Andromeda Galaxy

 The Andromeda Galaxy , also known as Messier 31, M31, or NGC 224 and originally the Andromeda Nebula (see below), is a Barred spiral galaxy  approximately 2.5 million light-years (770 Kiloparsecs) from Earth and the nearest large galaxy to the Milky way . The galaxy's name stems from the area of Earth's sky in which it appears, the Constellation of Andromeda  , which itself is named after the Ethiopian (or Phoenicia) who was the wife of Perseus in Greek mythology 

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The virial masses of the Andromeda Galaxy is of the same order of magnitude as that of the Milky Way, at 1 trillion solar masses (2.0×10⁴² kilograms). The mass of either galaxy is difficult to estimate with any accuracy, but it was long thought that the Andromeda Galaxy is more massive than the Milky Way by a margin of some 25% to 50%. This has been called into question by a 2018 study that cited a lower estimate on the mass of the Andromeda Galaxy, combined with preliminary reports on a 2019 study estimating a higher mass of the Milky Way. The Andromeda Galaxy has a diameter of about 220,000 ly (67 kpc), making it the largest member of the Local Group  in terms of extension.

The number of stars contained in the Andromeda Galaxy is estimated at one trillion (1×10¹²), or roughly twice the number estimated for the Milky Way.

The Milky Way and Andromeda galaxies are expected to collide  in around 4-5 billion years merging to form a giant elliptical galaxy .  or a large lenticular galaxy With an apparent magnitude  of 3.4, the Andromeda Galaxy is among the brightest of the Messier objects making it visible to the naked eye . from Earth on moonless nights, even when viewed from areas with moderate light pollution .

Around the year 964, the Persian astronomers Abd al-Rahaman al-sufi was the first to describe the Andromeda Galaxy. He referred to it in his Book of fixed stars  as a "nebulous smear".

star charts  of that period labeled it as the Little Cloud.  In 1612, the German astronomer Simon Marius  gave an early description of the Andromeda Galaxy based on telescopic observations.  Pierre Louis Maupertuis  conjectured in 1745 that the blurry spot was an island universe. In 1764, Charles Messier  cataloged Andromeda as object M31 and incorrectly credited Marius as the discoverer despite it being visible to the naked eye. In 1785, the astronomer William Herschel  noted a faint reddish hue in the core region of Andromeda. He believed Andromeda to be the nearest of all the "great nebulae ", and based on the color and magnitude of the nebula, he incorrectly guessed that it was no more than 2,000 times the distance of Sirius, or roughly 18,000 ly (5.5 kpc). In 1850, William Parsons , 3rd Earl of Rosse made the first drawing of Andromeda's spiral structure .

In 1864 Sir William Huggins noted that the spectrum  of Andromeda differed from that of a gaseous nebula. The spectra of Andromeda displays a  continuum of frequencies , superimposed with dark absorption lines  that help identify the chemical composition of an object. Andromeda's spectrum is very similar to the spectra of individual stars, and from this, it was deduced that Andromeda has a stellar nature. In 1885, a supernova  (known as S Andromedae ) was seen in Andromeda, the first and so far only one observed in that galaxy. At the time Andromeda was considered to be a nearby object, so the cause was thought to be a much less luminous and unrelated event called a nova, and was named accordingly; "Nova 1885".

In 1888, Issac Roberts  took one of the first photographs of Andromeda, which was still commonly thought to be a nebula within our galaxy. Roberts mistook Andromeda and similar "spiral nebulae" as star system being formed .

In 1912, Vesto slipher used Spectroscopy to measure the radial velocity of Andromeda with respect to our solar system —the largest velocity yet measured, at 300 km/s (190 mi/s).

Until 2018, mass estimates for the Andromeda Galaxy's halo (including dark matter) gave a value of approximately 1.5×10¹² M compared to 8×10¹¹ M_☉ for the Milky Way. This contradicted earlier measurements that seemed to indicate that the Andromeda Galaxy and Milky Way are almost equal in mass.

In 2018, the equality of mass was re-established by radio results as approximately 8×10¹¹ M_☉  In 2006, the Andromeda Galaxy's spheroid  was determined to have a higher stellar density than that of the Milky Way, and its galactic stellar disk was estimated at about twice the diameter of that of the Milky Way. The total mass of the Andromeda Galaxy is estimated to be between 8×10¹¹ M_☉ and 1.1×10¹² M_☉. The stellar mass of M31 is 10-15×10¹⁰ M_☉, with 30% of that mass in the central bulge, 56% in the disk, and the remaining 14% in the stellar  halo. 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 a number of research groups worldwide.

As of 2019, current calculations based on escape velocity and dynamical mass measurements put the Andromeda Galaxy at 0.8×10¹² M_☉, which is only half of the Milky Way's newer mass, calculated in 2019 at 1.5×10¹² M_☉.

In addition to stars, the Andromeda Galaxy's interstellar medium  contains at least 7.2×10⁹ M_☉ in the form of neutral hydrogen , at least 3.4×10⁸ M_☉ as molecular hydrogen (within its innermost 10 kiloparsecs), and 5.4×10⁷ M_☉ of dust .

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 , and its properties are those expected for a galaxy that lies in the "green valley" of the Galaxy color-magnitude diagram (see 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.