Andromeda
The Andromeda Galaxy (/ænˈdrɒmɪdə/), 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
of Andromeda.
The 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 Andromeda 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.
Around the year 964, the Persian astronomer Abd al-Rahman
al-Sufi described the Andromeda Galaxy, in his 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 Andromeda 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 cataloged Andromeda as object M31 and incorrectly credited
Marius as the discoverer despite it being visible to the naked eye. In 1785,
the astronomer William Herschel noted a faint reddish hue in the core region of
Andromeda. He believed Andromeda to be the nearest of all the "great
nebulae", and based on the color and magnitude of the nebula, he
incorrectly guessed that it 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.
In 1864, William Huggins noted that the spectrum of
Andromeda differs 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 Andromeda was considered to be a nearby object, so the cause was
thought to be a much less luminous and unrelated event called a nova, and was
named accordingly; "Nova 1885".
In 1887, Isaac Roberts took the first photographs of
Andromeda, which was still commonly thought to be a nebula within our galaxy.
Roberts mistook Andromeda and similar spiral nebulae as solar systems being
formed.[citation needed] 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).
Island universe
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 independent galaxies.
In 1920, the Great Debate between Harlow Shapley and Curtis
took place, concerning the nature of the Milky Way, spiral nebulae, and the
dimensions of the universe. To support his claim of the Great Andromeda Nebula
being, in fact, an external galaxy, Curtis also noted the appearance of dark
lanes within Andromeda which resembled the dust clouds in our own galaxy, as
well as historical observations of Andromeda Galaxy's significant Doppler
shift. In 1922 Ernst Öpik presented a method to estimate the distance of
Andromeda using the measured velocities of its stars. His result placed the
Andromeda Nebula far outside our galaxy at a distance of about 450,000 parsecs
(1,500,000 ly). 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 Great 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.
In 1943, Walter Baade was the first person to resolve stars
in the central region of the Andromeda Galaxy. Baade identified two distinct
populations of stars based on their metallicity, naming the young,
high-velocity stars in the disk Type I and the older, red stars in the bulge
Type II. This nomenclature was subsequently adopted for stars within the Milky
Way, and elsewhere. (The existence of two distinct populations had been noted
earlier by Jan Oort.) Baade also discovered that there were two types of
Cepheid 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 Andromeda Galaxy was
detected by Hanbury Brown and Cyril Hazard at Jodrell Bank Observatory. The
first radio maps of the galaxy were made in the 1950s by John Baldwin and
collaborators at the Cambridge Radio Astronomy Group. The core of the Andromeda
Galaxy is called 2C 56 in the 2C radio astronomy catalog. In 2009, the first
planet may have been discovered in the Andromeda Galaxy. This was detected
using a technique called microlensing, which is caused by the deflection of
light by a massive object.
General
The estimated distance of the Andromeda Galaxy from our own
was doubled in 1953 when it was discovered that there is another, dimmer type
of Cepheid. In the 1990s, measurements of both standard red giants as well as
red clump stars from the Hipparcos satellite measurements were used to
calibrate the Cepheid distances.
Formation and history
Andromeda Galaxy was formed roughly 10 billion years ago
from the collision and subsequent merger of smaller protogalaxies. This violent
collision formed most of the galaxy's (metal-rich) galactic halo and extended
disk. During this epoch, star formation would have been very high, to the point
of becoming a luminous infrared galaxy for roughly 100 million years. Andromeda
and the Triangulum Galaxy had 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 of Andromeda as well as the
presence there of a relatively young (100 million years old) stellar population.[citation
needed]
Distance estimate
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 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
2.52×106 ± 0.14×106 ly (1.594×1011 ± 8.9×109 AU) and the whole Andromeda Galaxy
at about 2.5×106 ly (1.6×1011 AU). This new value is in excellent agreement
with the previous, independent Cepheid-based distance value.
Averaged together, these distance estimates give a value of
2.54×106 ± 0.11×106 ly (1.606×1011 ± 7.0×109 AU). And, from this, the diameter
of 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
Mass
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.
Andromeda 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. Over 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.
Luminosity
Compared to the Milky Way, the Andromeda 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
Andromeda Galaxy (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 or close).
An estimation done with the help of Spitzer Space Telescope
published in 2010 suggests an absolute magnitude (in the blue) of −20.89 (that
with a color index of +0.63 translates to an absolute visual magnitude of
−21.52, compared to −20.9 for the Milky Way), and a total luminosity in that
wavelength of 3.64×1010 L☉.
The rate of star formation in the Milky Way is much higher,
with Andromeda 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 the Milky Way is experiencing more active
star formation. Should this continue, the luminosity of the Milky Way may
eventually overtake that of Andromeda Galaxy.
According to recent studies, the Andromeda Galaxy lies in
what in the galaxy color–magnitude diagram is known as the "green
valley", a region populated by galaxies like the Milky Way in transition
from the "blue cloud" (galaxies actively forming new stars) to the
"red sequence" (galaxies that lack star formation). Star formation
activity in green valley galaxies is slowing as they run out of star-forming
gas in the interstellar medium. In simulated galaxies with similar properties
to 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 Andromeda
Galaxy and the Milky Way.
Structure
A 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 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 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 in the Andromeda
Galaxy is three times larger in diameter than previously estimated. This
constitutes evidence that there is a vast, extended stellar disk that makes the
galaxy more than 220,000 light-years (67,000 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
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 the
Andromeda Galaxy. The Galaxy M33 could be responsible for some warp in
Andromeda's arms, though more precise distances and radial velocities are
required.
Spectroscopic studies have provided detailed measurements of
the rotational velocity of the Andromeda Galaxy as a function of radial
distance from the core. The rotational velocity has a maximum value of 225
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 Andromeda 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:
Since the Andromeda Galaxy is seen close to edge-on, it is
difficult to study its spiral structure. Rectified images of the galaxy seem to
show a fairly normal spiral galaxy, exhibiting two continuous trailing arms
that are separated from each other by a minimum of about 13,000 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 Infrared
Space Observatory demonstrated that the overall form of the Andromeda Galaxy
may be transitioning into a ring galaxy. The gas and dust within the galaxy is
generally formed into several overlapping rings, with a particularly prominent
ring formed at a radius of 32,000 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 Andromeda
Galaxy is concentrated there.
Later studies with the help of the 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 Andromeda
Galaxy with the same telescope also showed a smaller dust ring that is believed
to have been caused by the interaction with M32 more than 200 million years ago. Simulations show that the
smaller galaxy passed through the disk of the Andromeda Galaxy along the
latter's polar axis. This collision stripped more than half the mass from the
smaller M32 and created the ring structures in Andromeda. It is the
co-existence of the long-known large ring-like feature in the gas of Messier
31, together with this newly discovered inner ring-like structure, offset from
the barycenter, that suggested a nearly head-on collision with the satellite
M32, a milder version of the Cartwheel encounter.
Studies of the extended halo of the Andromeda Galaxy show
that it is roughly comparable to that of the Milky Way, with stars in the halo
being generally "metal-poor", and increasingly so with greater
distance. This evidence indicates that the two galaxies have followed similar
evolutionary paths. They are likely to have accreted and assimilated about
100–200 low-mass galaxies during the past 12
billion years. The stars in the extended halos of the Andromeda Galaxy
and the Milky Way may extend nearly one-third the distance separating the two
galaxies.
Nucleus
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 Space
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.
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 Andromeda Galaxy.
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 Andromeda Galaxy's 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.
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 billion years
In 2005, astronomers discovered a completely new type of
star cluster in the 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 extended clusters.
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}}} {\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 within the
Andromeda Galaxy and the first outside of the Milky Way Galaxy.
Satellites
Like the Milky Way, the Andromeda 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 Andromeda
Galaxy in the past. M32 may once have been a larger galaxy that had its stellar
disk removed by M31, and underwent a sharp increase of star formation in the
core region, which lasted until the relatively recent past.
M110 also appears to be interacting with the Andromeda
Galaxy, and astronomers have found in the halo of the latter a stream of
metal-rich stars that appear to have been stripped from these satellite
galaxies. M110 does contain a dusty lane, which may indicate recent or ongoing
star formation.
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
The Andromeda 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 Andromeda Galaxy one of about 100 observable
blueshifted galaxies. Andromeda Galaxy's tangential or sideways velocity with
respect to the Milky Way is relatively much smaller than the approaching
velocity and therefore it is expected to collide directly with the Milky Way in
about 4 billion years. A likely outcome of the collision is that the galaxies
will merge to form a giant elliptical galaxy or perhaps even a large disc
galaxy. Such events are frequent among the galaxies in galaxy groups. The fate
of the Earth and the Solar System in the event of a collision is currently
unknown. Before the galaxies merge, there is a small chance that the Solar
System could be ejected from the Milky Way or join the Andromeda Galaxy.
Amateur observing
The Andromeda 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 to observe. 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.
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