Editing Globular cluster (section)

===Properties===
Since globular clusters form at once from a single giant molecular cloud, a cluster's stars have roughly the same age and composition. A star's evolution is primarily determined by its initial mass, so the positions of stars in a cluster's H–R or color–magnitude diagram mostly reflect their initial masses. A cluster's H–R diagram, therefore, appears quite different from H–R diagrams containing stars of a wide variety of ages. Almost all stars fall on a well-defined curve in globular cluster H–R diagrams, and that curve's shape indicates the age of the cluster.<ref name="carroll_ostlie_cmd" /><ref>{{cite journal
 | author=Sandage, A. R. | author-link=Allan Sandage
 | title=Observational Approach to Evolution. III. Semiempirical Evolution Tracks for M67 and M3
 | journal=Astrophysical Journal | volume=126
 | page=326 | date=1957
 | bibcode=1957ApJ...126..326S | doi=10.1086/146405 }}</ref> A more detailed H–R diagram often reveals multiple stellar populations as indicated by the presence of closely separated curves, each corresponding to a distinct population of stars with a slightly different age or composition.<ref name="bastian" /> Observations with the [[Wide Field Camera 3]], [[STS-125|installed in 2009]] on the Hubble Space Telescope, made it possible to distinguish these slightly different curves.<ref>{{cite journal |last1=Piotto |first1=G. |last2=Milone |first2=A. P. |last3=Bedin |first3=L. R. |last4=Anderson |first4=J. |last5=King |first5=I. R. |last6=Marino |first6=A. F. |last7=Nardiello |first7=D. |last8=Aparicio |first8=A. |last9=Barbuy |first9=B. |last10=Bellini |first10=A. |last11=Brown |first11=T. M. |last12=Cassisi |first12=S. |last13=Cool |first13=A. M. |last14=Cunial |first14=A. |last15=Dalessandro |first15=E. |last16=D'Antona |first16=F. |last17=Ferraro |first17=F. R. |last18=Hidalgo |first18=S. |last19=Lanzoni |first19=B. |last20=Monelli |first20=M. |last21=Ortolani |first21=S. |last22=Renzini |first22=A. |last23=Salaris |first23=M. |last24=Sarajedini |first24=A. |last25=Marel |first25=R. P. van der |last26=Vesperini |first26=E. |last27=Zoccali |first27=M.|author27-link=Manuela Zoccali |title=The ''Hubble Space Telescope'' UV Legacy Survey of Galactic Globular Clusters. I. Overview of the Project and Detection of Multiple Stellar Populations |journal=The Astronomical Journal |date=February 5, 2015 |volume=149 |issue=3 |page=91 |doi=10.1088/0004-6256/149/3/91|arxiv=1410.4564 |bibcode=2015AJ....149...91P |s2cid=119194870 }}</ref>

The most massive main-sequence stars have the highest luminosity and will be the first to evolve into the [[giant star]] stage. As the cluster ages, stars of successively lower masses will do the same. Therefore, the age of a single-population cluster can be measured by looking for those stars just beginning to enter the giant star stage, which form a "knee" in the H–R diagram called the [[main-sequence turnoff]], bending to the upper right from the main-sequence line. The absolute magnitude at this bend is directly a function of the cluster's age; an age scale can be plotted on an axis parallel to the magnitude.<ref name="carroll_ostlie_cmd" />

The morphology and luminosity of globular cluster stars in H–R diagrams are influenced by numerous parameters, many of which are still actively researched. Recent observations have overturned the historical paradigm that all globular clusters consist of stars born at exactly the same time, or sharing exactly the same chemical abundance. Some clusters feature multiple populations, slightly differing in composition and age; for example, high-precision imagery of cluster [[NGC 2808]] discerned three close, but distinct, main sequences.<ref name="kr2010">{{cite journal |bibcode=2010RSPTA.368..755K |title=Star clusters as laboratories for stellar and dynamical evolution |last1=Kalirai |first1=J. S. |last2=Richer |first2=H. B. |journal=Philosophical Transactions of the Royal Society of London, Series A |year=2010 |volume=368 |issue=1913 |pages=755–82 |doi=10.1098/rsta.2009.0257 |pmid=20083505 |arxiv=0911.0789 |s2cid=5561270 |quote=Verification of the picture above came from extremely precise HST/ACS imaging observations of NGC 2808 by ''Piotto et al.'' (2007), who resolve three main sequences in the cluster for a single turnoff (see figure 3). This remarkable observation is consistent with multiple stellar populations of approximately the same age with varying helium abundances}}</ref> Further, the placements of the cluster stars in an H–R diagram (including the brightnesses of distance indicators) can be influenced by observational biases. One such effect, called blending, arises when the cores of globular clusters are so dense that observations see multiple stars as a single target. The brightness measured for that seemingly single star is thus incorrect{{snd}}too bright, given that multiple stars contributed.<ref name="ma2012">{{cite journal |bibcode=2012ApJ...752L..10M |title=The Impact of Contaminated RR Lyrae/Globular Cluster Photometry on the Distance Scale |last1=Majaess |first1=D. |last2=Turner |first2=D. |last3=Gieren |first3=W. |last4=Lane |first4=D. |journal=The Astrophysical Journal |year=2012 |volume=752 |issue=1 |pages=L10 |doi=10.1088/2041-8205/752/1/L10 |arxiv=1205.0255 |s2cid=118528078 }}</ref> In turn, the computed distance is incorrect, so the blending effect can introduce a systematic uncertainty into the [[cosmic distance ladder]] and may bias the estimated age of the universe and the [[Hubble constant]].<ref name="lee2014">{{cite journal |bibcode=2014ApJS..210....6L |title=Toward a Better Understanding of the Distance Scale from RR Lyrae Variable Stars: A Case Study for the Inner Halo Globular Cluster NGC 6723 |last1=Lee |first1=Jae-Woo |last2=López-Morales |first2=Mercedes |last3=Hong |first3=Kyeongsoo |last4=Kang |first4=Young-Woon |last5=Pohl |first5=Brian L. |last6=Walker |first6=Alistair |journal=The Astrophysical Journal Supplement Series |year=2014 |volume=210 |issue=1 |page=6 |doi=10.1088/0067-0049/210/1/6 |arxiv=1311.2054 |s2cid=119280050 }}</ref>