Editing Red-giant branch (section)

==Discovery==
[[File:NGC 288 HST.jpg|thumb|left|upright=1.0|The brightest stars in [[globular cluster]]s such as [[NGC 288]] are red giants.]]
[[Red giant]]s were identified early in the 20th century when the use of the [[Hertzsprung–Russell diagram]] made it clear that there were two distinct types of cool stars with very different sizes: dwarfs, now formally known as the [[main sequence]]; and [[giant star|giant]]s.<ref name=adamsjoy>{{cite journal|bibcode=1921ApJ....53...13A|title=The parallaxes of 1646 stars derived by the spectroscopic method|journal=Astrophysical Journal|volume=53|pages=13|last1=Adams|first1=W. S.|last2=Joy|first2=A. H.|last3=Stromberg|first3=G.|last4=Burwell|first4=C. G.|year=1921|doi=10.1086/142584}}</ref><ref name=trumpler>{{cite journal|bibcode=1925PASP...37..307T|title=Spectral Types in Open Clusters|journal=Publications of the Astronomical Society of the Pacific|volume=37|issue=220|pages=307|last1=Trumpler|first1=R. J.|year=1925|doi=10.1086/123509|doi-access=free}}</ref>

The term ''red-giant branch'' came into use during the 1940s and 1950s, although initially just as a general term to refer to the red-giant region of the Hertzsprung–Russell diagram. Although the basis of a thermonuclear main-sequence lifetime, followed by a thermodynamic contraction phase to a [[white dwarf]] was understood by 1940, the internal details of the various types of giant stars were not known.<ref name=gamow1939>{{cite journal|doi=10.1103/PhysRev.55.718|title=Physical Possibilities of Stellar Evolution|journal=Physical Review|volume=55|issue=8|pages=718–725|year=1939|last1=Gamow|first1=G.|bibcode = 1939PhRv...55..718G }}</ref>

In 1968, the name [[asymptotic giant branch]] (AGB) was used for a branch of stars somewhat more luminous than the bulk of red giants and more unstable, often large-amplitude [[variable star]]s such as [[Mira]].<ref name=sandage>{{cite journal|bibcode=1968ApJ...153L.129S|title=An Indication of Gaps in the Giant Branch of the Globular Cluster M15|journal=Astrophysical Journal|volume=153|pages=L129|last1=Sandage|first1=Allan|last2=Katem|first2=Basil|last3=Kristian|first3=Jerome|year=1968|doi=10.1086/180237}}</ref> Observations of a bifurcated giant branch had been made years earlier but it was unclear how the different sequences were related.<ref name=arp>{{cite journal|bibcode=1953AJ.....58....4A|title=The color-magnitude diagram of the globular cluster M 92|journal=Astronomical Journal|volume=58|pages=4|last1=Arp|first1=Halton C.|last2=Baum|first2=William A.|last3=Sandage|first3=Allan R.|year=1953|doi=10.1086/106800|doi-access=free}}</ref> By 1970, the red-giant region was well understood as being made up from [[subgiant]]s, the RGB itself, the [[horizontal branch]], and the AGB, and the evolutionary state of the stars in these regions was broadly understood.<ref name=strom>{{cite journal|bibcode=1970A&A.....8..243S|title=On the Evolutionary Status of Stars above the Horizontal Branch in Globular Clusters|journal=Astronomy and Astrophysics|volume=8|pages=243|last1=Strom|first1=S. E.|last2=Strom|first2=K. M.|author2-link=Karen Strom|last3=Rood|first3=R. T.|last4=Iben|first4=I.|year=1970}}</ref> The red-giant branch was described as the first giant branch in 1967, to distinguish it from the second or asymptotic giant branch,<ref name=iben>{{cite journal|bibcode=1967ARA&A...5..571I|title=Stellar Evolution Within and off the Main Sequence|journal=Annual Review of Astronomy and Astrophysics|volume=5|pages=571–626|last1=Iben|first1=Icko|year=1967|doi=10.1146/annurev.aa.05.090167.003035}}</ref> and this terminology is still frequently used today.<ref name=pols/>

Modern stellar physics has modelled the internal processes that produce the different phases of the post-main-sequence life of moderate-mass stars,<ref name=vassiliaid>{{cite journal|bibcode=1993ApJ...413..641V|title=Evolution of low- and intermediate-mass stars to the end of the asymptotic giant branch with mass loss|journal=Astrophysical Journal|volume=413|pages=641|last1=Vassiliadis|first1=E.|last2=Wood|first2=P. R.|year=1993|doi=10.1086/173033|doi-access=free}}</ref> with ever-increasing complexity and precision.<ref name=marigo>{{cite journal|bibcode=2008A&A...482..883M |doi=10.1051/0004-6361:20078467 |title=Evolution of asymptotic giant branch stars |journal=Astronomy and Astrophysics |volume=482 |issue=3 |pages=883–905 |year=2008 |last1=Marigo |first1=P. |last2=Girardi |first2=L. |last3=Bressan |first3=A. |last4=Groenewegen |first4=M. A. T. |last5=Silva |first5=L. |last6=Granato |first6=G. L. |arxiv = 0711.4922 |s2cid=15076538 }}</ref> The results of RGB research are themselves being used as the basis for research in other areas.<ref name=rizzi>{{cite journal|bibcode=2007ApJ...661..815R|arxiv=astro-ph/0701518|title=Tip of the Red Giant Branch Distances. II. Zero-Point Calibration|journal=The Astrophysical Journal|volume=661|issue=2|pages=815–829|last1=Rizzi|first1=Luca|last2=Tully|first2=R. Brent|last3=Makarov|first3=Dmitry|last4=Makarova|first4=Lidia|last5=Dolphin|first5=Andrew E.|last6=Sakai|first6=Shoko|last7=Shaya|first7=Edward J.|year=2007|doi=10.1086/516566|s2cid=12864247}}</ref>