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M31 has long been known to harbor a dense and compact star cluster at its very center. In a large telescope it creates a visual impression of a star embedded in the more diffuse surrounding bulge. The luminosity of the nucleus is in excess of the most luminous globular clusters.
In 1991 Tod R. Lauer used WFPC, then onboard the Hubble Space Telescope, to image Andromeda's nucleus. To everyone's surprise the nucleus was seen to be double, consisting of two concentrations of starlight separated by 1.5 parsecs. 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 the 108 M☉ black hole inferred to exist at the center of M31