Editing Andromeda Galaxy (section)

=== Nucleus ===
[[File:M31 nucleus (labels).jpg|thumb|[[Hubble Space Telescope]] image of the Andromeda Galaxy core showing P1, P2 and P3, with P3 containing M31*. [[NASA]]/[[European Space Agency|ESA]] photo]]
The Andromeda Galaxy is known to harbor a dense and compact star cluster at its very center, similar to the [[Milky Way]] galaxy. A large telescope creates a visual impression of a star embedded in the more diffuse surrounding bulge. In 1991, the [[Hubble Space Telescope]] was used to image the Andromeda Galaxy's inner nucleus. The nucleus consists of two concentrations separated by {{cvt|1.5|pc|lk=on}}. The brighter concentration, designated as P1, is offset from the center of the galaxy. The dimmer concentration, P2, falls at the true center of the galaxy and contains an embedded star cluster, called P3,<ref name="Bender et al 2005"/> containing many [[Ultraviolet|UV]]-bright [[A-type main-sequence star|A-stars]] and the [[supermassive black hole]], called [[M31*]].<ref name=":0">{{Cite journal |last1=Garcia |first1=Michael R. |last2=Hextall |first2=Richard |last3=Baganoff |first3=Frederick K. |last4=Galache |first4=Jose |last5=Melia |first5=Fulvio |last6=Murray |first6=Stephen S. |last7=Primini |first7=F. A. |last8=Sjouwerman |first8=Loránt O. |last9=Williams |first9=Ben |date=1 February 2010 |title=X-ray and Radio Variability of M31*, The Andromeda Galaxy Nuclear Supermassive Black Hole |url=https://ui.adsabs.harvard.edu/abs/2010ApJ...710..755G |journal=The Astrophysical Journal |volume=710 |issue=1 |pages=755–763 |arxiv=0907.4977 |bibcode=2010ApJ...710..755G |doi=10.1088/0004-637X/710/1/755 |hdl=1721.1/96091 |issn=0004-637X}}</ref><ref name=":1">{{Cite journal |last1=Yang |first1=Yang |last2=Li |first2=Zhiyuan |last3=Sjouwerman |first3=Loránt O. |last4=Yuan |first4=Feng |last5=Shen |first5=Zhi-Qiang |date=1 August 2017 |title=Very Large Array Multiband Monitoring Observations of M31* |journal=The Astrophysical Journal |volume=845 |issue=2 |pages=140 |arxiv=1707.08317 |bibcode=2017ApJ...845..140Y |doi=10.3847/1538-4357/aa8265 |doi-access=free |issn=0004-637X}}</ref> The black hole is classified as a low-luminosity [[Active galactic nucleus|AGN]] (LLAGN) and it was detected only in [[Radio astronomy|radio wavelengths]] and in [[X-ray astronomy|x-rays]].<ref name=":1"/> It was quiescent in 2004–2005, but it was highly variable in 2006–2007.<ref name=":0"/> An additional x-ray flare occurred in 2013.<ref>{{Cite journal |last1=DiKerby |first1=Stephen |last2=Zhang |first2=Shuo |last3=Irwin |first3=Jimmy |date=2025-03-01 |title=Fifteen Years of M31* X-Ray Variability and Flares |journal=The Astrophysical Journal |volume=981 |issue=1 |pages=50 |arxiv=2502.01365 |bibcode=2025ApJ...981...50D |doi=10.3847/1538-4357/adb1d5 |doi-access=free |issn=0004-637X}}</ref> The mass of M31* was measured at 3–5 × 10<sup>7</sup> {{Solar mass}} in 1993,<ref name="Lauer"/> and at 1.1–2.3 × 10<sup>8</sup> {{Solar mass}} in 2005.<ref name="Bender et al 2005"/> The [[velocity dispersion]] of material around it is measured to be ≈ {{cvt|160|km/s|round=10|lk=on}}.<ref name="Gebhardt et al 2000"/>

It has been proposed that the observed double nucleus could be explained if P1 is the projection of a disk of stars in an [[orbital eccentricity|eccentric orbit]] around the central black hole.<ref name="Tremaine 1995"/> The eccentricity is such that stars linger at the orbital [[apsis|apocenter]], creating a concentration of stars. It has been postulated that such an eccentric disk could have been formed from the result of a previous black hole merger, where the release of gravitational waves could have "kicked" the stars into their current eccentric distribution.<ref>{{Cite journal |last1=Akiba |first1=Tatsuya |last2=Madigan |first2=Ann-Marie |date=1 November 2021 |title=On the Formation of an Eccentric Nuclear Disk following the Gravitational Recoil Kick of a Supermassive Black Hole |journal=The Astrophysical Journal Letters |volume=921 |issue=1 |pages=L12 |doi=10.3847/2041-8213/ac30d9 |arxiv=2110.10163 |bibcode=2021ApJ...921L..12A |s2cid=239049969 |issn=2041-8205 |doi-access=free }}</ref> P2 also contains a compact disk of hot, [[stellar classification|spectral-class]] A stars. The A stars are not evident in redder filters, but in blue and ultraviolet light they dominate the nucleus, causing P2 to appear more prominent than P1.<ref name="hubblesite 1993-07-20"/>

While at the initial time of its discovery it was hypothesized that the brighter portion of the double nucleus is the remnant of a small galaxy "cannibalized" by the Andromeda Galaxy,<ref name="Schewe & Stein 1993"/> this is no longer considered a viable explanation, largely because such a nucleus would have an exceedingly short lifetime due to [[tidal force|tidal disruption]] by the central black hole. While this could be partially resolved if P1 had its own black hole to stabilize it, the distribution of stars in P1 does not suggest that there is a black hole at its center.<ref name="Tremaine 1995"/>