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Andromeda Galaxy

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

   CAPTION: Andromeda Galaxy

           Galaxy          List of galaxies
              Observation data
   ( Epoch J2000)
        Constellation      Andromeda
       Right ascension     00^h 42^m 44.3^s
         Declination       +41° 16′ 9″
          Redshift         -301 ± 1 km/ s
          Distance         2.52 ± 0.14 Mly
                           (770 ± 40 kpc)
            Type           SA(s)b I-II
   Apparent dimensions (V) 190′ × 60′
   Apparent magnitude (V)  +4.36
      Notable features
             Other designations

   M31, NGC 224, UGC 454, PGC 2557

   M31 in a small telescope
   Enlarge
   M31 in a small telescope

   The Andromeda Galaxy ( IPA: /ˌanˈdrɒmədə/, also known as Messier 31,
   M31, or NGC 224; older texts often called it the Andromeda Nebula) is a
   spiral galaxy approximately 2.5 million light-years away in the
   constellation Andromeda.

   Andromeda was believed to be the largest galaxy of the Local Group of
   galaxies, which consists of the Andromeda Galaxy, the Milky Way Galaxy,
   and the Triangulum Galaxy, and about 30 other smaller galaxies. Due to
   recent findings based on improved measurements and data, scientists now
   believe that the Milky Way contains more dark matter and may be the
   most massive in the grouping. However, recent observations by the
   Spitzer Space Telescope revealed that M31 contains one trillion (10^12)
   stars, greatly exceeding the number of stars in our own galaxy. 2006
   estimates put the mass of the Milky Way to be ~80% of the mass of
   Andromeda, which is estimated to be 7.1×10^11 M[☉].

   The Andromeda Galaxy is easily visible to the naked eye in a moderately
   dark sky, though such a sky is available only in smaller towns and
   isolated areas reasonably far from population centers and sources of
   light pollution. 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.

Observation history

   The earliest recorded observation of the Andromeda Galaxy was in 905 by
   the Persian astronomer 'Abd Al-Rahman Al Sufi, who described it as a
   "small cloud". Star charts of that period have it labeled as the Little
   Cloud. The first description of the object based on telescopic
   observation was given by Simon Marius (1612). In 1764, Charles Messier
   catalogued it as object M31 and incorrectly credited Simon Marius as
   the discoverer, unaware of Al Sufi's earlier work.

   In 1785, the astronomer William Herschel noted a faint reddish hue in
   the core region of the galaxy. He believed it to be the nearest of all
   the "great nebulae" and, based on the colour and magnitude of the
   nebula, he estimated (incorrectly) that it was no more than 2,000 times
   the distance of Sirius.

   William Huggins in 1864 observed the spectrum of Andromeda and noted
   that it differed from a gaseous nebula. The spectra of Andromeda
   displayed a continuum of frequencies, superimposed with dark lines.
   This was very similar to the spectra of individual stars. From this it
   was deduced that Andromeda had a stellar nature.

   In 1885, a supernova (known as " S Andromedae") was seen in the
   Andromeda Galaxy, the first and so far only one observed in that
   galaxy. At the time, it was thought to be a much less luminous and
   unrelated event called a nova, and was named accordingly Nova 1885.
   Great Andromeda Nebula by Isaac Roberts.
   Enlarge
   Great Andromeda Nebula by Isaac Roberts.

   The first photographs of this galaxy were taken in 1887 by Isaac
   Roberts from his private observatory in Sussex. The long-duration
   exposure allowed the spiral structure of the galaxy to be seen for the
   first time. 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 birthing 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. 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.

Island universe

   In 1917, Heber Curtis had observed a nova within M31. 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. 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 that held the spiral nebulae
   were actually independent galaxies.

   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. To support his claim that M31 was an
   external galaxy, Curtis also noted the appearance of dark lanes
   resembling the dust clouds in our own galaxies, as well as the
   significant doppler shift.

   Edwin Hubble settled the debate in 1925 when he identified
   extragalactic Cepheid variable stars for the first time on astronomical
   photos of this galaxy. These were made using a 2.5- metre (100 in.)
   reflecting telescope, and they enabled the distance of M31 to be
   determined. 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.

   This galaxy plays an important role in galactic studies, since it is
   the nearest giant spiral (although not the nearest galaxy). In 1943,
   Walter Baade was the first person to resolve stars in the central
   region of the Andromeda Galaxy. 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. 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.) 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.

   The first radio maps of the Andromeda 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 catalogue.

General information

   The Andromeda Galaxy is approaching the Sun at about 300 kilometres per
   second (186 miles/sec.), so it is one of the few blue shifted galaxies.
   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 kilometres per second (62–87 miles/sec.).
   However, this does not mean it will definitely collide with the Milky
   Way, since the galaxy's tangential velocity is unknown. If it is on a
   collision course, the impact is predicted to occur in about 3 billion
   (10^9) years. In that case the two galaxies will likely merge to form a
   giant elliptical galaxy. Such events are frequent among the galaxies in
   galaxy groups.

   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. 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
   Enlarge
   The Andromeda Galaxy pictured in ultraviolet light by GALEX

   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. The binary star, designated
   M31VJ00443799+4129236, has two luminous and hot blue stars of types O
   and B. 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. When the visual and absolute
   magnitudes are known, the distance to the star can be measured. The
   stars lie at the distance of 2.52 ± 0.14 million light-years and the
   whole Andromeda Galaxy at about 2.5 million light-years. This new value
   is in excellent agreement with the previous, independent Cepheid-based
   distance value.

   Current mass estimates for the Andromeda halo (including dark matter)
   give a value of approximately 1.23 × 10^12 M[☉] (or 1.2 million million
   solar masses) compared to 1.9 × 10^12 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. 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. 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. 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
   nearly directly along its long axis. Andromeda is also a LINER-type
   galaxy (Low-Ionization Nuclear Emission-line Region), the most common
   class of active nuclei galaxies.
   The Andromeda Galaxy seen in infrared by the Spitzer Space Telescope,
   one of NASA's four Great Space Observatories
   Enlarge
   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 are actually part
   of the main disk itself. 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. A possible
   cause of such a warp could be gravitational interaction with the
   satellite galaxies near M31.

   Spectroscopic studies have provided detailed measurements of the
   rotational velocity of this galaxy at various radii from the core. 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.). 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 × 10^9 M[☉] in the nucleus. The total mass of the
   galaxy increases linearly out to 45,000 light-years, then more slowly
   beyond that radius.

   The spiral arms of Andromeda are outlined by a series of H II regions
   that Baade described as resembling "beads on a string". They appear to
   be tightly wound, although they are more widely spaced than in our
   galaxy. 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. This
   can be seen by the displacement of the neutral hydrogen clouds from the
   stars.
   Image of Andromeda Galaxy (M31) taken by Spitzer in infrared, 24
   micrometres (Credit:NASA/JPL-Caltech/K. Gordon (University of Arizona)
   Enlarge
   Image of Andromeda Galaxy (M31) taken by Spitzer in infrared, 24
   micrometres (Credit: NASA/ JPL- Caltech/K. 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 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. 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 later's polar
   axis. This collision stripped more than half the mass from the smaller
   M32 and created the ring structures in Andromeda.

   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. 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 thousand million years The stars
   in the extended halos of M31 and the Milky Way may extend nearly ^1⁄[3]
   the distance separating the two galaxies.

Features

   HST image of Andromeda galaxy core showing possible double structure.
   NASA photo.
   Enlarge
   HST image of Andromeda galaxy core showing possible double structure.
   NASA photo.

   In 1991 the Planetary Camera then onboard the Hubble Space Telescope
   imaged Andromeda's core. To everyone's surprise its nucleus showed a
   double structure, with two nuclear hot-spots located within a few
   light-years of each other. Subsequent ground-based observations have
   led to speculation that indeed two nuclei exist and are moving with
   respect to each other, that one nucleus is slowly tidally disrupting
   the other, and that one nucleus may be the remnant of a smaller galaxy
   "eaten" by M31. The nuclei of many galaxies, including M31, are known
   to be quite violent places, and the existence of supermassive black
   holes is frequently postulated to explain them.

   Multiple X-ray sources have been detected in the Andromeda Galaxy,
   using observations from the ESA's XMM-Newton orbiting observatory. 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. The spectrum of the neutron stars is the same as
   the hypothesized black holes, but can be distinguished by their masses.

   There are approximately 460 globular clusters associated with the
   Andromeda galaxy The most massive of these clusters, identified as
   Mayall II, nicknamed Globular One, has a greater luminosity than any
   known globular cluster in the local group of galaxies. It contains
   several million stars, and is about twice as luminous as Omega
   Centauri, the brightest known globular cluster in the Milky Way.
   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. The globular with the greatest apparent brightness is G76
   which is located in the south-west arm's eastern half.

   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.

Satellites

   Like our Milky Way, Andromeda has satellite galaxies, consisting of 14
   known dwarf galaxies. 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 M31 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.

   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. 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.

   In 2006 it was discovered that nine of these galaxies lie along a plane
   that intersects the core of the Andromeda Galaxy, rather than being
   randomly generated. This may indicate a common origin for the
   satellites.

   CAPTION: Andromeda's satellites discovered before 1900

    Name     Type    Distance
                    from Sun
                    ( Mly)      Magnitude    Discovered by       Year
                                                              discovered
   M32     cE2      2.65 ± 0.10 +9.0      Guillaume Le Gentil 1749
   M110    E5 pec   2.9         +8.9      Charles Messier     1773
   NGC 185 dSph/dE3 2.08 ± 0.15 +10.1     William Herschel    1787
   NGC 147 dSph/dE5 2.67 ± 0.18 +10.5     John Herschel       1829

Andromeda Galaxy in fiction

   The Andromeda galaxy has had a role in a number of science fiction
   works. It has been used as a source of alien life forms that are in the
   process of invading the Milky Way galaxy, or as a setting for alien
   worlds and territories.

   It has been hinted that the Star Wars galaxy is the Andromeda Galaxy.

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