Editing Main sequence (section)

== History ==
{{Star nav}}
In the early part of the 20th century, information about the types and distances of [[star]]s became more readily available. The [[stellar spectrum|spectra]] of stars were shown to have distinctive features, which allowed them to be categorized. [[Annie Jump Cannon]] and [[Edward Charles Pickering]] at [[Harvard College Observatory]] developed a method of categorization that became known as the [[stellar classification|Harvard Classification Scheme]], published in the ''Harvard Annals'' in 1901.<ref name=longair06/>

In [[Potsdam]] in 1906, the Danish astronomer [[Ejnar Hertzsprung]] noticed that the reddest stars—classified as K and M in the Harvard scheme—could be divided into two distinct groups. These stars are either much brighter than the Sun or much fainter. To distinguish these groups, he called them "giant" and "dwarf" stars. The following year he began studying [[star cluster]]s; large groupings of stars that are co-located at approximately the same distance. For these stars, he published the first plots of color versus [[luminosity]]. These plots showed a prominent and continuous sequence of stars, which he named the Main Sequence.<ref name=brown/>

At [[Princeton University]], [[Henry Norris Russell]] was following a similar course of research. He was studying the relationship between the spectral classification of stars and their actual brightness as corrected for distance—their [[absolute magnitude]]. For this purpose, he used a set of stars that had reliable [[parallax]]es and many of which had been categorized at Harvard. When he plotted the spectral types of these stars against their absolute magnitude, he found that dwarf stars followed a distinct relationship. This allowed the real brightness of a dwarf star to be predicted with reasonable accuracy.<ref name=obs36/>

Of the red stars observed by Hertzsprung, the dwarf stars also followed the spectra-luminosity relationship discovered by Russell. However, giant stars are much brighter than dwarfs and so do not follow the same relationship. Russell proposed that "giant stars must have low density or great surface brightness, and the reverse is true of dwarf stars". The same curve also showed that there were very few faint white stars.<ref name=obs36/>

In 1933, [[Bengt Strömgren]] introduced the term Hertzsprung–Russell diagram to denote a luminosity-spectral class diagram.<ref name=zfa7/> This name reflected the parallel development of this technique by both Hertzsprung and Russell earlier in the century.<ref name=brown/>

As evolutionary models of stars were developed during the 1930s, it was shown that, for stars with the same composition, the star's mass determines its luminosity and radius. Conversely, when a star's chemical composition and its position on the main sequence are known, the star's mass and radius can be deduced. This became known as the [[Vogt–Russell theorem]]; named after [[Heinrich Vogt (astronomer)|Heinrich Vogt]] and Henry Norris Russell. It was subsequently discovered that this relationship breaks down somewhat for stars of the non-uniform composition.<ref name=schatzman33/>

A refined scheme for [[stellar classification]] was published in 1943 by [[William Wilson Morgan]] and [[Philip Childs Keenan]].<ref name=keenan_morgan43/> The MK classification assigned each star a spectral type—based on the Harvard classification—and a luminosity class.  The Harvard classification had been developed by assigning a different letter to each star based on the strength of the hydrogen spectral line before the relationship between spectra and temperature was known.  When ordered by temperature and when duplicate classes were removed,  the [[spectral type]]s of stars followed, in order of decreasing temperature with colors ranging from blue to red, the sequence O, B, A, F, G, K, and M. (A popular [[mnemonic]] for memorizing this sequence of stellar classes is "Oh Be A Fine Girl/Guy, Kiss Me".) The luminosity class ranged from I to V, in order of decreasing luminosity. Stars of luminosity class V belonged to the main sequence.<ref name=tnc/>

In April 2018, astronomers reported the detection of the most distant "ordinary" (i.e., main sequence) [[star]], named [[Icarus (star)|Icarus]] (formally, [[MACS J1149 Lensed Star 1]]), at 9 billion light-years away from [[Earth]].<ref name=" NA-20180402">{{cite journal |author=Kelly, Patrick L. |display-authors=etal |title=Extreme magnification of an individual star at redshift 1.5 by a galaxy-cluster lens |date=2 April 2018 |journal=[[Nature (journal) |Nature]] |volume=2 |issue=4 |pages=334–342 |doi=10.1038/s41550-018-0430-3 |arxiv=1706.10279 |bibcode=2018NatAs...2..334K |s2cid=125826925}}</ref><ref name=" SPC-20180402">{{cite web |last=Howell |first=Elizabeth |title=Rare Cosmic Alignment Reveals Most Distant Star Ever Seen |url=https://www.space.com/40171-cosmic-alignment-reveals-most-distant-star-yet.html |date=2 April 2018 |work=[[Space.com]] |access-date=2 April 2018}}</ref>