Editing Andromeda Galaxy (section)

== Globular clusters ==
[[File:Star cluster in the Andromeda galaxy.jpg|thumb|Star clusters in the Andromeda Galaxy<ref name="And-Clustr"/>|400x400px]]
There are approximately 460 [[globular cluster]]s associated with the Andromeda Galaxy.<ref name="Barmby & Huchra 2001"/> The most massive of these clusters, identified as [[Mayall II]], nicknamed Globular One, has a greater luminosity than any other known globular cluster in the [[Local Group]] of galaxies.<ref name="hubblesite 1996-04-24"/> It contains several million stars and is about twice as luminous as [[Omega Centauri]], the brightest known globular cluster in the Milky Way. [[Mayall II]] (also known as Globular One or G1) has several stellar populations and a structure too massive for an ordinary globular. As a result, some consider Mayall II to be the remnant core of a [[dwarf galaxy]] that was consumed by Andromeda in the distant past.<ref name="Meylan et al 2001"/> The cluster with the greatest apparent brightness is G76 which is located in the southwest arm's eastern half.<ref name="NSOG"/>
Another massive globular cluster, named 037-B327 (also known as Bol 37) and discovered in 2006 as is heavily reddened by the Andromeda Galaxy's [[Cosmic dust|interstellar dust]], was thought to be more massive than Mayall II and the largest cluster of the Local Group;<ref name="SuperStr"/> however, other studies have shown it is actually similar in properties to Mayall II.<ref name="Cohen06"/>

Unlike the globular clusters of the Milky Way, which show a relatively low age dispersion, Andromeda Galaxy's globular clusters have a much larger range of ages: from systems as old as the galaxy itself to much younger systems, with ages between a few hundred million years to five billion years.<ref name="Formations"/>

In 2005, astronomers discovered a completely new type of star cluster in the Andromeda Galaxy. The new-found clusters contain hundreds of thousands of stars, a similar number of stars that can be found in globular clusters. What distinguishes them from the globular clusters is that they are much larger—several hundred light-years across—and hundreds of times less dense. The distances between the stars are, therefore, much greater within the newly discovered extended clusters.<ref name="Huxor et al 2005"/>

The most massive globular cluster in the Andromeda Galaxy, B023-G078, likely has a central intermediate black hole of almost 100,000 solar masses.<ref name="central black hole">{{cite journal | last1=Pechetti | first1=Renuka | last2=Seth | first2=Anil | last3=Kamann | first3=Sebastian | last4=Caldwell | first4=Nelson | last5=Strader | first5= Jay |title=Detection of a 100,000 M ⊙ black hole in M31's Most Massive Globular Cluster: A Tidally Stripped Nucleus |journal=The Astrophysical Journal |date=January 2022 |volume=924 |issue=2 |pages=13 |doi=10.3847/1538-4357/ac339f | arxiv=2111.08720 | bibcode=2022ApJ...924...48P | s2cid=245876938 | doi-access=free }}</ref>

=== PA-99-N2 event and possible exoplanet in galaxy ===
{{Main|PA-99-N2}}
PA-99-N2 was a microlensing event detected in the Andromeda Galaxy in 1999. One of the explanations for this is the gravitational lensing of a red giant by a star with a mass between 0.02 and 3.6 times that of the [[Sun]], which suggested that the star is likely orbited by a planet. This possible exoplanet would have a mass 6.34 times that of Jupiter. If finally confirmed, it would be the first ever found [[extragalactic planet]]. However, anomalies in the event were later found.<ref name="Better needed"/>