies sell names of stars which are not recognized by the IAU, professional astronomers, or the amateur astronomy community.[47] The British Library calls this an unregulated commercial enterprise,[48][49] and the New York City Department of Consumer and Worker Protection issued a violation against one such star-naming company for engaging in a deceptive trade practice.[50][51]