Editing Globular cluster (section)

===Radii===
{{redirect-distinguish|Tidal radius|Roche limit}}
[[File:Appearances can be deceptive.jpg|thumb|alt=Hundreds of white-ish dots scattered on a black background, concentrated towards the center, with some brighter red and blue dots scattered across the frame|[[NGC 411]] is classified as an open cluster.<ref>{{cite news|title=Appearances can be deceptive|url=http://www.spacetelescope.org/images/potw1303a/|access-date=February 12, 2013|newspaper=ESO Picture of the Week | id= potw1303a}}</ref>]]

Astronomers characterize the morphology (shape) of a globular cluster by means of standard radii: the core radius (''r''<sub>''c''</sub>), the [[half-light radius]] (''r''<sub>''h''</sub>), and the tidal or Jacobi radius (''r''<sub>''t''</sub>). The radius can be expressed as a physical distance or as a subtended angle in the sky. Considering a radius around the core, the surface luminosity of the cluster steadily decreases with distance, and the core radius is the distance at which the apparent surface luminosity has dropped by half.<ref name="star clusters">{{cite web | url=http://www.astro.caltech.edu/~george/ay20/eaa-starclus.pdf |archive-url=https://web.archive.org/web/20060923134045/http://www.astro.caltech.edu/~george/ay20/eaa-starclus.pdf |archive-date=2006-09-23 |url-status=live | title=Star Clusters | publisher=Encyclopedia of Astronomy and Astrophysics | date=November 2000 | access-date=March 26, 2014 | author=Kenneth Janes | page=2}}</ref> A comparable quantity is the half-light radius, or the distance from the core containing half the total luminosity of the cluster; the half-light radius is typically larger than the core radius.<ref>{{cite web |last1=Rosen |first1=Anna |title=Understanding the Dynamical State of Globular Clusters |url=https://astrobites.org/2012/07/18/understanding-the-dynamical-state-of-globular-clusters/ |website=astrobites |date=July 18, 2012}}</ref><ref>{{cite journal |last1=Chatterjee |first1=Sourav |last2=Umbreit |first2=Stefan |last3=Fregeau |first3=John M. |last4=Rasio |first4=Frederic A. |title=Understanding the dynamical state of globular clusters: core-collapsed versus non-core-collapsed |journal=Monthly Notices of the Royal Astronomical Society |date=March 11, 2013 |volume=429 |issue=4 |pages=2881–2893 |doi=10.1093/mnras/sts464 |doi-access=free |bibcode=2013MNRAS.429.2881C |arxiv=1207.3063}}</ref>

Most globular clusters have a half-light radius of less than ten parsecs (pc), although some globular clusters have very large radii, like [[NGC 2419]] (r<sub>h</sub>&nbsp;= 18&nbsp;pc) and [[Palomar 14]] (r<sub>h</sub>&nbsp;= 25&nbsp;pc).<ref name="Bergh2007">{{Cite journal|last=Van den Bergh|first=Sidney|author-link=Sidney van den Bergh|date=November 2007|title=Globular Clusters and Dwarf Spheroidal Galaxies|journal=[[Monthly Notices of the Royal Astronomical Society]]|volume=385|issue=1|pages=L20–L22|arxiv=0711.4795|bibcode=2008MNRAS.385L..20V|doi=10.1111/j.1745-3933.2008.00424.x|doi-access=free |s2cid=15093329}}</ref> The half-light radius includes stars in the outer part of the cluster that happen to lie along the line of sight, so theorists also use the half-mass radius (''r''<sub>''m''</sub>){{snd}}the radius from the core that contains half the total mass of the cluster. A small half-mass radius, relative to the overall size, indicates a dense core. [[Messier&nbsp;3]] (M3), for example, has an overall visible dimension of about 18 [[arc minute]]s, but a half-mass radius of only 1.12&nbsp;arc minutes.<ref>{{cite journal
 | last1 =Buonanno |first1=R. |last2=Corsi |first2=C. E. |last3=Buzzoni |first3=A. |last4=Cacciari |first4=C. |last5=Ferraro |first5=F. R. |last6=Fusi Pecci |first6=F.
 | date= 1994
 | title = The Stellar Population of the Globular Cluster M 3. I. Photographic Photometry of 10 000 Stars
 | journal = Astronomy and Astrophysics | volume = 290
 | pages = 69–103
 | bibcode = 1994A&A...290...69B
}}</ref>

The tidal radius, or [[Hill sphere]], is the distance from the center of the globular cluster at which the external gravitation of the galaxy has more influence over the stars in the cluster than does the cluster itself.<ref name=Piatti-2019>{{cite journal| title = Characteristic radii of the Milky Way globular clusters| year = 2019| doi = 10.1093/mnras/stz2499| arxiv = 1909.01718| last1 = Piatti| first1 = Andrés E.| last2 = Webb| first2 = Jeremy J.| last3 = Carlberg| first3 = Raymond G.| journal = Monthly Notices of the Royal Astronomical Society| volume = 489| issue = 3| pages = 4367–4377| doi-access = free}}</ref> This is the distance at which the individual stars belonging to a cluster can be separated away by the galaxy. The tidal radius of M3, for example, is about forty arc minutes,<ref name="DaCosta">{{cite journal
| last1       = Da Costa
| first1      = G. S.
| last2      = Freeman
| first2     = K. C.
| date       = May 1976
| title      = The structure and mass function of the globular cluster M3
| journal    = Astrophysical Journal
| volume     = 206
| issue      = 1
| pages      = 128–137
| bibcode    = 1976ApJ...206..128D
| doi        = 10.1086/154363
}}</ref> or about 113&nbsp;pc.<ref name=Brosche-1999>{{cite journal |last1=Brosche|first1=P. |last2=Odenkirchen|first2=M. |last3=Geffert|first3=M. |date=March 1999|title=Instantaneous and average tidal radii of globular clusters|journal=New Astronomy|volume=4|issue=2|pages=133–139 |bibcode=1999NewA....4..133B|doi=10.1016/S1384-1076(99)00014-7}}</ref>