View source for Messier 55

{{short description|Globular cluster in the constellation Sagittarius}}
{{Use mdy dates|date=April 2012}}
{{Infobox globular cluster
| name = [[Messier object|Messier]] 55
| image = [[File:Messier 55 VISTA.jpg|300px]]
| caption = Globular cluster Messier 55 in Sagittarius
| credit = 
| epoch = [[J2000]]
| class = XI<ref name=hcob849_11/>
| constellation = [[Sagittarius (constellation)|Sagittarius]]
| ra = {{RA|19|39|59.71}}<ref name=aj140_6_1830/>
| dec = {{DEC|−30|57|53.1}}<ref name=aj140_6_1830/>
| dist_ly = {{Convert|17.6|kly|kpc|abbr=on|lk=on}}<ref name=apj742_1_51/>
| dist_pc = 
| appmag_v = 6.3<ref>{{cite web |url=https://messier.seds.org/m/m055.html |title=Messier 55 |access-date=29 April 2022 |website=SEDS Messier Catalog}}</ref>
| size_v = 19.0{{prime}}
| mass_msol = {{Val|2.69|e=5}}<ref name=apj742_1_51/>
| radius_ly = 48&nbsp;ly<ref>From [[trigonometry]]: radius = {{nowrap|distance × sin( diameter_angle / 2 )}} = {{nowrap|17,600 × sin(19{{prime}}/2)}} = 48.6&nbsp;ly.</ref>
| metal_fe = –1.94<ref name=apj742_1_51/>
| v_hb = 
| age = 12.3&nbsp;[[Gigayear|Gyr]]<ref name=aaa481_3_701/>
| notes = 
| names = M55, [[New General Catalogue|NGC]] 6809, GCl 113, C 1936-310<ref name="simbad" />
}}
'''Messier 55''' (also known as '''M55''', '''[[New General Catalogue|NGC]] 6809''', or '''Specter Cluster''') is a [[globular cluster]] in the south of the [[constellation]] [[Sagittarius (constellation)|Sagittarius]]. It was discovered by [[Nicolas Louis de Lacaille]] in 1752{{efn|On June 16th}} while observing from what today is [[South Africa]].<ref name=thompson2007/>  Starting in 1754, [[Charles Messier]] made several attempts to find this object from [[Paris]] but its low declination meant from there it rises daily very little above the horizon, hampering observation.{{efn|Specifically in the south of this constellation which makes it visible from everywhere below about the [[50th parallel north]]. However the Sun passes through Sagittarius (or technically the Earth orbits so as to make the Sun seem to do so) throughout December. This also makes the cluster mostly risen during day, not night, in the nearest months.}}  He observed and catalogued it in 1778.  The cluster can be seen with 50&nbsp;mm binoculars; resolving individual stars needs a medium-sized telescope.<ref name=thompson2007/>

It is about 17,600 [[light-year]]s away from [[Earth]]. It contains about 269,000 [[solar mass]]es ({{solar mass}}).<ref name=apj742_1_51/> As with other Milky Way globular clusters, it has few elements other than hydrogen and helium compared to the Sun. Messier 55 therefore has "low [[metallicity]]". This quantity is normally listed as the base 10 [[logarithm]] of the proportion of the Sun; for NGC 6809 the metallicity is given by: [Fe/H] = &minus;1.94 [[decimal exponent|dex]], whereby &minus;2 would be 100 times less iron than the Sun.<ref name=apj742_1_51/> This means the cluster has 1.1% of the proportion of the Sun's iron compared to hydrogen and helium.

Only about 55 [[variable star]]s have been found in the central part of M55.<ref name=aa60_3_245/>

==Gallery==
{{Gallery|width=200
|File:M55HunterWilson.jpg|Messier 55, wide field view
|File:M55map.png|Map showing the location of M55
}}

==See also==
* [[List of Messier objects]]

==References and footnotes==
{{reflist|refs=

<ref name="simbad">{{cite simbad | title=M 55 | access-date=2006-11-16 }}</ref>

<ref name=apj742_1_51>{{citation | display-authors=1 | last1=Boyles | first1=J. | last2=Lorimer | first2=D. R. | last3=Turk | first3=P. J. | last4=Mnatsakanov | first4=R. | last5=Lynch | first5=R. S. | last6=Ransom | first6=S. M. | last7=Freire | first7=P. C. | last8=Belczynski | first8=K. | title=Young Radio Pulsars in Galactic Globular Clusters | journal=The Astrophysical Journal | volume=742 | issue=1 | page=51 |date=November 2011 | doi=10.1088/0004-637X/742/1/51 | bibcode=2011ApJ...742...51B |arxiv = 1108.4402 | s2cid=118649860 }}</ref>

<ref name=thompson2007>{{citation | first1=Robert Bruce | last1=Thompson | first2=Barbara Fritchman | last2=Thompson | title=Illustrated guide to astronomical wonders | series=DIY science | publisher=[[O'Reilly Media, Inc.]] | year=2007 | isbn=978-0-596-52685-6 | page=413 | url=https://books.google.com/books?id=ymt9nj_uPhwC&pg=PA413 }}</ref>

<ref name=aj140_6_1830>{{citation | display-authors=1 | last1=Goldsbury | first1=Ryan | last2=Richer | first2=Harvey B. | last3=Anderson | first3=Jay | last4=Dotter | first4=Aaron | last5=Sarajedini | first5=Ata | last6=Woodley | first6=Kristin | title=The ACS Survey of Galactic Globular Clusters. X. New Determinations of Centers for 65 Clusters | journal=The Astronomical Journal | volume=140 | issue=6 | pages=1830–1837 |date=December 2010 | doi=10.1088/0004-6256/140/6/1830 | bibcode=2010AJ....140.1830G |arxiv = 1008.2755 | s2cid=119183070 }}</ref>

<ref name=aa60_3_245>{{citation | display-authors=1 | last1=Kaluzny | first1=J. | last2=Thompson | first2=I. B. | last3=Krzeminski | first3=W. | last4=Zloczewski | first4=K. | title=The Cluster AgeS Experiment (CASE). Variable Stars in the Globular Cluster M55 | journal=Acta Astronomica | volume=60 | issue=3 | pages=245–260 |date=September 2010 | bibcode=2010AcA....60..245K |arxiv = 1011.0831 }}</ref>

<ref name=hcob849_11>{{citation | last1=Shapley | first1=Harlow | last2=Sawyer | first2=Helen B. | title=A Classification of Globular Clusters | journal=Harvard College Observatory Bulletin | volume=849 | issue=849 | pages=11–14 |date=August 1927 | bibcode=1927BHarO.849...11S | postscript=. }}</ref>

<ref name=aaa481_3_701>{{citation | display-authors=1 | last1=Sollima | first1=A. | last2=Lanzoni | first2=B. | last3=Beccari | first3=G. | last4=Ferraro | first4=F. R. | last5=Fusi Pecci | first5=F. | title=The correlation between blue straggler and binary fractions in the core of Galactic globular clusters | journal=Astronomy and Astrophysics | volume=481 | issue=3 | pages=701–704 |date=April 2008 | doi=10.1051/0004-6361:20079082 | bibcode=2008A&A...481..701S |arxiv = 0801.4511 | s2cid=3088769 }}</ref>

}}
{{notelist}}

==External links==
{{Commons category|Messier 55}}
* [http://messier.seds.org/m/m055.html Messier 55, SEDS Messier pages]
* [http://gclusters.altervista.org/cluster_4.php?ggc=M+55 Messier 55, Galactic Globular Clusters Database page]
* {{APOD |date=23 February 2001|title=M55 Color Magnitude Diagram}}
* {{WikiSky}}

{{Sky|19|39|59.40|-|30|57|43.5|17300}}
{{Portal bar|Astronomy|Stars|Outer space}}
{{Messier objects}}
{{catalogs|M=55|NGC=6809}}
{{Ngc70}}

[[Category:Carina–Sagittarius Arm]]
[[Category:Globular clusters|Messier 055]]
[[Category:Sagittarius (constellation)|Messier 055]]
[[Category:Messier objects|055]]
[[Category:NGC objects|Messier 055]]
[[Category:Astronomical objects discovered in 1752|17520616]]