Andromeda Galaxy, Neighbouring Our Own, The Milky Way Galaxy.

The Andromeda Galaxy, also known as Messier 31, M31, or NGC 224 and originally the Andromeda Nebula, is a barred spiral galaxy with diameter of about 220,000 ly 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 Phoenician) princess who was the wife of Perseus in Greek mythology.

Conversely, our solar system and the planets including our Earth, alongwith the sun……Exists inside the Milky Way Galaxy. So now we have two Galaxies existing besides each other. One is the Milky Way Galaxy wherein our planet exists. The other is the Andromeda Galaxy about which only the essential factors are known to Modern Man, the Earth Based Humans. With our limited hi tech instruments we Humans have only barely been able to probe the vastness ot the universe. The same applies to Andromeda Galaxy.

There are no conclusive results as to the existence of an Oxygen Sustaining atmosphere on planets in Andromeda Galaxy, where humans may one day travel to, when Planet Earth becomes non viable for Mankind’s survival mainly due to green house gasses, micro fibres and plastics pollution, Global Warming turning to Global Heating, paucity of drinking water, etc.

As a matter of fact, Mankind has not yet been able to find out whether there are worlds or planets like our own Earth with its own Oxygen rich Atmosphere……Planets which may be existing inside our own Milky Way Galaxy.

There are likewise, millions of galaxies existing within the Universe, where Man has not yet reached nor travelled.

ANDROMEDA GALAXY.

The virial mass 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 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, and is visible to the naked eye from Earth on moonless nights, even when viewed from areas with moderate light pollution.

Pronunciation /ænˈdrɒmɪdə/

Constellation Andromeda

Right ascension00h 42m 44.3s

Declination+41° 16′ 9″

Redshift z = −0.001004
(minus sign
indicates blueshift)

Helio radial velocity−301 ± 1 km/s

Distance 752 kpc (2.45 Mly)

Apparent magnitude (V)3.44[4]

Absolute magnitude (V)−21.5

Type SA (s)b

Mass (1.5±0.5)×1012  M☉

Number of stars~1 trillion (1012)

Size~220 kly (67 kpc) (diameter)

Apparent size (V)3.167° × 1°

Around the year 964, the Persian astronomer Abd al-Rahman al-Sufi was the first to formally describe the Andromeda Galaxy. He referred to it in his Book of Fixed Stars as a “nebulous smear” or “small cloud”.
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 its 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, Isaac 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 systems being formed.

In 1912, Vesto Slipher used spectroscopy to measure the radial velocity of Andromeda with respect to the Solar System—the largest velocity yet measured, at 300 km/s (190 mi/s).

As early as 1755 the German philosopher Immanuel Kant proposed the hypothesis that the Milky Way is only one of many galaxies, in his book Universal Natural History and Theory of the Heavens. Arguing that a structure like the Milky Way would look like a circular nebula viewed from above and like an elliptical if viewed from an angle, he concluded that the observed elliptical nebulae like Andromeda, which could not be explained otherwise at the time, were indeed galaxies similar to the Milky Way.

In 1917, Heber Curtis observed a nova within Andromeda. Searching the photographic record, 11 more novae were discovered. Curtis noticed that these novae were, on average, 10 magnitudes fainter than those that occurred elsewhere in the sky. As a result, he was able to come up with a distance estimate of 500,000 ly (3.2×1010 AU). He became a proponent of the so-called “island universes” hypothesis, which held that spiral nebulae were actually independent galaxies.

In 1920, the Great Debate between Harlow Shapley and Curtis took place concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the universe. To support his claim of the Great Andromeda Nebula being, in fact, an external galaxy, Curtis also noted the appearance of dark lanes within Andromeda which resembled the dust clouds in our own galaxy, as well as historical observations of Andromeda Galaxy’s significant Doppler shift. In 1922 Ernst Öpik presented a method to estimate the distance of Andromeda using the measured velocities of its stars. His result placed the Andromeda Nebula far outside our galaxy at a distance of about 450 kpc (1,500 kly).

Edwin Hubble settled the debate in 1925 when he identified extragalactic Cepheid variable stars for the first time on astronomical photos of Andromeda. These were made using the 100-inch (2.5 m) Hooker telescope, and they enabled the distance of Great Andromeda Nebula to be determined. His measurement demonstrated conclusively that this feature was not a cluster of stars and gas within our own galaxy, but an entirely separate galaxy located a significant distance from the Milky Way.

In 1943, Walter Baade was the first person to resolve stars in the central region of the Andromeda Galaxy. Baade identified two distinct populations of stars based on their metallicity, naming the young, high-velocity stars in the disk Type I and the older, red stars in the bulge Type II. This nomenclature was subsequently adopted for stars within the Milky Way, and elsewhere. (The existence of two distinct populations had been noted earlier by Jan Oort.) Baade also discovered that there were two types of Cepheid variable stars, which resulted in a doubling of the distance estimate to Andromeda, as well as the remainder of the universe.

In 1950, radio emission from the Andromeda Galaxy was detected by Hanbury Brown and Cyril Hazard at Jodrell Bank Observatory. The first radio maps of the galaxy were made in the 1950s by John Baldwin and collaborators at the Cambridge Radio Astronomy Group. The core of the Andromeda Galaxy is called 2C 56 in the 2C radio astronomy catalog. In 2009, the first planet may have been discovered in the Andromeda Galaxy. This was detected using a technique called microlensing, which is caused by the deflection of light by a massive object.

Observations of linearly polarized radio emission with the Westerbork Synthesis Radio Telescope, the Effelsberg 100-m Radio Telescope, and the Very Large Array revealed ordered magnetic fields aligned along the “10-kpc ring” of gas and star formation. The total magnetic field has a strength of about 0.5 nT, of which 0.3 nT are ordered.

The estimated distance of the Andromeda Galaxy from our own was doubled in 1953 when it was discovered that there is another, dimmer type of Cepheid variable star. In the 1990s, measurements of both standard red giants as well as red clump stars from the Hipparcos satellite measurements were used to calibrate the Cepheid distances.

The Andromeda Galaxy was formed roughly 10 billion years ago from the collision and subsequent merger of smaller protogalaxies.

This violent collision formed most of the galaxy’s (metal-rich) galactic halo and extended disk. During this epoch, its rate of star formation would have been very high, to the point of becoming a luminous infrared galaxy for roughly 100 million years. Andromeda and the Triangulum Galaxy (M33) had a very close passage 2–4 billion years ago. This event produced high rates of star formation across the Andromeda Galaxy’s disk — even some globular clusters — and disturbed M33’s outer disk.

Over the past 2 billion years, star formation throughout Andromeda’s disk is thought to have decreased to the point of near-inactivity. There have been interactions with satellite galaxies like M32, M110, or others that have already been absorbed by the Andromeda Galaxy. These interactions have formed structures like Andromeda’s Giant Stellar Stream. A galactic merger roughly 100 million years ago is believed to be responsible for a counter-rotating disk of gas found in the center of Andromeda as well as the presence there of a relatively young (100 million years old) stellar population.

Distance estimate.

At least four distinct techniques have been used to estimate distances from Earth to the Andromeda Galaxy. In 2003, using the infrared surface brightness fluctuations (I-SBF) and adjusting for the new period-luminosity value and a metallicity correction of −0.2 mag dex−1 in (O/H), an estimate of 2.57 ± 0.06 million light-years (1.625×1011 ± 3.8×109 astronomical units) was derived. A 2004 Cepheid variable method estimated the distance to be 2.51 ± 0.13 million light-years (770 ± 40 kpc). In 2005, an eclipsing binary star was discovered in the Andromeda Galaxy. The binary is two hot blue stars of 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 visual and absolute magnitudes are known, the distance to the star can be calculated. The stars lie at a distance of 2.52×106 ± 0.14×106 ly (1.594×1011 ± 8.9×109 AU) and the whole Andromeda Galaxy at about 2.5×106 ly (1.6×1011 AU). This new value is in excellent agreement with the previous, independent Cepheid-based distance value. The TRGB method was also used in 2005 giving a distance of 2.56×106 ± 0.08×106 ly (1.619×1011 ± 5.1×109 AU). Averaged together, these distance estimates give a value of 2.54×106 ± 0.11×106 ly (1.606×1011 ± 7.0×109 AU).[c] And, from this, the diameter of Andromeda at the widest point is estimated to be 220 ± 3 kly (67,450 ± 920 pc). Applying trigonometry (angular diameter), this is equivalent to an apparent 4.96° angle in the sky.

Luminosity estimates.

Compared to the Milky Way, the Andromeda Galaxy appears to have predominantly older stars with ages >7×109 years. The estimated luminosity of the Andromeda Galaxy, ~2.6×1010 L☉, is about 25% higher than that of our own galaxy. However, the galaxy has a high inclination as seen from Earth and its 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 megaparsecs of the Milky Way, after the Sombrero Galaxy, with an absolute magnitude of around −22.21 or close.

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,
compared to −20.9 for the Milky Way), and a total luminosity in that wavelength of 3.64×1010 L☉.

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 novae in the Milky Way is also double that of the Andromeda Galaxy. 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. 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 in the Galaxy color–magnitude diagram is known 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.

Based on its appearance in visible light, the Andromeda Galaxy is classified as an SA(s)b galaxy in the de Vaucouleurs–Sandage extended classification system of spiral galaxies. However, infrared data from the 2MASS survey and from the Spitzer Space Telescope showed that Andromeda is actually a barred spiral galaxy, like the Milky Way, with Andromeda’s bar major axis oriented 55 degrees anti-clockwise from the disc major axis.
In 2005, astronomers used the Keck telescopes to show that the tenuous sprinkle of stars extending outward from the galaxy is actually part of the main disk itself. This means that the spiral disk of stars in the Andromeda Galaxy is three times larger in diameter than previously estimated. This constitutes evidence that there is a vast, extended stellar disk that makes the galaxy more than 220,000 light-years (67 kiloparsecs) in diameter. Previously, estimates of the Andromeda Galaxy’s size ranged from 70,000 to 120,000 light-years (21 to 37 kpc) across.

The galaxy is inclined an estimated 77° relative to Earth (where an angle of 90° would be edge-on). Analysis of the cross-sectional shape of the galaxy appears to demonstrate a pronounced, S-shaped warp, rather than just a flat disk. A possible cause of such a warp could be gravitational interaction with the satellite galaxies near the Andromeda Galaxy. The Galaxy M33 could be responsible for some warp in Andromeda’s arms, though more precise distances and radial velocities are required.

Spectroscopic studies have provided detailed measurements of the rotational velocity of the Andromeda Galaxy as a function of radial distance from the core. The rotational velocity has a maximum value of 225 km/s (140 mi/s) at 1,300 ly (82,000,000 AU) from the core, and it has its minimum possibly as low as 50 km/s (31 mi/s) at 7,000 ly (440,000,000 AU) from the core. Further out, rotational velocity rises out to a radius of 33,000 ly (2.1×109 AU), where it reaches a peak of 250 km/s (160 mi/s). The velocities slowly decline beyond that distance, dropping to around 200 km/s (120 mi/s) at 80,000 ly (5.1×109 AU). These velocity measurements imply a concentrated mass of about 6×109 M☉ in the nucleus. The total mass of the galaxy increases linearly out to 45,000 ly (2.8×109 AU), then more slowly beyond that radius.

The spiral arms of the Andromeda Galaxy are outlined by a series of HII regions, first studied in great detail by Walter Baade and described by him as resembling “beads on a string”. His studies show two spiral arms that appear to be tightly wound, although they are more widely spaced than in our galaxy. His descriptions of the spiral structure, as each arm crosses the major axis of the Andromeda Galaxy, are as follows.

Since the Andromeda Galaxy is seen close to edge-on, it is difficult to study its spiral structure. Rectified images of the galaxy seem to show a fairly normal spiral galaxy, exhibiting two continuous trailing arms that are separated from each other by a minimum of about 13,000 ly (820,000,000 AU) and that can be followed outward from a distance of roughly 1,600 ly (100,000,000 AU) from the core. Alternative spiral structures have been proposed such as a single spiral arm or a flocculent pattern of long, filamentary, and thick spiral arms.
The most likely cause of the distortions of the spiral pattern is thought to be interaction with galaxy satellites M32 and M110.[81] This can be seen by the displacement of the neutral hydrogen clouds from the stars.

In 1998, images from the European Space Agency’s Infrared Space Observatory demonstrated that the overall form of the Andromeda Galaxy may be transitioning into a ring galaxy. The gas and dust within the galaxy is generally formed into several overlapping rings, with a particularly prominent ring formed at a radius of 32,000 ly (9.8 kpc) from the core, nicknamed by some astronomers the ring of fire. This ring is hidden from visible light images of the galaxy because it is composed primarily of cold dust, and most of the star formation that is taking place in the Andromeda Galaxy is concentrated there.

Later studies with the help of the Spitzer Space Telescope showed how the Andromeda Galaxy’s spiral structure in the infrared appears to be composed of two spiral arms that emerge from a central bar and continue beyond the large ring mentioned above. Those arms, however, are not continuous and have a segmented structure.

Close examination of the inner region of the Andromeda Galaxy with the same telescope also showed a smaller dust ring that is believed to have been caused by the interaction with M32 more than 200  million years ago. Simulations show that the smaller galaxy passed through the disk of the Andromeda Galaxy along the latter’s polar axis. This collision stripped more than half the mass from the smaller M32 and created the ring structures in Andromeda. It is the co-existence of the long-known large ring-like feature in the gas of Messier 31, together with this newly discovered inner ring-like structure, offset from the barycenter, that suggested a nearly head-on collision with the satellite M32, a milder version of the Cartwheel encounter.

Studies of the extended halo of the Andromeda Galaxy show that it is roughly comparable to that of the Milky Way, with stars in the halo being generally “metal-poor”, and increasingly so with greater distance. This evidence indicates that the two galaxies have followed similar evolutionary paths. They are likely to have accreted and assimilated about 100–200 low-mass galaxies during the past 12 billion years. The stars in the extended halos of the Andromeda Galaxy and the Milky Way may extend nearly one third the distance separating the two galaxies.

Collision with the Milky Way Galaxy.

Main article, Andromeda / Milky Way collision.

The Andromeda Galaxy is approaching the Milky Way at about 110 kilometres per second (68 miles per second).  It has been measured approaching relative to the Sun at around 300 km/s (190 mi/s) as the Sun orbits around the center of the galaxy at a speed of approximately 225 km/s (140 mi/s). This makes the Andromeda Galaxy one of about 100 observable blueshifted galaxies.

Andromeda Galaxy’s tangential or sideways velocity with respect to the Milky Way is relatively much smaller than the approaching velocity and therefore it is expected to collide directly with the Milky Way in about 4 billion years. A likely outcome of the collision is that the galaxies will merge to form a giant elliptical galaxy or perhaps even a large disc galaxy. Such events are frequent among the galaxies in galaxy groups. The fate of the Earth and the Solar System in the event of a collision is currently unknown. Before the galaxies merge, there is a small chance that the Solar System could be ejected from the Milky Way or join the Andromeda Galaxy.

Like the Milky Way, the Andromeda Galaxy has satellite galaxies, consisting of over 20 known dwarf galaxies. The Andromeda Galaxy’s dwarf galaxy population is very similar to the Milky Way’s, but the galaxies are much more numerous. The best known and most readily observed satellite galaxies are M32 and M110. Based on current evidence, it appears that M32 underwent a close encounter with the Andromeda Galaxy in the past. M32 may once have been a larger galaxy that had its stellar disk removed by M31, and underwent a sharp increase of star formation in the core region, which lasted until the relatively recent past.

M110 also appears to be interacting with the Andromeda Galaxy, and astronomers have found in the halo of the latter a stream of metal-rich stars that appear to have been stripped from these satellite galaxies. M110 does contain a dusty lane, which may indicate recent or ongoing star formation. M32 has a young stellar population as well.

The Triangulum Galaxy is a non-dwarf galaxy that lies 750,000 light years from Andromeda. It is currently unknown whether it is a satellite of Andromeda.

In 2006, it was discovered that nine of the satellite galaxies lie in a plane that intersects the core of the Andromeda Galaxy; they are not randomly arranged as would be expected from independent interactions. This may indicate a common tidal origin for the satellites.

SUMMARY.

The immense Andromeda galaxy, also known as Messier 31 (M31), is captured in full in this new image from NASA’s Wide-field Infrared Survey Explorer (WISE). The mosaic covers an area equivalent to more than 100 full moons, or five degrees across the sky. WISE used all four of its infrared detectors to capture this picture (3.4- and 4.6-micron light is colored blue; 12-micron light is green; and 22-micron light is red). Blue highlights mature stars, while yellow and red show dust heated by newborn, massive stars.

Andromeda is the closest large galaxy to our Milky Way galaxy, and is located 2.5 million light-years from our sun. It is close enough for telescopes to spy the details of its ringed arms of new stars and hazy blue backbone of older stars. Also seen in the mosaic are two satellite galaxies: M32, located just a bit above Andromeda to the left of center, and the fuzzy blue M110, located below the center of the great spiral arms. These satellites are the largest of several that are gravitationally bound to Andromeda.Andromeda is larger than the Milky Way and contains more stars, but the Milky Way is thought to perhaps have more mass due to its larger proportion of a mysterious substance called dark matter. Both galaxies belong to our so-named Local Group, a collection of more than 50 galaxies, most of which are tiny dwarf systems. In its quest to map the whole sky, WISE will capture the entire Local Group.

Courtesy, Author, NASA/JPL-Caltech/UCLA.

Source, thanks Wikipedia.