Number of stars
~1 trillion (1012)
~220 kly (diameter)
Apparent size (V)
3.167° × 1°
M31, NGC 224, UGC 454, PGC 2557, 2C 56 (Core), CGCG 535-17, MCG
+07-02-016, IRAS 00400+4059, 2MASX J00424433+4116074, GC 116, h 50,
Bode 3, Flamsteed 58, Hevelius 32, Ha 3.3, IRC +40013
See also: Galaxy, List of galaxies
Galaxy (/ænˈdrɒmɪdə/), named after the mythological
Princess Andromeda, also known as Messier 31, M31, or NGC 224, is a
spiral galaxy approximately 780 kiloparsecs (2.5 million light-years)
from Earth, and the nearest major galaxy to the Milky Way. Its name
stems from the area of the sky in which it appears, the constellation
The 2006 observations by the
Spitzer Space Telescope
Spitzer Space Telescope revealed that the
Galaxy contains approximately one trillion stars, more
than twice the number of the Milky Way’s estimated 200-400 billion
Andromeda Galaxy, spanning approximately 220,000 light
years, is the largest galaxy in our Local Group, which is also home to
Galaxy and other minor galaxies. The
mass is estimated to be around 1.76 times that of the
Milky Way Galaxy
(~0.8-1.5×1012 solar masses  vs the Milky Way's 8.5×1011
Milky Way and
Andromeda galaxies are expected to collide in ~4.5
billion years, merging to form a giant elliptical galaxy or a
large disc galaxy. With an apparent magnitude of 3.4, the
Galaxy is among the brightest of the Messier objects -
making it visible to the naked eye on moonless nights, even when
viewed from areas with moderate light pollution.
1 Observation history
1.1 Island universe
2.1 Formation and history
2.2 Distance estimate
Mass and luminosity estimates
5 Discrete sources
7 Collision with the Milky Way
8 Amateur observing
9 See also
12 External links
Andromeda Nebula by Isaac Roberts, 1899.
Around the year 964, the Persian astronomer Abd al-Rahman al-Sufi
Andromeda Galaxy, in his
Book of Fixed Stars
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
Galaxy based on telescopic
observations. The German philosopher
Immanuel Kant in 1755 in his
work Universal Natural History and Theory of the Heavens conjectured
that the blurry spot was an island universe. In 1764, Charles Messier
Andromeda as object M31 and incorrectly credited Marius as
the discoverer despite it being visible to the naked eye. In 1785, the
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 is no more than 2,000 times the
distance of Sirius. In 1850, William Parsons, 3rd Earl of Rosse,
saw and made the first drawing of Andromeda's spiral structure.
William Huggins noted that the spectrum of
from 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
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
Isaac Roberts took the first photographs of Andromeda, which
was still commonly thought to be a nebula within our galaxy. Roberts
Andromeda and similar spiral nebulae as solar systems 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 kilometres per
second (190 mi/s).
Location of the
Galaxy (M31) in the
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 light-years
(3.2×1010 AU). He became a proponent of the so-called "island
universes" hypothesis, which held that spiral nebulae were actually
Galaxy above the Very Large Telescope. You can also see
Galaxy on the top.
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 450,000 parsecs
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
2.5-metre (100-in) Hooker telescope, and they enabled the distance of
Andromeda Nebula to be determined. His measurement demonstrated
conclusively that this feature is not a cluster of stars and gas
within our own Galaxy, but an entirely separate galaxy located a
significant distance from the Milky Way.
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 variables, 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
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
Galaxy is called 2C 56 in the 2C radio
astronomy catalog. In 2009, the first planet may have been discovered
Andromeda Galaxy. This was detected using a technique called
microlensing, which is caused by the deflection of light by a massive
The estimated distance of the
Galaxy from our own was
doubled in 1953 when it was discovered that there is another, dimmer
type of Cepheid. In the 1990s, measurements of both standard red
giants as well as red clump stars from the
measurements were used to calibrate the
Formation and history
Galaxy as seen by NASA's Wide-field
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, 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
a very close passage 2–4 billion years ago. This event produced high
levels 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
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
Andromeda as well as the presence there of a relatively young (100
million years old) stellar population.
At least four distinct techniques have been used to estimate distances
Earth to the
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 AU) was derived.
In 2004, using the
Cepheid variable method, the distance was estimated
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[c] 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
measured. The stars lie at a distance of
2.52×10^6 ± 0.14×10^6 ly
(1.594×1011 ± 8.9×109 AU) and the whole Andromeda
Galaxy at about 2.5×10^6 ly (1.6×1011 AU). This new
value is in excellent agreement with the previous, independent
Cepheid-based distance value.
In 2005, using the TRGB method, the distance was estimated to be
2.56×10^6 ± 0.08×10^6 ly
(1.619×1011 ± 5.1×109 AU).
Averaged together, these distance estimates give a value of
2.54×10^6 ± 0.11×10^6 ly
(1.606×1011 ± 7.0×109 AU).[a] And, from this, the
Andromeda at the widest point is estimated to be
220 ± 3 kly
(67,450 ± 920 pc).[original research?] Applying
trigonometry (angular diameter), this is equivalent to an apparent
4.96° angle in the sky.
Mass and luminosity estimates
Galaxy pictured in ultraviolet light by GALEX.
Illustration showing both the size of each galaxy and the distance
between the two galaxies, to scale.
Giant halo around
Mass estimates for the
Andromeda Galaxy's halo (including dark matter)
give a value of approximately 1.5×1012 M☉ (or 1.5 trillion
solar masses) compared to 8×1011 M☉ for the Milky Way. This
contradicts earlier measurements, that seem to indicate that Andromeda
Galaxy and the
Milky Way are almost equal in mass. Even so, Andromeda
Galaxy's spheroid actually has a higher stellar density than that of
the Milky Way and its galactic stellar disk is about twice the
size of that of the Milky Way. The total stellar mass of Andromeda
Galaxy is estimated to be between 1.1×1011 M☉., (i.e.,
around twice as massive as that of the Milky Way) and
1.5×1011 M☉, with around 30% of that mass in the central
bulge, 56% in the disk, and the remaining 14% in the halo.
In addition to it,
Andromeda Galaxy's interstellar medium contains at
least around 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.
Galaxy is surrounded by a large and 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 supernovae 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 Andromeda
Galaxy's star-filled disk and eject these heavier elements into space.
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.
Compared to the Milky Way, the
Galaxy appears to have
predominantly older stars with ages >7×109
years.[clarification needed] The estimated luminosity of 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
(some authors even propose it is the second-brightest galaxy within a
radius of 10 mega-parsecs of the Milky Way, after the Sombrero
Galaxy, with an absolute magnitude of around -22.21[d] or
An estimation done with the help of
Spitzer Space Telescope
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,[b] 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
Galaxy producing only about one solar mass per year compared
to 3–5 solar masses for the Milky Way. The rate of supernovae in the
Milky Way is also double that of
Andromeda Galaxy.[not in citation
given] This suggests that the latter once experienced a great star
formation phase, but is now in a relative state of quiescence, whereas
Milky Way is experiencing more active star formation. Should
this continue, the luminosity of the
Milky Way may eventually overtake
According to recent studies, the
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
Andromeda Galaxy, star formation is expected to
extinguish within about five billion years from the now, even
accounting for the expected, short-term increase in the rate of star
formation due to the collision between
Galaxy and the Milky
Galaxy seen in infrared by the Spitzer Space Telescope,
one of NASA's four Great Space Observatories.
Image of the
Galaxy taken by Spitzer in infrared, 24
micrometres (Credit:NASA/JPL–Caltech/K. Gordon, University of
A Swift Tour of
Galaxy Evolution Explorer image of the
Andromeda Galaxy. The bands
of blue-white making up the galaxy's striking rings are neighborhoods
that harbor hot, young, massive stars. Dark blue-grey lanes of cooler
dust show up starkly against these bright rings, tracing the regions
where star formation is currently taking place in dense cloudy
cocoons. When observed in visible light,
Andromeda Galaxy’s rings
look more like spiral arms. The ultraviolet view shows that these arms
more closely resemble the ring-like structure previously observed in
infrared wavelengths with NASA’s Spitzer Space Telescope.
Astronomers using the latter interpreted these rings as evidence that
the galaxy was involved in a direct collision with its neighbor, M32,
more than 200 million years ago.
Based on its appearance in visible light, the
classified as an SA(s)b galaxy in the de Vaucouleurs–Sandage
extended classification system of spiral galaxies. However, data
2MASS survey showed that
Andromeda is actually a barred
spiral galaxy, like the Milky Way, with Andromeda's bar oriented along
its long 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
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 (67,000 pc) in diameter. Previously, estimates of the
Andromeda Galaxy's size ranged from 70,000 to 120,000 light-years
(21,000 to 37,000 pc) across.
The galaxy is inclined an estimated 77° relative to the
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 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
Galaxy as a function of radial
distance from the core. The rotational velocity has a maximum value of
225 kilometres per second (140 mi/s) at 1,300 light-years
(82,000,000 AU) from the core, and it has its minimum possibly as
low as 50 kilometres per second (31 mi/s) at 7,000 light-years
(440,000,000 AU) from the core. Further out, rotational velocity
rises out to a radius of 33,000 light-years (2.1×109 AU), where
it reaches a peak of 250 kilometres per second (160 mi/s). The
velocities slowly decline beyond that distance, dropping to around 200
kilometres per second (120 mi/s) at 80,000 light-years
(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 light-years
(2.8×109 AU), then more slowly beyond that radius.
The spiral arms of the
Galaxy are outlined by a series of H
II 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§pp1062§pp92:
Baade's spiral arms of M31
Arms (N=cross M31's major axis at north, S=cross M31's major axis at
Distance from center (arcminutes) (N*/S*)
Distance from center (kpc) (N*/S*)
Dust arms with no OB associations of HII regions.
Dust arms with some OB associations.
As per N2/S2, but with some HII regions too.
Large numbers of OB associations, HII regions, and little dust.
As per N4/S4 but much fainter.
Loose OB associations. No dust visible.
As per N6/S6 but fainter and inconspicuous.
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
light-years (820,000,000 AU) and that can be followed outward
from a distance of roughly 1,600 light-years (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.
This can be seen by the displacement of the neutral hydrogen clouds
from the stars.
In 1998, images from the European Space Agency's
Observatory demonstrated that the overall form of the
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 light-years
(2.0×109 AU) (10 kiloparsecs) 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
Galaxy is concentrated there.
Later studies with the help of the
Spitzer Space Telescope
Spitzer Space Telescope showed how
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
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
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
Studies of the extended halo of the
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
Galaxy and the
Milky Way may extend nearly one-third the
distance separating the two galaxies.
Hubble image of the
Galaxy core showing possible double
structure. NASA/ESA photo.
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. In 1991, the Hubble
Telescope was used to image
Andromeda Galaxy's inner nucleus.
The nucleus consists of two concentrations separated by 1.5 parsecs
(4.9 ly). 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 black hole measured at
3–5 × 107 M☉ in 1993, and at 1.1–2.3 × 108 M☉ in
2005. The velocity dispersion of material around it is measured to
be ≈ 160 km/s.
Chandra X-ray telescope image of the center of
Andromeda Galaxy. A
number of X-ray sources, likely X-ray binary stars, within the
galaxy's central region appear as yellowish dots. The blue source at
the center is at the position of the supermassive black hole.
It has been proposed that the observed double nucleus could be
explained if P1 is the projection of a disk of stars in an eccentric
orbit around the central black hole. The eccentricity is such that
stars linger at the orbital apocenter, creating a concentration of
stars. P2 also contains a compact disk of hot, spectral class A stars.
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.
While at the initial time of its discovery it was hypothesized that
the brighter portion of the double nucleus is the remnant of a small
galaxy "cannibalized" by
Andromeda Galaxy, this is no longer
considered a viable explanation, largely because such a nucleus would
have an exceedingly short lifetime due to tidal disruption by the
central black hole. While this could be partially resolved if P1 had
its own black hole to stabilize it, the distribution of stars in P1
does not suggest that there is a black hole at its center.
Galaxy in high-energy X-ray and ultraviolet light
(released 5 January 2016).
Artist's concept of the
Galaxy core showing a view across a
disk of young, blue stars encircling a supermassive black hole.
Apparently, by late 1968, no X-rays had been detected from the
Andromeda Galaxy. A balloon flight on October 20, 1970, set an
upper limit for detectable hard X-rays from the
Multiple X-ray sources have since been detected in the Andromeda
Galaxy, using observations from the European Space Agency's (ESA)
XMM-Newton orbiting observatory. Robin Barnard et al. hypothesized
that these are candidate black holes or neutron stars, which are
heating the 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 other 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 Andromeda
in the distant past. The globular with the greatest apparent
brightness is G76 which is located in the south-west arm's eastern
half. Another massive globular cluster, named 037-B327 and
discovered in 2006 as is heavily reddened by the
interstellar dust, was thought to be more massive than G1 and the
largest cluster of the Local Group; however, other studies have
shown it is actually similar in properties to G1.
Star clusters in the
Unlike the globular clusters of the Milky Way, which show a relatively
low age dispersion,
Andromeda Galaxy's globular clusters have a much
larger range of ages: from systems as old as the galaxy itself to much
younger systems, with ages between a few hundred million years to five
In 2005, astronomers discovered a completely new type of star cluster
Andromeda Galaxy. 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
In 2012, a microquasar, a radio burst emanating from a smaller black
hole, was detected in the
Andromeda Galaxy. The progenitor black hole
is located near the galactic center and has about 10
displaystyle begin smallmatrix M_ odot end smallmatrix
. Discovered through a data collected by the European Space Agency's
XMM-Newton probe, and subsequently observed by NASA's Swift Gamma-Ray
Burst Mission and Chandra X-Ray Observatory, the Very Large Array, and
the Very Long Baseline Array, the microquasar was the first observed
Galaxy and the first outside of the Milky Way
Main article: Andromeda's satellite galaxies
Messier 32 is to the left of the center,
Messier 110 is to the
bottom-right of the center.
Like the Milky Way, the
Galaxy 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 the
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.
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.
Collision with the Milky Way
Main article: Andromeda–
Milky Way collision
Galaxy is approaching the
Milky Way at about 110
kilometres per second (68 mi/s). It has been measured
approaching relative to our Sun at around 300 kilometres per second
(190 mi/s) as the Sun orbits around the center of our galaxy
at a speed of approximately 225 kilometres per second (140 mi/s).
This makes the
Galaxy one of about 100 observable
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
Earth and the
Solar System in the event of a collision is
currently unknown. Before the galaxies merge, there is a small chance
Solar System could be ejected from the
Milky Way or join the
Galaxy is bright enough to be seen with the naked eye,
even with some light pollution.
Andromeda is best seen during
autumn nights in the Northern Hemisphere, when from mid-latitudes the
galaxy reaches zenith (its highest point at midnight) so can be seen
almost all night. From the Southern Hemisphere, it is most visible at
the same months, that is in spring, and away from our equator does not
reach a high altitude over the northern horizon, making it difficult
Binoculars can reveal some larger structures and its two
brightest satellite galaxies, M32 and M110. An amateur telescope
can reveal Andromeda's disk, some of its brightest globular clusters,
dark dust lanes and the large star cloud NGC 206.
Galaxy in fiction
Andromeda Nebula in fiction
List of Messier objects
List of galaxies
New General Catalogue
NGC 206 – the brightest star cloud in the
^ a b average(787 ± 18, 770 ± 40, 772 ± 44, 783 ± 25) = ((787 +
770 + 772 + 783) / 4) ± (182 + 402 + 442 + 252)0.5 / 2 = 778 ± 33.
^ a b Blue absolute magnitude of −20.89 –
Color index of 0.63 =
^ J00443799+4129236 is at celestial coordinates R.A.
00h 44m 37.99s, Dec. +41° 29′ 23.6″.
^ Blue absolute magnitude of −21.58 (see reference) – Color index
of 0.63 = absolute visual magnitude of −22.21
^ a b c d e f g h i "Results for Messier 31". NASA/IPAC Extragalactic
Database. NASA/IPAC. Retrieved 2006-11-01.
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Wikimedia Commons has media related to
Galaxy on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α,
X-Ray, Astrophoto, Sky Map, Articles and images
StarDate: M31 Fact Sheet
SEDS Messier pages
Astronomy Picture of the Day
A Giant Globular Cluster in M31 1998 October 17.
Galaxy 2004 July 18.
Universe 2005 December 22.
Universe 2010 January 9.
Andromeda 2010 February 19
M31 and its central Nuclear Spiral
Amateur photography – M31
Globular Clusters in M31 at The Curdridge Observatory
First direct distance to
Andromeda − Astronomy magazine article
Galaxy at SolStation.com
Galaxy at The Encyclopedia of Astrobiology, Astronomy, &
Galaxy at NightSkyInfo.com
Than, Ker (January 23, 2006). "Strange Setup: Andromeda's Satellite
Galaxies All Lined Up". Space.com.
Hubble Finds Mysterious Disk of Blue Stars Around Black Hole Hubble
observations (September 20, 2005) put the mass of the
black hole at 140 million solar masses
M31 (Apparent) Novae Page (IAU)
Andromeda Project (crowd-source)
Gray, Meghan; Szymanek, Nik; Merrifield, Michael. "M31 – Andromeda
Galaxy". Deep Sky Videos. Brady Haran.
Galaxy (M31) at
APOD – 2013 August 1 (M31's angular size compared with full Moon)
Hubble's High-Definition Panoramic View of the
NationalGeographic.com video of the Hubble High-Definition Panoramic
View of the
Catalogue of Nebulae and Clusters of Stars
Herschel 400 Catalogue
New General Catalogue
Revised New General Catalogue
New General Catalogue
New General Catalogue 1 to 499
List of NGC objects
Andromeda subgroup → Local Group →
Virgo Supercluster → Laniakea Supercluster → Observable
universe → Universe
Each → may be read as "within" or "part of".
*It is uncertain whether it is a companion galaxy of the Andromeda
Milky Way collision
Coordinates: 00h 42m 44.3s, +41°