Editing Blue straggler (section)

==Formation==
[[Image:Ngc6397 hst blue straggler.jpg|thumb|right|A [[Hubble Space Telescope]] image of [[NGC 6397]], with a number of bright blue stragglers present<ref>{{cite web
 | title=Too Close for Comfort | date=August 7, 2003
 | work=Hubble Site | publisher=NASA
 | url=http://hubblesite.org/newscenter/archive/releases/2003/21/
 | access-date=2010-01-21 }}</ref>]]
Several explanations have been put forth to explain the existence of blue stragglers. The simplest is that blue stragglers formed later than the rest of the stars in the cluster, but evidence for this is limited.<ref name = "hst">{{cite news | title=NASA's Hubble Space Telescope Finds "Blue Straggler" Stars in the Core of a Globular Cluster | publisher=Hubble News Desk | date=1991-07-24 | url=http://hubblesite.org/newscenter/newsdesk/archive/releases/1991/12/text/ | access-date=2006-05-24 }}</ref> Another simple proposal is that blue stragglers are either field stars which are not actually members of the clusters to which they seem to belong, or are field stars which were captured by the cluster. This too seems unlikely, as blue stragglers often reside at the very center of the clusters to which they belong. The most likely explanation is that blue stragglers are the result of stars that come too close to another star or similar mass object and [[Stellar collision|collide]].<ref name=pjtl>{{cite journal | author = Leonard, Peter J. T. | title =  Stellar collisions in globular clusters and the blue straggler problem | journal = The Astronomical Journal | volume = 98 | pages = 217–226 | bibcode = 1989AJ.....98..217L | year = 1989 | doi = 10.1086/115138}}</ref> The newly formed star has thus a higher mass, and occupies a position on the HR diagram which would be populated by genuinely young stars.

===Cluster interactions===
[[File:Evolution of globular clusters.ogv|thumb|left|upright=1.2|Video showing the movement of blue straggler stars in globular clusters over time]]
The two most viable explanations put forth for the existence of blue stragglers both involve interactions between cluster members. One explanation is that they are current or former [[binary star]]s that are in the process of merging or have already done so. The [[Stellar collision|merger]] of two stars would create a single more massive star, potentially with a mass larger than that of stars at the [[Main sequence#Evolutionary tracks|main-sequence turn-off point]]. While a star born with a mass larger than that of stars at the turn-off point would evolve quickly off the main sequence, the components forming a more massive star (via merger) would thereby delay such a change. There is evidence in favor of this view, notably that blue stragglers appear to be much more common in dense regions of clusters, especially in the cores of [[globular cluster]]s. Since there are more stars per unit volume, collisions and close encounters are far more likely in clusters than among field stars and calculations of the expected number of collisions are consistent with the observed number of blue stragglers.<ref name=pjtl />
[[File:NGC 6752 HST.jpg|thumb|right|[[NGC 6752]], a [[globular cluster]] that contains a high number of blue straggler stars<ref>{{cite news|title=Young Stars at Home in an Ancient Cluster|url=http://www.spacetelescope.org/images/potw1205a/|access-date=30 January 2012|newspaper=ESA/Hubble Picture of the Week}}</ref> ]]
One way to test this hypothesis is to study the [[Stellar pulsation]] of [[variable star|variable]] blue stragglers. The [[asteroseismology|asteroseismological]] properties of merged stars may be measurably different from those of typical pulsating variables of similar mass and luminosity. However, the measurement of pulsations is very difficult, given the scarcity of variable blue stragglers, the small [[Photometry (astronomy)|photometric]] amplitudes of their pulsations and the crowded fields in which these stars are often found. Some blue stragglers have been observed to [[rotate]] quickly, with one example in [[47 Tucanae]] observed to rotate 75 times faster than the [[Sun]], which is consistent with formation by collision.<ref>{{cite news | title = Hubble Catches up with a Blue Straggler Star | publisher = Hubble News Desk | date = 1997-10-29 | access-date = 2022-04-28 | url = https://hubblesite.org/contents/news-releases/1997/news-1997-35.html}}</ref>

The other explanation relies on mass transfer between two stars born in a [[binary star]] system. The more massive of the two stars in the system will evolve first and as it expands, will overflow its [[Roche lobe]]. Mass will quickly transfer from the initially more massive companion onto the less massive; like the collision hypothesis, this would explain why there are main-sequence stars more massive than other stars in the cluster which have already evolved off the main sequence.<ref name=shu>{{cite book | author = Shu, Frank | title = The Physical Universe | url = https://archive.org/details/physicaluniverse00shuf | url-access = registration | year = 1982 | publisher = University Science Books | isbn = 978-0-935702-05-7}}</ref> Observations of blue stragglers have found that some have significantly less [[carbon]] and [[oxygen]] in their [[photosphere]]s than is typical, which is evidence of their outer material having been dredged up from the interior of a companion.<ref name = "Space.com2006">{{cite news | title = Origin of Strange 'Blue Straggler' Stars Pinned Down | url = http://www.space.com/2967-origin-strange-blue-straggler-stars-pinned.html  | publisher = Space.com | date = 2006-10-05 | access-date = 2014-03-23}}</ref><ref name="Ferraro2006">{{Cite journal | doi = 10.1086/507327| title = Discovery of Carbon/Oxygen-depleted Blue Straggler Stars in 47 Tucanae: The Chemical Signature of a Mass Transfer Formation Process| journal = The Astrophysical Journal| volume = 647| issue = 1| pages = L53–L56| date = 2006-08-10| last1 = Ferraro | first1 = F. R.| last2 = Sabbi | first2 = E.| last3 = Gratton | first3 = R.| last4 = Piotto | first4 = G.| last5 = Lanzoni | first5 = B.| last6 = Carretta | first6 = E.| last7 = Rood | first7 = R. T.| last8 = Sills | first8 = A. | last9 = Fusi Pecci | first9 = F.| last10 = Moehler | first10 = S.| last11 = Beccari | first11 = G.| last12 = Lucatello | first12 = S.| last13 = Compagni | first13 = N.|arxiv = astro-ph/0610081 |bibcode = 2006ApJ...647L..53F | s2cid = 119450832}}</ref>

Overall, there is evidence in favor of both collisions and mass transfer between binary stars.<ref>{{cite news | title = Blue Stragglers Can Be Either Vampires or Stellar Bad Boys | author = Nancy Atkinson | publisher = Universe Today | url = http://www.universetoday.com/2009/12/23/blue-stragglers-can-be-either-vampires-or-stellar-bad-boys/ | date = 2009-12-23 | access-date = 2010-01-18}}</ref> In [[Messier 3|M3]], [[47 Tucanae]], and [[NGC 6752]], both mechanisms seem to be operating, with collisional blue stragglers occupying the cluster cores and mass transfer blue stragglers at the outskirts.<ref>{{cite journal | author = Mapelli, M. | title = The radial distribution of blue straggler stars and the nature of their progenitors | journal = [[Monthly Notices of the Royal Astronomical Society]] | volume = 373 | issue = 1 | pages = 361–368 | year = 2006 | bibcode = 2006MNRAS.373..361M | doi = 10.1111/j.1365-2966.2006.11038.x| doi-access = free |arxiv = astro-ph/0609220 | s2cid = 14214665 |display-authors=etal}}</ref> The discovery of low-mass [[white dwarf]] companions around two blue stragglers in the [[Kepler Mission|Kepler]] field suggests these two blue stragglers gained mass via stable mass transfer.<ref>{{cite journal | title = Transits and Lensing by Compact Objects in the Kepler Field: Disrupted Stars Orbiting Blue Stragglers | author = Di Stefano, Rosanne | journal =  The Astronomical Journal| volume = 141 | issue = 5 | pages = 142 | year = 2010 | bibcode = 2011AJ....141..142D | doi=10.1088/0004-6256/141/5/142 | arxiv=1002.3009| s2cid = 118647532 }}</ref>

===Field formation===
[[File:New VISTA snap of star cluster 47 Tucanae.jpg|thumb|right|[[47 Tucanae]] contains at least 21 blue stragglers near its core.<ref name="hst"/>]]
Blue stragglers are also found among field stars, as a result of close binary interaction. Since the fraction of close binaries increases with decreasing [[metallicity]], blue stragglers are increasingly likely to be found across metal poor stellar populations. The identification of blue stragglers among field stars however is more difficult than in stellar clusters, because of the mix of stellar ages and metallicities among field stars. Field blue stragglers however can be identified among old stellar populations, like the Galactic halo, or dwarf galaxies.<ref name=":0" />