Andromeda Galaxy

The Andromeda
, 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 2,500,000 light-years away in the constellation Andromeda.

It is the nearest spiral galaxy to our own, the Milky Way. As it is
visible as a faint smudge on a moonless night, it is one of the farthest
objects visible to the naked eye, and can be seen even from urban areas
with binoculars. It is named after the princess Andromeda in Greek

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. Although the largest, Andromeda 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.

The 2006 observations by the Spitzer Space Telescope revealed that M31
contains one trillion (1012) stars, several times more than the
of stars in our own galaxy, which is estimated to be c. 200-400 billion.

While the 2006 estimates put the mass of the Milky Way to be ~80% of the
mass of Andromeda, which is estimated to be 7.1 X 1011 solar masses, a
2009 study concluded that Andromeda and the Milky Way are about equal in

At an apparent magnitude of 3.4, the Andromeda Galaxy is notable for
being one of the brightest Messier objects, making it easily visible to
the naked eye even when viewed from areas with moderate light pollution.
Although it appears more than six times as wide as the full moon when
photographed through a larger telescope, only the brighter central
region is visible with the naked eye.


The earliest recorded observation of the Andromeda Galaxy was in 964 CE
by the Persian astronomer, Abd al-Rahman al-Sufi (Azophi), who described
it as a "small cloud" in his Book of Fixed Stars. Other 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 in 1612.

Charles Messier catalogued it as object M31 in 1764 and incorrectly
credited 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 M31. He believed it to be the nearest of all the
"great nebulae" and, based on the color and magnitude of the nebula, he
incorrectly guessed that it was no more than 2,000 times the distance of

William Huggins in 1864 observed the spectrum of M31 and noted that it
differed from a gaseous nebula. The spectra of M31 displayed a continuum
of frequencies, superimposed with dark absorption lines that help
identify the chemical composition of an object. The Andromeda nebula was
very similar to the spectra of individual stars, and 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. At the time M31 was
considered to be a nearby object, so the cause was thought to be a much
less luminous and unrelated event called a nova, and was named
accordingly "Nova 1885".

The first photographs of M31 were taken in 1887 by Isaac Roberts from
his private observatory in Sussex, England. 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 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. The result was the largest velocity recorded at that time,
at 300 kilometres per second (190 mi/s), moving in the direction of the

Island Universe

In 1917, Heber Curtis 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 (3.2X1010 AU). He became a
proponent of the so-called "island universes" hypothesis, which held
that spiral nebulae were actually independent galaxies.

In 1920 the Great Debate between Harlow Shapley and Heber Curtis took
place, concerning the nature of the Milky Way, spiral nebulae, and the
dimensions of the universe. 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.

In 1922 Ernst Opik presented a very elegant and simple astrophysical
method to estimate the distance of M31, his result (450 kpc (1,500 kly))
put Andromeda Nebula far outside our Galaxy.

Edwin Hubble settled the debate in 1925 when he identified
extragalactic Cepheid variable stars for the first time on astronomical
photos of M31. These were made using the 2.5 metres (98 in) Hooker
telescope, and they enabled the distance of Great Andromeda Nebula to be
determined. His measurement demonstrated conclusively that this feature
was not a cluster of stars and gas within our Galaxy, but an entirely
separate galaxy located a significant distance from our own.

Andromeda plays an important role in galactic studies, since it is the
nearest spiral galaxy (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
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.) 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.

Radio emission from the Andromeda Galaxy was first detected by Grote
Reber in 1940. The first radio maps of the galaxy were made in the 1950s
by John Baldwin and collaborators at the Cambridge Radio Astronomy
Group. The core of the Andromeda Galaxy is called 2C 56 in the 2C radio
astronomy catalogue.

In 2009, the first planet may have been discovered in the Andromeda
Galaxy. This candidate was detected using a technique called
microlensing, which is caused by the deflection of light by a massive


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
almost directly 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 in
Andromeda is three times larger in diameter than estimated.
This constitutes 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 Andromeda’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 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. 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 M31 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 mi/s) at a radius of 1,300 light-years
(82,000,000 AU) light-years, then descends to a minimum at 7,000
light-years (440,000,000 AU) where the rotation velocity may be as low
as 50 kilometres per second (31 mi/s).

Thereafter the velocity steadily climbs again 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 x 109 M in the nucleus. The total mass of
the galaxy increases linearly out to 45,000 light-years (2.8 x109 AU),
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

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 (8.2E+8 AU). These can be followed outward from a
distance of roughly 1,600 light-years (100,000,000 AU) 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.

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
(2.0×109 AU) 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 latter’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 billion years. The stars in the
extended halos of M31 and the Milky Way may extend nearly one third the
distance separating the two galaxies.


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. 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 3-5×107 M black hole.

Scott Tremaine has 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 was the remnant of a small galaxy
"cannibalized" by Andromeda, this is no longer considered to be a
viable explanation. The primary reason is that 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.

Artist’s concept of Andromeda galaxy core showing a view across a

mysterious disk of young, blue stars encircling a supermassive black

Discrete Sources

Multiple X-ray sources have been detected in the Andromeda Galaxy, using
observations from the ESA’s XMM-Newton orbiting observatory. 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
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 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.

Future Collision of the Milky Way with Andromeda

The Andromeda Galaxy is approaching the Sun at about 100 to 140
kilometres per second (62 to 87 mi/s),[56] so it is one of the few blue
shifted galaxies. The Andromeda Galaxy and the Milky Way are thus
expected to collide in perhaps 2.5 billion years, although the
are uncertain since Andromeda’s tangential velocity with respect to the
Milky Way is only known to within about a factor of two.

A likely outcome of the collision is that the galaxies will merge to
form a giant elliptical galaxy. Such are frequent among the
galaxies in galaxy groups. The fate of the Earth and the Solar System in
the event of a collision are presently unknown. If the galaxies do not
merge, there is a small chance that the Solar System could be ejected
from the Milky Way or join Andromeda.

Satellite Galaxies
Like the Milky Way, Andromeda Galaxy has satellite galaxies, consisting
of 14 known dwarf galaxies.