Editing Andromeda Galaxy (section)

== General ==
The estimated distance of the Andromeda Galaxy from our own was doubled in 1953 when it was discovered that there is a second, dimmer type of [[Cepheid variable|Cepheid variable star]]. In the 1990s, measurements of both standard [[red giant]]s as well as [[red clump]] stars from the ''[[Hipparcos]]'' satellite measurements were used to calibrate the Cepheid distances.<ref name="Holland 1998"/><ref name="Stanek & Garnavich 1998"/>

=== Formation and history ===

[[File:Andromeda Galaxy 560mm FL.jpg|thumb|Processed image of the Andromeda Galaxy, with enhancement of [[H-alpha]] to highlight its star-forming regions]]
A major [[Galaxy merger|merger]] occurred 2 to 3&nbsp;billion years ago at the Andromeda location, involving two galaxies with a mass ratio of approximately 4.<ref name="Hammer et al. 2018">{{cite journal |last1=Hammer |first1=F |last2=Yang |first2=Y B |last3=Wang |first3=J L |last4=Ibata |first4=R |last5=Flores |first5=H |last6=Puech |first6=M |title=A 2–3 billion year old major merger paradigm for the Andromeda galaxy and its outskirts |journal=Monthly Notices of the Royal Astronomical Society |date=1 April 2018 |volume=475 |issue=2 |pages=2754–2767 |doi=10.1093/mnras/stx3343|doi-access=free |arxiv=1801.04279 }}</ref><ref name="D'Souza and Bell">{{cite journal |last1=D’Souza |first1=Richard |last2=Bell |first2=Eric F. |title=The Andromeda galaxy's most important merger about 2 billion years ago as M32's likely progenitor |journal=Nature Astronomy |date=September 2018 |volume=2 |issue=9 |pages=737–743 |doi=10.1038/s41550-018-0533-x |arxiv=1807.08819 |bibcode=2018NatAs...2..737D |s2cid=256713163 |url=https://www.nature.com/articles/s41550-018-0533-x |language=en |issn=2397-3366}}</ref>

The discovery of a recent merger in the Andromeda galaxy was first based on interpreting its anomalous age-velocity dispersion relation,<ref name="Dorman et al. 2014">{{Cite journal |last1=Dorman |first1=Claire E. |last2=Guhathakurta |first2=Puragra |last3=Seth |first3=Anil C. |last4=Weisz |first4=Daniel R. |last5=Bell |first5=Eric F. |last6=Dalcanton |first6=Julianne J. |last7=Gilbert |first7=Karoline M. |last8=Hamren |first8=Katherine M. |last9=Lewis |first9=Alexia R. |last10=Skillman |first10=Evan D. |last11=Toloba |first11=Elisa |last12=Williams |first12=Benjamin F. |date=9 April 2015 |title=A clear age-velocity dispersion correlation in Andromeda's stellar disk |journal=The Astrophysical Journal |volume=803 |issue=1 |pages=24 |arxiv=1502.03820 |bibcode=2015ApJ...803...24D |doi=10.1088/0004-637X/803/1/24 |s2cid=119223754}}</ref> as well as the fact that 2&nbsp;billion years ago, star formation throughout Andromeda's disk was much more active than today.<ref name="Williams et al. 2015">{{Cite journal |last1=Williams |first1=Benjamin F. |last2=Dalcanton |first2=Julianne J. |last3=Dolphin |first3=Andrew E. |last4=Weisz |first4=Daniel R. |last5=Lewis |first5=Alexia R. |last6=Lang |first6=Dustin |last7=Bell |first7=Eric F. |last8=Boyer |first8=Martha |last9=Fouesneau |first9=Morgan |last10=Gilbert |first10=Karoline M. |last11=Monachesi |first11=Antonela |last12=Skillman |first12=Evan |date=5 June 2015 |title=A Global Star-forming Episode in M31 2-4 Gyr Ago |journal=The Astrophysical Journal |volume=806 |issue=1 |pages=48 |arxiv=1504.02120 |bibcode=2015ApJ...806...48W |doi=10.1088/0004-637X/806/1/48 |s2cid=118435748}}</ref>

Modeling<ref name="Hammer et al. 2018"/> of this violent collision shows that it has formed most of the galaxy's (metal-rich) [[galactic halo]], including the Giant Stream,<ref name="Ibata et al. 2001">{{cite journal |last1=Ibata |first1=Rodrigo |last2=Irwin |first2=Michael |last3=Lewis |first3=Geraint |last4=Ferguson |first4=Annette M. N. |last5=Tanvir |first5=Nial |title=A giant stream of metal-rich stars in the halo of the galaxy M31 |journal=Nature |date=July 2001 |volume=412 |issue=6842 |pages=49–52 |doi=10.1038/35083506|pmid=11452300 |arxiv=astro-ph/0107090 |bibcode=2001Natur.412...49I |s2cid=4413139 }}</ref> and also the extended thick disk, the young age thin disk, and the static 10 kpc ring. During this epoch, its rate of [[star formation]] would have been [[Starburst galaxy|very high]], to the point of becoming a [[luminous infrared galaxy]] for roughly 100 million years. Modeling also recovers the bulge profile, the large bar, and the overall halo density profile.

Andromeda and the [[Triangulum Galaxy]] (M33) might have had a very close passage 2–4&nbsp;billion years ago, but it seems unlikely from the last measurements from the Hubble Space Telescope.<ref name="Patel et al. 2017">{{cite journal |last1=Patel |first1=Ekta |last2=Besla |first2=Gurtina |last3=Sohn |first3=Sangmo Tony |title=Orbits of massive satellite galaxies – I. A close look at the Large Magellanic Cloud and a new orbital history for M33 |journal=Monthly Notices of the Royal Astronomical Society |date=1 February 2017 |volume=464 |issue=4 |pages=3825–3849 |doi=10.1093/mnras/stw2616|doi-access=free |hdl=10150/623269 |hdl-access=free }}</ref>

=== Distance estimate ===
[[File:Milky Way and Andromeda in space, to scale.jpg|thumb|left|Illustration showing both the size of each galaxy and the distance between the two galaxies, to scale]]
At least four distinct techniques have been used to estimate distances from Earth to the Andromeda Galaxy. In 2003, using the infrared [[surface brightness fluctuation]]s (I-SBF) and adjusting for the new period-luminosity value and a metallicity correction of −0.2 mag dex<sup>−1</sup> in (O/H), an estimate of {{convert|2.57|+/-|0.06|e6ly|e9AU|lk=on|abbr=unit}} was derived. A 2004 Cepheid variable method estimated the distance to be 2.51 ± 0.13&nbsp;million light-years (770 ± 40 kpc).<ref name="Karachentsevetal2006"/><ref name="karachentsevetal2004"/>

In 2005, an eclipsing [[binary star]] was discovered in the Andromeda Galaxy. The binary{{efn|name=M31VJ}} is made up of two hot blue stars of [[stellar classification|types]] O and B. By studying the eclipses of the stars, astronomers were able to measure their sizes. Knowing the sizes and temperatures of the stars, they were able to measure their [[absolute magnitude]]. When the [[apparent magnitude|visual]] and absolute magnitudes are known, the distance to the star can be calculated. The stars lie at a distance of {{convert|2.52|+/-|0.14|e6ly|e9AU|abbr=unit}} and the whole Andromeda Galaxy at about {{convert|2.5|e6ly|e9AU|abbr=unit}}.<ref name="Ribas2005"/> This new value is in excellent agreement with the previous, independent Cepheid-based distance value. The [[tip of the red-giant branch|TRGB]] method was also used in 2005 giving a distance of {{convert|2.56|+/-|0.08|e6ly|e9AU|abbr=unit}}.<ref name="McConnachieetal2005"/> Averaged together, these distance estimates give a value of {{convert|2.54|+/-|0.11|e6ly|e9AU|abbr=unit}}.{{efn|name=avg dist}}

=== 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"/>

=== Luminosity estimates ===
The estimated [[luminosity]] of the Andromeda Galaxy, {{Solar luminosity|~2.6{{e|10}}|link=y}}, is about 25% higher than that of our own galaxy.<ref name="vdb"/><ref name="Moskvitch 2010"/> However, the galaxy has a high [[Orbital inclination|inclination]] as seen from Earth, and its [[Cosmic dust|interstellar dust]] absorbs an unknown amount of light, so it is difficult to estimate its actual brightness and other authors have given other values for the luminosity of the Andromeda Galaxy (some authors even propose it is the second-brightest galaxy within a radius of 10 [[megaparsec]]s of the Milky Way, after the [[Sombrero Galaxy]],<ref name="Karachentsev2004"/> with an absolute magnitude of around −22.21{{efn|name=bright m31}} or close<ref name="McCall2014"/>).

An estimation done with the help of [[Spitzer Space Telescope]] published in 2010 suggests an [[absolute magnitude]] (in the blue) of −20.89 (that with a [[color index]] of +0.63 translates to an absolute visual magnitude of −21.52,{{efn|name=blue mag}} compared to −20.9 for the Milky Way), and a total luminosity in that [[wavelength]] of {{Solar luminosity|3.64{{e|10}}}}.<ref name="Tempel2010"/>

The rate of star formation in the Milky Way is much higher, with the Andromeda Galaxy producing only about one solar mass per year compared to 3–5 solar masses for the Milky Way. The rate of [[nova]]e in the Milky Way is also double that of the Andromeda Galaxy.<ref name="Liller & Mayer 1987"/> This suggests that the latter once experienced a great star formation phase, but is now in a relative state of quiescence, whereas the Milky Way is experiencing more active star formation.<ref name="vdb"/> Should this continue, the luminosity of the Milky Way may eventually overtake that of the Andromeda Galaxy.

According to recent studies, the Andromeda Galaxy lies in what is known in the [[galaxy color–magnitude diagram]] as the "green valley", a region populated by galaxies like the Milky Way in transition from the "blue cloud" (galaxies actively forming new stars) to the "red sequence" (galaxies that lack star formation). Star formation activity in green valley galaxies is slowing as they run out of star-forming gas in the interstellar medium. In simulated galaxies with similar properties to the Andromeda Galaxy, star formation is expected to extinguish within about five billion years, even accounting for the expected, short-term increase in the rate of star formation due to the collision between the Andromeda Galaxy and the Milky Way.<ref name="MWAY"/>