Editing Andromeda Galaxy (section)

=== Mass estimates ===
[[File:Hubble Finds Giant Halo Around the Andromeda Galaxy.jpg|thumb|right|Giant halo around Andromeda Galaxy<ref name="Hubble-halo"/>]]

Until 2018, mass estimates for the Andromeda Galaxy's halo (including [[dark matter]]) gave a value of approximately {{Solar mass|1.5{{e|12}}|link=y}},<ref name="Jorge Peñarrubia2014"/> compared to {{Solar mass|8{{e|11}}}} for the Milky Way. This contradicted even earlier measurements that seemed to indicate that the Andromeda Galaxy and Milky Way are almost equal in mass. In 2018, the earlier measurements for equality of mass were re-established by radio results as approximately {{Solar mass|8{{e|11}}}}.<ref name="Kafle et al. 2018"/><ref name="Need4Speed-1"/><ref name="cosmic"/><ref name="Space.com"/> In 2006, the Andromeda Galaxy's [[Spiral galaxy#Galactic spheroid|spheroid]] was determined to have a higher stellar density than that of the Milky Way,<ref name="Kalirai et al 2006"/> and its galactic stellar disk was estimated at twice the diameter of that of the Milky Way.<ref name="Chapman et al 2006"/> The total mass of the Andromeda Galaxy is estimated to be between {{Solar mass|8{{e|11}}}}<ref name="Kafle et al. 2018"/> and {{Solar mass|1.1{{e|12}}}}.<ref name="Barmby 2006"/><ref name="Barmby2007"/> The stellar mass of M31 is {{Solar mass|10–15{{e|10}}}}, with 30% of that mass in the central [[Bulge (astronomy)|bulge]], 56% in the [[Disc (galaxy)|disk]], and the remaining 14% in the [[Spiral galaxy#Galactic spheroid|stellar halo]].<ref name="Tamm2012"/> The radio results (similar mass to the Milky Way Galaxy) should be taken as likeliest as of 2018, although clearly, this matter is still under active investigation by several research groups worldwide.

As of 2019, current calculations based on escape velocity and dynamical mass measurements put the Andromeda Galaxy at {{Solar mass|0.8{{e|12}}}},<ref name="Need4Speed"/> which is only half of the Milky Way's newer mass, calculated in 2019 at {{Solar mass|1.5{{e|12}}}}.<ref name="daily"/><ref name="SA-20190308"/><ref name="ARX-20190208"/>

In addition to stars, the Andromeda Galaxy's [[interstellar medium]] contains at least {{Solar mass|7.2{{e|9}}}}<ref name="BraunThilkerWalterbosCorbelli2009"/> in the form of [[neutral hydrogen]], at least {{Solar mass|3.4{{e|8}}}} as molecular [[hydrogen]] (within its innermost 10 kiloparsecs), and {{Solar mass|5.4{{e|7}}}} of [[interstellar dust|dust]].<ref name="Andromedandust"/>

The Andromeda Galaxy is surrounded by a massive halo of hot gas that is estimated to contain half the mass of the stars in the galaxy. The nearly invisible halo stretches about a million light-years from its host galaxy, halfway to our Milky Way Galaxy. Simulations of galaxies indicate the halo formed at the same time as the Andromeda Galaxy. The halo is enriched in elements heavier than hydrogen and helium, formed from [[supernova]]e, and its properties are those expected for a galaxy that lies in the "green valley" of the [[Galaxy color–magnitude diagram|Galaxy color-magnitude diagram]] (see [[#Luminosity estimates|below]]). Supernovae erupt in the Andromeda Galaxy's star-filled disk and eject these heavier elements into space. Over the Andromeda Galaxy's lifetime, nearly half of the heavy elements made by its stars have been ejected far beyond the galaxy's 200,000-light-year-diameter stellar disk.<ref name="Hubble-halo-a"/><ref name="Massive"/><ref name="Evidence"/><ref name="HaloFind"/>