Editing Horizontal branch (section)

==Discovery==
Horizontal branch stars were discovered with the first deep photographic [[Photometry (astronomy)|photometric]] studies of [[globular clusters]]<ref>
{{Citation
  | last1 = Arp  | first1 = H. C.
  | author1-link = Halton Arp
  | last2 = Baum | first2 = W. A.
  | last3 = Sandage | first3 = A. R.
  | author3-link = Allan Sandage
  | title = The HR diagrams for the globular clusters M 92 and M 3
  | journal = [[Astronomical Journal]]
  | volume = 57
  | pages = 4–5
  | year = 1952
  | doi = 10.1086/106674
| bibcode=1952AJ.....57....4A
}}</ref><ref>
{{Citation
  | last1 = Sandage | first1 = A. R.
  
  | title = The color-magnitude diagram for the globular cluster M 3
  | journal = [[Astronomical Journal]]
  | volume = 58
  | pages = 61–75
  | year = 1953
  | doi = 10.1086/106822
| bibcode=1953AJ.....58...61S
}}
</ref>
and were notable for being absent from all [[open clusters]] that had been studied up to that time. The horizontal branch is so named because in low-[[metallicity]] star collections like [[globular clusters]], HB stars lie along a roughly horizontal line in a [[Hertzsprung–Russell diagram]]. Because the stars of one globular cluster are all at essentially the same distance from us, their apparent magnitudes all have the same relationship to their absolute magnitudes, and thus absolute-magnitude-related properties are plainly visible on an H-R diagram confined to stars of that cluster, undiffused by distance and thence magnitude uncertainties.