View source for Messier 4

{{short description|Globular cluster in Scorpius}}
{{Infobox globular cluster
| name = Messier 4
| image = [[File:Globular star cluster Messier 4.jpg|280px]]
| caption = Globular star cluster Messier 4
| credit = 
| epoch = [[J2000]]
| class = IX<ref name=hcob849_11/>
| constellation = [[Scorpius]]
| ra = {{RA|16|23|35.22}}<ref name=aj140_6_1830/>
| dec = {{DEC|−26|31|32.7}}<ref name=aj140_6_1830/>
| dist_ly = {{Convert|6.033|kly|kpc|abbr=on|lk=on}}<ref>{{cite journal|last=Baumgardt|first=H.|display-authors=etal|title=Accurate distances to Galactic globular clusters through a combination of Gaia EDR3, HST, and literature data|journal=Monthly Notices of the Royal Astronomical Society|date=2021|volume=505|issue=4 |pages=5957–5977|doi=10.1093/mnras/stab1474|doi-access=free |arxiv =2105.09526 |bibcode = 2021MNRAS.505.5957B}}</ref>
| appmag_v = 5.6<ref>{{cite web |url=http://www.messier.seds.org/m/m004.html |title=Messier 4 |access-date=21 July 2024 |website=SEDS Messier Catalog}}</ref>
| size_v = 26′.0
| mass_msol = {{Val|8.4|e=4}}<ref name="Vitral+23">{{cite journal|last=Vitral|first=E.|display-authors=etal|title=An elusive dark central mass in the globular cluster M4|journal=Monthly Notices of the Royal Astronomical Society|date=2023|volume=522|issue=4 |pages=5740–5757|doi=10.1093/mnras/stad1068|doi-access=free |arxiv =2305.12702 |bibcode = 2023MNRAS.522.5740V }}</ref>
| radius_ly = 35 [[light-year]]s {{Citation needed|reason=Did not specify which radius it is (half-mass / core / tidal), and cannot find source. |date=September 2021}}
| v_hb =
| age = {{nowrap|(12.2 ± 0.2) [[Byr|Gyr]]}}<ref name=aaa143_1/>
| metal_fe = −1.07<ref name=aaa490_2/>
| notes = Closest [[globular cluster]]
| names = [[New General Catalogue|NGC]] 6121<ref name="simbad"/>
}}

'''Messier 4''' or '''M4''' (also known as '''NGC 6121''' or the '''Spider Globular Cluster''') is a [[globular cluster]] in the [[constellation]] of [[Scorpius]]. It was discovered by [[Philippe Loys de Chéseaux]] in 1745 and catalogued by [[Charles Messier]] in 1764.<ref name=":0">{{Cite web |title=Messier Object 4 |url=http://www.messier.seds.org/m/m004.html |access-date=2023-11-13 |website=www.messier.seds.org}}</ref> It was the first globular cluster in which individual [[star]]s were resolved.<ref name=":0" />

==Visibility==
M4 is conspicuous in even the smallest of [[telescope]]s as a fuzzy ball of light. It appears about the same size as the [[Moon]] in the sky. It is one of the easiest globular clusters to find, being located only 1.3 degrees west of the bright star [[Antares]], with both objects being visible in a wide-field telescope. Modestly sized telescopes will begin to resolve individual stars, of which the brightest in M4 are of [[apparent magnitude]] 10.8.<ref name=":0" />

==Characteristics==
M4 is a rather loosely concentrated cluster of [[Shapley–Sawyer Concentration Class|class IX]] and measures 75 [[light-year]]s across. It features a characteristic "bar" structure across its core, visible to moderate sized telescopes. The structure consists of 11th-magnitude stars and is approximately 2.5' long and was first noted by [[William Herschel]] in 1783. At least 43 [[variable star]]s have been observed within M4.<ref name=":0" />

M4 is approximately 6,000 light-years away,<ref>{{cite journal|last=Baumgardt|first=H.|display-authors=etal|title=Accurate distances to Galactic globular clusters through a combination of Gaia EDR3, HST, and literature data|journal=Monthly Notices of the Royal Astronomical Society|date=2021|volume=505|issue=4 |pages=5957–5977|doi=10.1093/mnras/stab1474|doi-access=free |arxiv =2105.09526 |bibcode = 2021MNRAS.505.5957B}}</ref> making it the closest globular cluster to the [[Solar System]]. It has an estimated age of 12.2 billion years.<ref name=aaa143_1/>

In astronomy, the abundance of elements other than [[hydrogen]] and [[helium]] is called the [[metallicity]], and it is usually denoted by the abundance ratio of [[iron]] to hydrogen as compared to the [[Sun]]. For this cluster, the measured abundance of iron is equal to:
:<math>\left[\frac{Fe}{H}\right]\ =\ -1.07 \pm 0.01</math>
This value is the [[logarithm]] of the ratio of iron to hydrogen relative to the same ratio in the Sun. Thus the cluster has an abundance of iron equal to 8.5% of the iron abundance in the Sun. This strongly suggests this cluster hosts two distinct stellar populations, differing by age. Thus the cluster probably saw two main cycles or phases of star formation.<ref name=aaa490_2/>

The [[Stellar kinematics|space velocity]] components are (U, V, W) = ({{nowrap|–57 ± 3}}, {{nowrap|–193 ± 22}}, {{nowrap|–8 ± 5}}) km/s. This confirms an orbit around the [[Milky Way]] of a period of {{nowrap|(116 ± 3) million years}} with [[Orbital eccentricity|eccentricity]] 0.80 ± 0.03: during peri[[apsis]] it comes within {{nowrap|(0.6 ± 0.1) [[Parsec#Parsecs and kiloparsecs|kpc]]}} from the galactic core, while at apo[[apsis]] it travels out to {{nowrap|(5.9 ± 0.3) kpc}}. The [[inclination]] is at (an angle of) {{nowrap|23° ± 6°}} from the galactic plane, thus it reaches as much as {{nowrap|1.5 ± 0.4 kpc}} above the disk.<ref name=aj117_4/> When passing through the disk, this cluster does so at less than 5 kpc from the [[galactic nucleus]]. The cluster undergoes [[tidal shock]] during each passage, which can cause the repeated shedding of stars. Thus the cluster may have been much more massive.<ref name=aaa490_2/>

==Notable stars==
Photographs by the [[Hubble Space Telescope]] in 1995 found [[white dwarf]] stars in M4 that are among the oldest known stars in [[Milky Way Galaxy|our galaxy]]; aged 13 billion years. One has been found to be a [[binary star]] with a pulsar companion, [[PSR B1620−26]] and a [[PSR B1620−26 b|planet]] orbiting it with a mass of 2.5 times that of [[Jupiter mass|Jupiter]] ({{jupiter mass}}).<ref>{{cite news|title=Ancient orbs|url=http://www.spacetelescope.org/images/potw1236a/|access-date=3 September 2012|newspaper=ESA/Hubble Picture of the Week}}</ref> One star in Messier 4 was also found to have much more of the rare light element lithium than expected.<ref>{{cite news|title=A Cluster with a Secret|url=http://www.eso.org/public/news/eso1235/|access-date=5 September 2012|newspaper=ESO Press Release}}</ref>

CX-1 Is located in M4. It is known as a possible millisecond pulsar/neutron star binary. It orbits in 6.31 hours.<ref name="Kaluzny"/>

==Spinthariscope analogy==
The view of Messier 4 through a good telescope was likened by [[Robert Burnham Jr.]] to that of hyperkinetic luminous alpha particles seen in a [[spinthariscope]].<ref>''Celestial Handbook, Volume three: Pavo through Vulpecula'' Robert Burnham Jr. page 1703.</ref>

==Central black hole==
[[File:STScI-01H0MY22SC4HPS3SYE78B4EZ4F AdobeExpress.gif|thumb|alt=Simulation of stellar motions in Messier 4|Simulation of stellar motions in Messier 4, where astronomers suspect that an [[intermediate-mass black hole]] could be present.<ref name="Vitral+23"/><ref name="NASAV23">{{cite news |date=23 May 2023|title=NASA's Hubble Hunts for Intermediate-Sized Black Hole Close to Home | url=https://www.nasa.gov/feature/goddard/2023/nasas-hubble-hunts-for-intermediate-sized-black-hole-close-to-home/ |work=NASA|access-date=23 May 2023}}</ref> If confirmed, the black hole would be in the center of the cluster, and would have a [[sphere of influence (black hole)]] limited by the red circle.]]
In 2023, an analysis of [[Hubble Space Telescope]] and European Space Agency's Gaia spacecraft data from Messier 4 revealed an excess mass of roughly 800 solar masses in the center of this cluster, which appears to not be extended. This could thus be considered as kinematic evidence for an [[intermediate-mass black hole]]<ref name="Vitral+23"/><ref name="NASAV23"/> (even if an unusually compact cluster of compact objects like [[white dwarfs]], [[neutron stars]] or stellar-mass [[black holes]] cannot be completely discounted).

==References==
{{reflist|refs=
<ref name="simbad">{{cite simbad | title=M 4 | access-date=2010-03-25}}</ref>

<ref name=aj140_6_1830>{{Cite journal | 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=aaa143_1>{{cite journal | author=Caputo, F.| author2=Castellani, V.| author3= Quarta, M. L. | title=Reddening, distance modulus and age of the globular cluster NGC 6121 (M4) from the properties of RR Lyrae variables | journal=Astronomy and Astrophysics | volume=143| issue=1 |date=February 1985 | pages=8–12 | bibcode=1985A&A...143....8C 
}}</ref>

<ref name=aj117_4>{{cite journal | author=Dinescu, Dana I.| author2= Girard, Terrence M.| author3= van Altena, William F. | title=Space Velocities of Globular Clusters. III. Cluster Orbits and Halo Substructure | journal=The Astronomical Journal | volume=117| issue=4 | pages=1792–1815 |date=April 1999 | doi=10.1086/300807 | bibcode=1999AJ....117.1792D | doi-access=free }}</ref>

<ref name=aaa490_2>{{cite journal | author=Marino, A. F. |display-authors=etal | title=Spectroscopic and photometric evidence of two stellar populations in the Galactic globular cluster NGC 6121 (M 4) | journal=Astronomy and Astrophysics | volume=490 | issue=2 | pages=625–640 |date=November 2008 | doi=10.1051/0004-6361:200810389 | bibcode=2008A&A...490..625M |arxiv = 0808.1414 |s2cid=15786238 }}</ref>

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

<!--<ref name=mnras406_3_2000>{{Cite journal | last1=Marks | first1=Michael | last2=Kroupa | first2=Pavel | title=Initial conditions for globular clusters and assembly of the old globular cluster population of the Milky Way | journal=[[Monthly Notices of the Royal Astronomical Society]] | volume=406 | issue=3 | pages=2000–2012 |date=August 2010 | doi=10.1111/j.1365-2966.2010.16813.x | bibcode=2010MNRAS.406.2000M |arxiv = 1004.2255 | s2cid=118652005 }} Mass is from M<sub>PD</sub> on Table 1.</ref>  -->

<ref name="Kaluzny">{{cite journal |last1=Kaluzny |first1=J. |last2=Rozanska |first2=A. |last3=Rozyczka |first3=M. |last4=Krzeminski |first4=W. |last5=Thompson |first5=Ian B. |title=A second neutron star in M4? |journal=The Astrophysical Journal Letters |date=May 2012 |volume=750 |issue=1 |page=L3 |doi=10.1088/2041-8205/750/1/L3 |url=https://ui.adsabs.harvard.edu/abs/2012ApJ...750L...3K |access-date=1 March 2021|arxiv=1203.5971 |bibcode=2012ApJ...750L...3K |s2cid=118323035 }}</ref>

}}

==See also==
* [[Messier object#Messier_objects|List of Messier objects]]

== External links ==
{{Commons category|Messier 4}}
* [http://www.messier.seds.org/m/m004.html M4, SEDS Messier pages]
* [http://gclusters.altervista.org/cluster_4.php?ggc=M+4 M4, Galactic Globular Clusters Database page]
* {{cite news | title=Hubble Space Telescope Finds Stellar Graveyard | publisher=Hubble News Desk | date=1995-08-28 | url=http://hubblesite.org/newscenter/newsdesk/archive/releases/1995/32/text/ | access-date=2006-05-24 }}
* {{cite web|last=Gray|first=Meghan|title=M4 – Globular Cluster|url=http://www.deepskyvideos.com/videos/messier/M4.html|work=Deep Sky Videos|publisher=[[Brady Haran]]}}
* {{WikiSky}}

{{Portal bar|Astronomy|Stars|Outer space}}
{{Catalogs|M=4|NGC=6121}}
{{Messier objects}}
{{Ngc65}}{{scorpius}}
{{Sky|16|23|35.41|-|26|31|31.9|7200}}

[[Category:Globular clusters|Messier 004]]
[[Category:Scorpius|Messier 004]]
[[Category:Messier objects|004]]
[[Category:NGC objects|Messier 004]]
[[Category:Astronomical objects discovered in 1745|?]]