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See Andromeda (disambiguation) for other uses of "Andromeda".
Andromeda Galaxy

A visible light image of the Andromeda Galaxy.
Observation data (J2000 epoch)
Constellation Andromeda
Right ascension 00h 42m 44. In Astronomy, an epoch is a moment in time used as a reference for the Orbital elements of a Celestial body. In Astronomy, an epoch is a moment in time used as a reference for the Orbital elements of a Celestial body. In common usage a constellation is a group of celestial bodies that are connected together in some arrangement typically stars to form a visible figure or picture Andromeda ( is a Constellation named for the princess Andromeda ( Greek Ανδρομέδη = guardian of the men) a character in Right ascension (abbrev RA; symbol α) is the Astronomical term for one of the two Coordinates of a point on the Celestial sphere 3s[1]
Declination +41° 16′ 9″[1]
Redshift −301 ± 1 km/s[2]
Distance 2.54 ± 0.06 Mly
(778 ± 17 kpc)[3][2][4][5][6][a]
Type SA(s)b[1]
Apparent dimensions (V) 190′ × 60′[1]
Apparent magnitude (V) 4. In Astronomy, declination (abbrev dec or δ) is one of the two coordinates of the Equatorial coordinate system, the other being either In Physics and Astronomy, redshift occurs when Electromagnetic radiation – usually Visible light – emitted or reflected by The kilometre ( American spelling: kilometer) symbol km is a unit of Length in the Metric system, equal to one thousand The second ( SI symbol s) sometimes abbreviated sec, is the name of a unit of Time, and is the International System of Units A light-year or light year (symbol ly) is a unit of Length, equal to just under ten trillion Kilometres As defined by History The first direct measurements of an object at interstellar distances were undertaken by German Astronomer Friedrich Wilhelm Bessel in 1838 Galaxy morphological classification is a system used by Astronomers to divide galaxies into groups based on their visual appearance The angular diameter of an object as seen from a given position is the "visual diameter" of the object measured as an angle The apparent magnitude ( m) of a celestial body is a measure of its Brightness as seen by an observer on Earth, normalized to the value 4[1]
Other designations
M31, NGC 224, UGC 454, PGC 2557, 2C 56 (Core)[1]
See also: Galaxy, List of galaxies

The Andromeda Galaxy (IPA: /ænˈdrɒmədə/, also known as Messier 31, M31, or NGC 224; often referred to as the Great Andromeda Nebula in older texts) is a spiral galaxy approximately 2. The Messier objects are a set of Astronomical objects first listed by French astronomer Charles Messier in his "Catalogue des Nébuleuses The New General Catalogue ( NGC) is the best-known catalogue of Deep sky objects in Amateur astronomy. The Uppsala General Catalogue of Galaxies (UGC is a catalogue of 12921 galaxies visible from the northern hemisphere The Catalogue of Principal Galaxies ( PGC) is an Astronomical catalogue published in 1989 that lists B1950 and J2000 Equatorial A galaxy is a massive gravitationally bound system consisting of Stars an Interstellar medium of gas and dust, and Dark matter This is a list of notable galaxies. List 4C 3711 Abell 1835 IR1916 AM 0644-741 A nebula (from Latin: "mist" pl nebulae or nebulæ, with ligature or nebulas) is an Interstellar cloud of A spiral galaxy is a Galaxy belonging to one of the three main classes of galaxy originally described by Edwin Hubble in his 1936 work “The Realm of the 5 million light-years away[4] in the constellation Andromeda. A light-year or light year (symbol ly) is a unit of Length, equal to just under ten trillion Kilometres As defined by In common usage a constellation is a group of celestial bodies that are connected together in some arrangement typically stars to form a visible figure or picture Andromeda ( is a Constellation named for the princess Andromeda ( Greek Ανδρομέδη = guardian of the men) a character in It is the nearest spiral galaxy to our own, the Milky Way, and is visible as a faint smudge on a moonless night to the naked eye. A spiral galaxy is a Galaxy belonging to one of the three main classes of galaxy originally described by Edwin Hubble in his 1936 work “The Realm of the The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply

Andromeda is the largest galaxy of the Local Group, which consists of the Andromeda Galaxy, the Milky Way Galaxy, the Triangulum Galaxy, and about 30 other smaller galaxies. The Local Group is the group of galaxies that includes our galaxy the Milky Way. The Triangulum Galaxy (also known as Messier 33 or NGC 598) is a Spiral galaxy approximately 3 million light-years away in the Although the largest, it may not be the most massive, as recent findings suggest that the Milky Way contains more dark matter and may be the most massive in the grouping. In Physics and cosmology, dark matter is hypothetical Matter that does not interact with the electromagnetic force but whose presence can be inferred from [7] However, recent observations by the Spitzer Space Telescope revealed that M31 contains one trillion (1012) stars, greatly exceeding the number of stars in our own galaxy. The Spitzer Space Telescope (formerly the Space Infrared Telescope Facility, SIRTF) is an Infrared Space observatory. [8] 2006 estimates put the mass of the Milky Way to be ~80% of the mass of Andromeda, which is estimated to be 7. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. 1×1011 solar masses. The solar mass is a standard way to express Mass in Astronomy, used to describe the masses of other Stars and galaxies. [2]

At an apparent magnitude of 4. The apparent magnitude ( m) of a celestial body is a measure of its Brightness as seen by an observer on Earth, normalized to the value 4, the Andromeda Galaxy is notable for being one of the brightest Messier objects,[9] making it easily visible to the naked eye even when viewed from areas with moderate light pollution. The Messier objects are a set of Astronomical objects first listed by French astronomer Charles Messier in his "Catalogue des Nébuleuses The naked eye is a Figure of speech referring to human Visual perception that is unaided by enhancing equipment such as a Telescope or Light pollution, also known as photopollution or luminous pollution, is excess or obtrusive Light created mainly by Humans Among other effects It appears quite small without a telescope because only the central part is bright enough to be visible, but the full angular diameter of the galaxy is seven times that of the full moon. The angular diameter of an object as seen from a given position is the "visual diameter" of the object measured as an angle

Contents

Observation history

The earliest recorded observation of the Andromeda Galaxy was in 964 CE by the Persian astronomer 'Abd Al-Rahman Al Sufi,[10] who described it as a "small cloud". Historically Astronomy was more concerned with the classification and description of phenomena in the sky while Astrophysics attempted to explain these phenomena Star charts of that period have it labeled as the Little Cloud. [10] The first description of the object based on telescopic observation was given by Simon Marius[10] in 1612. Simon Marius ( Latinized from German Simon Mayr) ( January 10, 1573 &ndash December 26, 1624) was a German Charles Messier catalogued it as object M31 in 1764 and incorrectly credited Marius as the discoverer, unaware of Al Sufi's earlier work. Charles Messier ( June 26, 1730 &ndash April 12, 1817) was a French astronomer most notable for publishing an In 1785, the astronomer William Herschel noted a faint reddish hue in the core region of the M31. Sir Frederick William Herschel FRS KH ( 15 November 1738 – 25 August 1822) was a German -born British He believed it to be the nearest of all the "great nebulae" and, based on the color and magnitude of the nebula, he estimated (incorrectly) that it was no more than 2,000 times the distance of Sirius. Sirius is the brightest star in the night sky with a visual Apparent magnitude of &minus1 [11]

William Huggins in 1864 observed the spectrum of M31 and noted that it differed from a gaseous nebula. Sir William Huggins, OM, FRS ( February 7 1824 &ndash May 12 1910) was an English Astronomer A spectrum (plural spectra or spectrums) is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. [12] The spectra of M31 displayed a continuum of frequencies, superimposed with dark lines. Frequency is a measure of the number of occurrences of a repeating event per unit Time. A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow frequency range compared This was very similar to the spectra of individual stars. From this it was deduced that M31 had a stellar nature.

In 1885, a supernova (known as "S Andromedae") was seen in M31, the first and so far only one observed in that galaxy. A supernova (plural supernovae or supernovas) is a stellar Explosion. |- style="background-color #A0B0FF" colspan="3"| Database References |- bgcolor="#FFFAFA" | Simbad || Data|- bgcolor="#FFFAFA" At the time, since M31 was considered to be a "near by" object, it was thought to be a much less luminous and unrelated event called a nova, and was named accordingly Nova 1885. A nova (pl novae or novas) is a Cataclysmic nuclear explosion caused by the accretion of hydrogen onto the surface of a White

Great Andromeda Nebula by Isaac Roberts.
Great Andromeda Nebula by Isaac Roberts. Isaac Roberts ( January 27 1829 - July 17 1904) was a Welsh Astronomer who was a pioneer in Photography of

The first photographs of M31 were taken in 1887 by Isaac Roberts from his private observatory in Sussex. A photograph (often shortened to photo) is an Image created by Light falling on a light-sensitive surface usually Photographic film or an electronic Isaac Roberts ( January 27 1829 - July 17 1904) was a Welsh Astronomer who was a pioneer in Photography of Sussex is a historic county in South East England corresponding roughly in area to the ancient Kingdom of Sussex. The long-duration exposure allowed the spiral structure of the galaxy to be seen for the first time. [13] However, at the time this object was commonly believed to be a nebula within our galaxy, and Roberts mistakenly believed that M31 and similar spiral nebulae were actually solar systems being formed, with the satellites nascent planets.

The radial velocity of this object with respect to our solar system was measured in 1912 by Vesto Slipher at the Lowell Observatory, using spectroscopy. Radial velocity is the Velocity of an object in the direction of the line of sight (i The Solar System consists of the Sun and those celestial objects bound to it by Gravity. Vesto Melvin Slipher ( November 11, 1875 &ndash November 8, 1969) was an American Astronomer. Lowell Observatory is an astronomical Observatory in Flagstaff Arizona. Spectroscopy was originally the study of the interaction between Radiation and Matter as a function of Wavelength (λ The result was the largest velocity recorded at that time, at 300 kilometres per second (186 miles/sec. ), moving in the direction of the Sun. [14]

Island universe

In 1917, Heber Curtis had observed a nova within M31. Heber Doust Curtis ( June 27 1872 – January 9 1942) was an American Astronomer. Searching the photographic record, 11 more novae were discovered. Curtis noticed that these novae were, on average, 10 magnitudes fainter than those that occurred within our Galaxy. In Astronomy, magnitude refers to the Logarithmic measure of the brightness of an object measured in a specific Wavelength or Passband, usually As a result he was able to come up with a distance estimate of 500,000 light-years. He became a proponent of the so-called "island universes" hypothesis, which held that spiral nebulae were actually independent galaxies. A spiral galaxy is a Galaxy belonging to one of the three main classes of galaxy originally described by Edwin Hubble in his 1936 work “The Realm of the [15]

In 1920 the Great Debate between Harlow Shapley and Heber Curtis took place, concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the universe. This is about the infamous discussion of astronomy For the Dream Theater song about Stem cell research, see The Great Debate (song. Harlow Shapley ( November 2 1885 &ndash October 20 1972) was an American Astronomer. Heber Doust Curtis ( June 27 1872 – January 9 1942) was an American Astronomer. The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply A spiral galaxy is a Galaxy belonging to one of the three main classes of galaxy originally described by Edwin Hubble in his 1936 work “The Realm of the The Universe is defined as everything that Physically Exists: the entirety of Space and Time, all forms of Matter, Energy To support his claim that "Great Andromeda Nebula" (M31) was an external galaxy, Curtis also noted the appearance of dark lanes resembling the dust clouds in our own Galaxy, as well as the significant Doppler shift. The Doppler effect (or Doppler shift) named after Christian Doppler, is the change in Frequency and Wavelength of a Wave for Edwin Hubble settled the debate in 1925 when he identified extragalactic Cepheid variable stars for the first time on astronomical photos of M31. Edwin Powell Hubble ( November 20, 1889 – September 28, 1953) was an American astronomer. A Cepheid variable (pron ˈse-f(ē-id or ˈsē-f(ē-id or Cepheid is a member of a particular class of Variable stars notable for a fairly tight correlation For the astronomical object see Variable star. Variable Star is a 2006 novel written by Spider Robinson These were made using a 2. 5 metre (100 in) reflecting telescope, and they enabled the distance of Great Andromeda Nebula to be determined. The metre or meter is a unit of Length. It is the basic unit of Length in the Metric system and in the International A reflecting telescope (also called a reflector) is an Optical telescope which uses a single or combination of Curved mirrors that reflect Light His measurement demonstrated conclusively that this feature was not a cluster of stars and gas within our Galaxy, but an entirely separate galaxy located a significant distance from our own. [16]

This galaxy plays an important role in galactic studies, since it is the nearest giant spiral (although not the nearest galaxy). This is a list of galaxies nearest to Earth. Note that 'near' in intergalactic terms still means thousands or millions of light years away (in comparison to say The In 1943, Walter Baade was the first person to resolve stars in the central region of the Andromeda Galaxy. Biography Along with Fritz Zwicky, he proposed that Supernovae could create Neutron stars He took advantage of wartime blackout conditions during Based on his observations of this galaxy, he was able to discern two distinct populations of stars, naming the young, high velocity stars in the disk Type I and the older, red stars in the bulge Type II. In Astronomy and Physical cosmology, the metallicity of an object is the proportion of its matter made up of Chemical elements other than Hydrogen In Astronomy and Physical cosmology, the metallicity of an object is the proportion of its matter made up of Chemical elements other than Hydrogen 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. Jan Hendrik Oort ( April 28, 1900 &ndash November 5, 1992) was a Dutch Astronomer. )[17] Dr. Baade also discovered that there were two types of Cepheid variables, which resulted in a doubling of the distance estimate to M31, as well as the remainder of the Universe. A Cepheid variable (pron ˈse-f(ē-id or ˈsē-f(ē-id or Cepheid is a member of a particular class of Variable stars notable for a fairly tight correlation

The first radio maps of the Andromeda Galaxy were made in the 1950s by John Baldwin and collaborators at the Cambridge Radio Astronomy Group. Radio astronomy is a subfield of Astronomy that studies celestial objects at radio frequencies. Professor John Evan Baldwin has worked at the Cavendish Astrophysics Group (formerly Mullard Radio Astronomy Observatory since 1954 The Cavendish Astrophysics Group (formerly the Radio Astronomy Group is based at the Cavendish Laboratory at the University of Cambridge. The core of the Andromeda Galaxy is called 2C 56 in the 2C radio astronomy catalogue.

General information

The Andromeda Galaxy is approaching the Sun at about 300 kilometers per second (186 miles/sec. The Sun (Sol is the Star at the center of the Solar System. The kilometre ( American spelling: kilometer) symbol km is a unit of Length in the Metric system, equal to one thousand ), so it is one of the few blue shifted galaxies. Blue shift is the shortening of a transmitted signal's Wavelength, and/or an increase in its Frequency, due to the Doppler Effect, which indicates that Given the motion of the Solar System inside the Milky Way, one finds that the Andromeda Galaxy and the Milky Way are approaching one another at a speed of 100 to 140 kilometers per second (62–87 miles/sec. The Solar System consists of the Sun and those celestial objects bound to it by Gravity. ). [18] The impact is predicted to occur in about 2. 5 billion years. In that case the two galaxies will likely merge to form a giant elliptical galaxy. An elliptical galaxy is a Galaxy belonging to one of the three main classes of galaxy originally described by Edwin Hubble (whose name was dedicated [19] However, Andromeda's tangential velocity with respect to the Milky Way is only known to within about a factor of two, which creates uncertainty about the details of when the collision will take place and how it will proceed. [20] Such events are frequent among the galaxies in galaxy groups. Galaxy groups and clusters are the largest Gravitationally bound objects to have arisen thus far in the process of cosmic structure formation

The measured distance to the Andromeda Galaxy was doubled in 1953 when it was discovered that there is another, dimmer type of Cepheid. In the 1990s, Hipparcos satellite measurements were used to calibrate the Cepheid distances. Hipparcos (an Acronym for Hi gh P recision Par allax Co llecting S atellite) was an Astrometry mission The corrected value gives the Andromeda Galaxy a distance of 2. 9 million light-years. Unfortunately, all Cepheids lie further than Hipparcos could measure accurately, and it became clear that Hipparcos-calibrated values for Cepheids were not reliable.

The Andromeda Galaxy pictured in ultraviolet light by GALEX
The Andromeda Galaxy pictured in ultraviolet light by GALEX
See also: Andromeda-Milky Way collision

Recent distance estimates

At least four distinct techniques have been used to measure distances to M31. Ultraviolet ( UV) light is Electromagnetic radiation with a Wavelength shorter than that of Visible light, but longer than X-rays The Galaxy Evolution Explorer is an Orbiting Ultraviolet Space telescope that was launched on April 28, 2003. The Andromeda-Milky Way collision is a predicted Galaxy collision, due to take place in approximately three billion years time between the two largest galaxies in the Local

In 2003, using the infrared surface brightness fluctuations (I-SBF) and adjusting for the new period-luminosity value of Freedman et al. Surface brightness fluctuation (SBF is a secondary distance indicator used to estimate distances to galaxies 2001 and using a metallicity correction of -0. 2 mag dex-1 in (O/H), an estimate of 2. 57 ± 0. 06 Mly (787 ± 18 kpc) was derived. A light-year or light year (symbol ly) is a unit of Length, equal to just under ten trillion Kilometres As defined by History The first direct measurements of an object at interstellar distances were undertaken by German Astronomer Friedrich Wilhelm Bessel in 1838

Using the Cepheid variable method, an estimate of 2. A Cepheid variable (pron ˈse-f(ē-id or ˈsē-f(ē-id or Cepheid is a member of a particular class of Variable stars notable for a fairly tight correlation 51 ± 0. 13 Mly (770 ± 40 kpc) was achieved in 2004. [3][2]

In 2005, a group of astronomers consisting of Ignasi Ribas (CSIC, IEEC) and his colleagues announced the discovery of an eclipsing binary star in the Andromeda Galaxy. A binary star is a Star system consisting of two Stars orbiting around their Center of mass. The binary star, designated M31VJ00443799+4129236,[c] has two luminous and hot blue stars of types O and B. In Astronomy, stellar classification is a classification of Stars based initially on photospheric temperature and its associated Spectral characteristics By studying the eclipses of the stars, which occur every 3. 54969 days, the astronomers were able to measure their sizes. Knowing the sizes and temperatures of the stars they were able to measure the absolute magnitude of the stars. In Astronomy, absolute magnitude (also known as absolute visual magnitude) is the Apparent magnitude an object would have if it were at a standard When the visual and absolute magnitudes are known, the distance to the star can be measured. The apparent magnitude ( m) of a celestial body is a measure of its Brightness as seen by an observer on Earth, normalized to the value The stars lie at the distance of 2. 52 ± 0. 14 Mly (770 ± 40 kpc) and the whole Andromeda Galaxy at about 2. 5 Mly. [4] This new value is in excellent agreement with the previous, independent Cepheid-based distance value.

Andromeda is close enough that the Tip of the Red Giant Branch (TRGB) method may also be used to estimate its distance. Tip of the red giant branch ( TRGB) is a primary distance indicator used in Astronomy. The estimated distance to M31 using this technique in 2005 yielded 2. 56 ± 0. 08 Mly (785 ± 25 kpc). [5]

Averaged together, all these distance measurements give a combined distance estimate of 2. 54 ± 0. 06 Mly (778 ± 17 kpc). [a] Based upon the above distance, the diameter of M31 at the widest point is estimated to be 141 ± 3 kly. [d]

Mass estimates

Mass estimates for the Andromeda halo (including dark matter) give a value of approximately 1. In Physics and cosmology, dark matter is hypothetical Matter that does not interact with the electromagnetic force but whose presence can be inferred from 23×1012 M☉[21] (or 1. The solar mass is a standard way to express Mass in Astronomy, used to describe the masses of other Stars and galaxies. 2 million million solar masses) compared to 1. The solar mass is a standard way to express Mass in Astronomy, used to describe the masses of other Stars and galaxies. 9×1012 M☉ for the Milky Way. Thus M31 may be less massive than our own galaxy, although the error range is still too large to say for certain. M31 does contain many more stars than our own galaxy and has a much larger size.

In particular, M31 appears to have significantly more common stars than the Milky Way, and the estimated luminosity of M31 is double that of our own galaxy. [22] However the rate of star formation in the Milky Way is much higher, with M31 only producing 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 M31. A nova (pl novae or novas) is a Cataclysmic nuclear explosion caused by the accretion of hydrogen onto the surface of a White [23] This suggests that M31 has experienced a great star formation phase in its past, while the Milky Way is in the middle of a current star formation phase. This could mean that in the future, the number of stars in the Milky Way will match the number observed in M31.

Structure

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. Galaxy morphological classification is a system used by Astronomers to divide galaxies into groups based on their visual appearance [1] However, data from the 2MASS survey showed that the bulge of M31 has a box-like appearance, which implies that the galaxy is actually a barred galaxy with the bar viewed almost directly along its long axis. Observations for the Two Micron All-Sky Survey ( 2MASS) began in 1997 and were completed in 2001 at two Telescopes located one each in the northern and [24]

The Andromeda Galaxy seen in infrared by the Spitzer Space Telescope, one of NASA's four Great Space Observatories
The Andromeda Galaxy seen in infrared by the Spitzer Space Telescope, one of NASA's four Great Space Observatories

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. Infrared ( IR) radiation is Electromagnetic radiation whose Wavelength is longer than that of Visible light, but shorter than that of The Spitzer Space Telescope (formerly the Space Infrared Telescope Facility, SIRTF) is an Infrared Space observatory. The National Aeronautics and Space Administration ( NASA, ˈnæsə is an agency of the United States government, responsible for the nation's public space program NASA 's series of Great Observatories Satellites are four large powerful space-based Telescopes Each of the Great Observatories has had a similar size and The W M Keck Observatory is a two-telescope Astronomical observatory at the 4145 meter (13600 ft summit of Mauna Kea in Hawai'i. [25] This means that the spiral disk of stars in Andromeda 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 in diameter. Previously, estimates of Andromeda's size ranged from 70,000 to 120,000 light-years across.

The galaxy is inclined an estimated 77° relative to the Earth (where an angle of 90° would be viewed directly from the side). Analysis of the cross-sectional shape of the galaxy appears to demonstrate a pronounced, S-shaped warp, rather than just a flat disk. [26] A possible cause of such a warp could be gravitational interaction with the satellite galaxies near M31. It also should be noted that the galaxy M33 could be responsible for some warp in M31's arms, though more precise distances and radial velocities are required.

Spectroscopic studies have provided detailed measurements of the rotational velocity of this galaxy at various radii from the core. The rotation curve of a Galaxy can be represented by a graph that plots the Orbital velocity of the Stars or Gas in the galaxy In the vicinity of the core, the rotational velocity climbs to a peak of 225 kilometres per second (140 miles/sec. ) at a radius of 1,300 light-years, then descends to a minimum at 7,000 light-years where the rotation velocity may be as low as 50 kilometres per second (31 miles/sec. A light-year or light year (symbol ly) is a unit of Length, equal to just under ten trillion Kilometres As defined by ). Thereafter the velocity steadily climbs again out to a radius of 33,000 light-years, where it reaches a peak of 250 kilometres per second (155 miles/sec. ). The velocities slowly decline beyond that distance, dropping to around 200 kilometres per second (124 miles/sec. ) at 80,000 light-years. These velocity measurements imply a concentrated mass of about 6×109 M☉ in the nucleus. The solar mass is a standard way to express Mass in Astronomy, used to describe the masses of other Stars and galaxies. A galaxy is a massive gravitationally bound system consisting of Stars an Interstellar medium of gas and dust, and Dark matter The total mass of the galaxy increases linearly out to 45,000 light-years, then more slowly beyond that radius. The word linear comes from the Latin word linearis, which means created by lines. [27]

The spiral arms of Andromeda are outlined by a series of H II regions that Baade described as resembling "beads on a string". A spiral galaxy is a Galaxy belonging to one of the three main classes of galaxy originally described by Edwin Hubble in his 1936 work “The Realm of the An H II region (also known as Emission nebula) is a cloud of glowing Gas and plasma, sometimes several hundred Light-years across They appear to be tightly wound, although they are more widely spaced than in our galaxy. [28] Rectified images of the galaxy show a fairly normal spiral galaxy with the arms wound up in a clockwise direction. There are two continuous trailing arms that are separated from each other by a minimum of about 13,000 light-years. These can be followed outward from a distance of roughly 1,600 light-years from the core. The most likely cause of the spiral pattern is thought to be interaction with M32. Messier 32 (also known as NGC 221 and Le Gentil) is a Dwarf elliptical galaxy about 2 This can be seen by the displacement of the neutral hydrogen clouds from the stars. An H I region is an Interstellar cloud composed of neutral atomic Hydrogen (H I [29]

Image of Andromeda Galaxy (M31) taken by Spitzer in infrared, 24 micrometres (Credit:NASA/JPL-Caltech/K. Gordon (University of Arizona)
Image of Andromeda Galaxy (M31) taken by Spitzer in infrared, 24 micrometres (Credit:NASA/JPL-Caltech/K. The Spitzer Space Telescope (formerly the Space Infrared Telescope Facility, SIRTF) is an Infrared Space observatory. A micrometre ( American spelling: micrometer; symbol µm) is one millionth of a Metre, or equivalently one thousandth of a Millimetre The National Aeronautics and Space Administration ( NASA, ˈnæsə is an agency of the United States government, responsible for the nation's public space program The California Institute of Technology (commonly referred to as Caltech) is a private, Coeducational research university located in Pasadena Gordon (University of Arizona)

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 University of Arizona (also referred to as UA, U of A, or Arizona) is a Land-grant and space-grant public institution The European Space Agency ( ESA) established in 1975 is an intergovernmental organisation dedicated to the exploration of space, currently with 17 member The Infrared Space Observatory ( ISO) was a Space telescope for Infrared light designed and operated by the European Space Agency (ESA A ring galaxy is a Galaxy with a ring-like appearance The ring consists of massive relatively young blue Stars which are extremely bright The gas and dust within Andromeda is generally formed into several overlapping rings, with a particularly prominent ring formed at a radius of 32,000 light-years from the core. [30] This ring is hidden from visible light images of the galaxy because it is composed primarily of cold dust.

Close examination of the inner region of Andromeda 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 Andromeda 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. [31]

Studies of the extended halo of M31 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. [32] This evidence indicates that the two galaxies have followed similar evolutionary paths. They are likely to have accreted and assimilated about 1–200 low-mass galaxies during the past 12 billion years. [33] The stars in the extended halos of M31 and the Milky Way may extend nearly one-third the distance separating the two galaxies.

Nucleus

HST image of Andromeda galaxy core showing possible double structure. NASA/ESA photo.
HST image of Andromeda galaxy core showing possible double structure. NASA/ESA photo. The National Aeronautics and Space Administration ( NASA, ˈnæsə is an agency of the United States government, responsible for the nation's public space program The European Space Agency ( ESA) established in 1975 is an intergovernmental organisation dedicated to the exploration of space, currently with 17 member
Artist's concept of Andromeda galaxy core showing a view across a mysterious disk of young, blue stars encircling a supermassive black hole. NASA/ESA photo.
Artist's concept of Andromeda galaxy core showing a view across a mysterious disk of young, blue stars encircling a supermassive black hole. NASA/ESA photo. The National Aeronautics and Space Administration ( NASA, ˈnæsə is an agency of the United States government, responsible for the nation's public space program The European Space Agency ( ESA) established in 1975 is an intergovernmental organisation dedicated to the exploration of space, currently with 17 member

M31 is 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 on board the Hubble Space Telescope, to image Andromeda's inner nucleus. Tod R Lauer is an associate astronomer on the research staff of the National Optical Astronomy Observatory. The Wide Field/Planetary Camera ( WFPC) (pronounced as wiffpick was a Camera installed on the Hubble Space Telescope. The Hubble Space Telescope ( HST; also known colloquially as "the Hubble" or just "Hubble" is a space telescope that was carried into The nucleus is double, consisting of two concentrations separated by 1. 5 parsecs. History The first direct measurements of an object at interstellar distances were undertaken by German Astronomer Friedrich Wilhelm Bessel in 1838 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 a 108 M☉ black hole. The solar mass is a standard way to express Mass in Astronomy, used to describe the masses of other Stars and galaxies. A black hole is a theoretical region of space in which the Gravitational field is so powerful that nothing not even Electromagnetic radiation (e

Scott Tremaine has proposed the following explanation of the double nucleus: P1 is the projection of a disk of stars in an eccentric orbit around the central black hole. Scott Duncan Tremaine (born 1950) is a Canadian -born astrophysicist. In Astrodynamics, under standard assumptions, any Orbit must be of Conic section shape The eccentricity is such that stars "linger" longer at the orbital apocenter, creating a concentration of stars. In Celestial mechanics, an apsis, plural apsides (ˈæpsɨdɪːz is the point of greatest or least distance of the Elliptical orbit of an object from P2 also contains a compact disk of hot, spectral class A-stars. In Astronomy, stellar classification is a classification of Stars based initially on photospheric temperature and its associated Spectral characteristics The A-stars are not evident in redder filters, but in blue and ultraviolet light they dominate the nucleus, causing P2 to appear more prominent than P1. [34]

While at the initial time of its discovery it was hypothesized that the brighter portion of the double nucleus was the remnant of a small galaxy 'cannibalized' by Andromeda,[35] this is no longer considered to be a viable explanation. Such a nucleus would have an exceedingly short lifetime to tidal disruption by the central black hole, nor does it have its own black hole to stabilize it. Further the clump does not resemble a galactic nucleus, nor is there any evidence of a merger at larger radii in the bulge.

Discrete sources

Multiple X-ray sources have been detected in the Andromeda Galaxy, using observations from the ESA's XMM-Newton orbiting observatory. The European Space Agency ( ESA) established in 1975 is an intergovernmental organisation dedicated to the exploration of space, currently with 17 member The XMM-Newton (X-ray Multi-Mirror Mission - Newton is an orbiting X-ray observatory, named in honor of Isaac Newton. Dr. Robin Barnard et al hypothesized that these are candidate black holes or neutron stars, which are heating incoming gas to millions of kelvins and emitting X-rays. A neutron star is a type of remnant that can result from the Gravitational collapse of a massive Star during a Type II, Type Ib or Type The spectrum of the neutron stars is the same as the hypothesized black holes, but can be distinguished by their masses. [36]

There are approximately 460 globular clusters associated with the Andromeda galaxy. A globular cluster is a spherical collection of Stars that orbits a galactic core as a Satellite. [37] The most massive of these clusters, identified as Mayall II, nicknamed Globular One, has a greater luminosity than any other known globular cluster in the local group of galaxies. Mayall II ( M31 G1) also known as NGC-224-G1, SKHB 1, GSC 27882139, HBK 0-1, M31GC J003247+393440 The Local Group is the group of galaxies that includes our galaxy the Milky Way. [38] It contains several million stars, and is about twice as luminous as Omega Centauri, the brightest known globular cluster in the Milky Way. Omega Centauri or NGC 5139 is a Globular cluster of Stars seen in the constellation of Centaurus, discovered by Edmond Halley in The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply Globular One (or G1) has several stellar populations and a structure too massive for an ordinary globular. As a result, some consider G1 to be the remnant core of a dwarf galaxy that was consumed by M31 in the distant past. A dwarf galaxy is a small Galaxy composed of up to several billion Stars a small number compared to our own Milky Way 's 200-400 billion stars [39] The globular with the greatest apparent brightness is G76 which is located in the south-west arm's eastern half. [10]

In 2005, astronomers discovered a completely new type of star cluster in M31. The new-found clusters contain hundreds of thousands of stars, a similar number of stars that can be found in globular clusters. What distinguishes them from the globular clusters is that they are much larger – several hundred light-years across – and hundreds of times less dense. The distances between the stars are, therefore, much greater within the newly discovered extended clusters. [40]

Satellites

Main article: Andromeda's satellite galaxies

Like the Milky Way, Andromeda Galaxy has satellite galaxies, consisting of 14 known dwarf galaxies. The Andromeda Galaxy has satellite galaxies just like the Milky Way Galaxy. The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply A satellite galaxy Orbits a larger galaxy due to gravitational attraction. A dwarf galaxy is a small Galaxy composed of up to several billion Stars a small number compared to our own Milky Way 's 200-400 billion stars The best known and most readily observed satellite galaxies are M32 and M110. Messier 32 (also known as NGC 221 and Le Gentil) is a Dwarf elliptical galaxy about 2 Messier 110 (also known as M110 and NGC 205) is a Dwarf elliptical galaxy that is a satellite of the Andromeda Galaxy.

Based on current evidence, it appears that M32 underwent a close encounter with M31 (Andromeda) 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 relative recent past. Star Formation is the process by which dense parts of Molecular clouds collapse into a ball of plasma to form a Star. [41]

M110 also appears to be interacting with M31, and astronomers have found a stream of metal-rich stars in the halo of M31 that appears to have been stripped from these satellite galaxies. [42] M110 does contain a dusty lane, which is a hint for recent or ongoing star formation. This is unusual in elliptical galaxies, which are usually fairly low in dust and gas. Dwarf elliptical galaxies, or dE's are elliptical galaxies that are much smaller than others classified as dE.

In 2006 it was discovered that nine of these galaxies lay along a plane that intersects the core of the Andromeda Galaxy, rather than being randomly arranged as would be expected from independent interactions. This may indicate a common tidal origin for the satellites. [43]

Andromeda's satellites discovered before 1900
Name Type Distance
from Sun
(Mly)		 Magnitude Discovered by Year
discovered
M32 cE2 2. Galaxy morphological classification is a system used by Astronomers to divide galaxies into groups based on their visual appearance The Sun (Sol is the Star at the center of the Solar System. A light-year or light year (symbol ly) is a unit of Length, equal to just under ten trillion Kilometres As defined by The apparent magnitude ( m) of a celestial body is a measure of its Brightness as seen by an observer on Earth, normalized to the value Messier 32 (also known as NGC 221 and Le Gentil) is a Dwarf elliptical galaxy about 2 65 ± 0. 10 +9. 0 Guillaume Le Gentil 1749
M110 E5 pec 2. Guillaume Joseph Hyacinthe Jean-Baptiste Le Gentil de la Galaisière ( September 12 1725 &ndash October 22 1792) was a French Astronomer Messier 110 (also known as M110 and NGC 205) is a Dwarf elliptical galaxy that is a satellite of the Andromeda Galaxy. 69 ± 0. 09 +8. 9 Charles Messier 1773
NGC 185 dSph/dE3 2. Charles Messier ( June 26, 1730 &ndash April 12, 1817) was a French astronomer most notable for publishing an NGC 185 is a Dwarf elliptical galaxy about 208 million Light-years away in the Constellation Cassiopeia. 08 ± 0. 15 +10. 1 William Herschel 1787
NGC 147 dSph/dE5 2. Sir Frederick William Herschel FRS KH ( 15 November 1738 – 25 August 1822) was a German -born British NGC 147 (DDO3 is a Dwarf spheroidal galaxy about 258 Mly away in the Constellation Cassiopeia. 67 ± 0. 18 +10. 5 John Herschel 1829

See also

Notes

  1. ^ average(787 ± 18, 770 ± 40, 772 ± 44, 783 ± 25) = ((787 + 770 + 772 + 783) / 4) ± ((182 + 402 + 442 + 252)0. Sir John Frederick William Herschel 1st Baronet KH, FRS ( March 7, 1792 &ndash May 11, 1871)was an Galaxies other than the Milky Way are popular settings for creators of Science fiction, particularly those working with broad-scale Space opera settings Mayall II ( M31 G1) also known as NGC-224-G1, SKHB 1, GSC 27882139, HBK 0-1, M31GC J003247+393440 A globular cluster is a spherical collection of Stars that orbits a galactic core as a Satellite. NGC 206 is a bright Star cloud in the Andromeda Galaxy. It is notable for being the brightest star cloud in Andromeda as viewed from Earth. A star cloud is a group of stars that appear to be in the same position in the sky The Messier objects are a set of astronomical objects catalogued by Charles Messier in his "Catalogue des Nébuleuses et des Amas d'Étoiles" ("Catalogue of Nebulae The New General Catalogue ( NGC) is the best-known catalogue of Deep sky objects in Amateur astronomy. 5 / 4) = 778 ± 17
  2. ^ Apparent Magnitude of 4. 36 - distance modulus of 24. The distance modulus is a way of expressing distances that is often used in Astronomy. 4 = −20. 0
  3. ^ J00443799+4129236 is at celestial coordinates R.A. 00h 44m 37. In Astronomy, a celestial coordinate system is a Coordinate system for mapping positions in the sky Right ascension (abbrev RA; symbol α) is the Astronomical term for one of the two Coordinates of a point on the Celestial sphere 99s, Dec. +41° 29′ 23. In Astronomy, declination (abbrev dec or δ) is one of the two coordinates of the Equatorial coordinate system, the other being either 6″.
  4. ^ distance × tan( diameter_angle = 190′ ) = 141 ± 3 kly diameter

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