The Milky Way,
or simply the
Galaxy, is the galaxy in
which the Solar
System is
located. It is a barred
spiral galaxy that
is part of the Local
Group of
galaxies. It is one of billions of galaxies in the observable
universe. Its name is a translation of theLatin Via
Lactea, in turn translated from the Greek Γαλαξίας
(Galaxias), referring to the pale band of light formed by
stars in the galactic plane as seen from Earth (see etymology
of galaxy).

Some sources hold that, strictly speaking, the term Milky
Way should refer
exclusively to the band of light that the galaxy forms in the night
sky, while the galaxy should receive the full name Milky
Way Galaxy, or alternatively the
Galaxy.^[9][10][11] However,
it is unclear how widespread this convention is, and the term Milky
Way is routinely
used in either context.

All the stars that
the eye can distinguish in the night sky are part of the Milky
Way Galaxy, but aside from these relatively nearby stars, the
galaxy appears as a hazy band of white light arching around the
entire celestial
sphere. The light originates from stars and other material
that lie within the galactic plane. Dark regions within the
band, such as the Great
Rift and theCoalsack,
correspond to areas where light from distant stars is blocked by
dark nebulae. The Milky Way has a relatively lowsurface
brightness due to
the interstellar
medium that fills
the galactic disk, which prevents us from seeing the bright
galactic center. It is thus difficult to see from any urban or suburban location
suffering from light
pollution.

The center of
the galaxy lies in the direction of Sagittarius,
and it is here that Milky Way looks brightest. From Sagittarius,
the Milky Way appears to pass westward through the
constellations of Scorpius, Ara, Norma, Triangulum
Australe, Circinus, Centaurus, Musca, Crux, Carina, Vela, Puppis, Canis
Major, Monoceros, Orion and Gemini, Taurus, Auriga,Perseus, Andromeda, Cassiopeia, Cepheus and Lacerta, Cygnus, Vulpecula, Sagitta, Aquila, Ophiuchus, Scutum,
and back to Sagittarius.
The fact that the Milky Way divides the night sky into two
roughly equal hemispheres indicates
that the Solar System lies close to the galactic
plane.

The galactic plane is inclined by about 60 degrees to the ecliptic (the
plane of the Earth’s orbit). Relative to the celestial
equator, it passes as far north as the constellation of Cassiopeiaand
as far south as the constellation of Crux,
indicating the high inclination of Earth’s equatorial
plane and the
plane of the ecliptic relative
to the galactic plane. In the Galactic
Coordinate System (in
which the equator corresponds with the galactic plane), the
north galactic pole is situated at right
ascension 12h
49m, declination +27.4°
(B1950)
near beta
Comae Berenices, and the south galactic pole is near alpha
Sculptoris.

Physical size

The stellar disk of the Milky Way Galaxy is approximately
100,000 light-years (9×10¹⁷ km)
(6×10¹⁷ mi)
in diameter, and is considered to be, on average, about 1,000 ly
(9×10¹⁵ km)
thick.^[1] It
is estimated to contain at least 200 billion stars^[12] and
possibly up to 400 billion stars,^[13]the
exact figure depending on the number of very low-mass stars,
which is highly uncertain. This can be compared to the one
trillion (10¹²) stars of
the neighbouring Andromeda
Galaxy.^[14] The
stellar disc does not have a sharp edge, a radius beyond which
there are no stars. Rather, the number of stars drops smoothly
with distance from the centre of the Galaxy. Beyond a radius of
roughly 40,000 light-years(4×10¹⁷ km)
the number of stars drops much faster with radius^[15],
for reasons that are not understood.

Extending beyond the stellar disk is a much thicker disk of gas.
Recent observations indicate that the gaseous disk of the Milky
Way has a thickness of around 12,000 ly (1×10¹⁷ km)—twice
the previously accepted value.^[16] As
a guide to the relative physical scale of
the Milky Way, if it were reduced to 10m in diameter, the Solar
System, including the hypothesized Oort
cloud, would be no more than 0.1mm in width.

The Galactic
Halo extends
outward, but is limited in size by the orbits of two Milky Way
satellites, the Large and the Small Magellanic
Clouds, whose perigalacticon is
at ~180,000 ly (2×10¹⁸ km).^[17] At
this distance or beyond, the orbits of most halo objects would
be disrupted by the Magellanic Clouds, and the objects would
likely be ejected from the vicinity of the Milky Way.

Age

In 2007, a star in the Galactic halo, HE
1523-0901, was estimated to be about 13.2 billion years old,
nearly as old
as the Universe. As the oldest known object in the Milky Way
at that time, it placed a lower limit on the age of the Milky
Way.^[5] This
estimate was determined using the UV-Visual Echelle Spectrograph
of the Very
Large Telescope to measure the
relative strengths of spectral
lines caused by
the presence ofThorium and
other elements created
by the R-process.
The line strengths yield abundances of different elemental isotopes,
from which an estimate of the age of the star can be derived
using nucleocosmochronology.^[5]

The age of stars in the Galactic thin disk can be estimated in
the same way as HE 1523-0901. Measurements of thin disk stars
yield an estimate that the thin disk formed between 8.8 ± 1.7
billion years ago. This suggest that there was a hiatus of
almost 5 billion years between the formation of the Galactic
halo and the thin disk.^[18]

Composition and structure

he galaxy consists of a bar-shaped core region surrounded by a
disk of gas,
dust and stars
forming four distinct arm structures spiralling outward in a logarithmic
spiral shape (see Spiral
arms). The mass distribution within the galaxy closely
resembles the Sbc Hubble
classification, which is a spiral galaxy with relatively
loosely-wound arms.^[19] Astronomers
first began to suspect that the Milky Way is abarred
spiral galaxy, rather than an ordinary spiral
galaxy, in the 1990s^[20].
Their suspicions were confirmed by the Spitzer
Space Telescopeobservations in 2005^[21] which
showed the galaxy’s central bar to be larger than previously
suspected.

Estimates for the mass of the Milky Way vary, depending upon the
method and data used. Recent estimates at the low end have
placed the mass of the Milky Way at 5.8 × 10¹¹ solar
masses (M_☉),
somewhat smaller than the Andromeda
Galaxy.^[22][23][24] Other
measurements by the Very
Long Baseline Array (VLBA)
have found velocities as large as 254 km/s for stars at the edge
of the Milky Way, higher than the previously accepted value of
220 km/s.^[25] As
the orbital velocity depends on the mass enclosed, this implies
that the Milky Way is more massive, roughly equaling the mass of
Andromeda Galaxy at 7 × 10¹¹ solar
masses (M_☉ within
50 kiloparsecs of
its center.^[26] Most
of the mass of the galaxy is dark
matter, which forms a dark
matter halo that
is spread out relatively uniformly to a distance beyond one
hundred kiloparsecs from the Galactic center. The overall mass
of the entire galaxy is estimated at 600–1000 billion M_☉ ^[24]

This mass in baryonic matter
is estimated to include 200 to 400 billion stars.^[27] Its
integrated absolute
visual magnitude has
been estimated to be −20.9.^[28]

Galactic Center

The galactic disc, which bulges outward at the galactic center,
has a diameter of between 70,000 and 100,000 light-years.^[29] The
distance from the Sun to the galactic center is now estimated at
26,000 ± 1,400 light-years, while older estimates could put the
Sun as far as 35,000 light-years from the central bulge.

The galactic center harbors a compact object of very large mass
as determined by the motion of material around the center.^[30] The
intense radio source named Sagittarius
A*, thought to mark the center of the Milky Way, is newly
confirmed to be a supermassive
black hole. For a photo see Chandra
X-ray Observatory; Jan. 6, 2003. Most galaxies are believed
to have a supermassive black hole at their center.^[31]

The galaxy’s bar is thought to be about 27,000 light-years long,
running through its center at a 44 ± 10 degree angle to the line
between the Sun and the center of the galaxy. It is composed
primarily of red stars, believed to be ancient (see red
dwarf, red
giant). The bar is surrounded by a ring called the “5-kpc ring”
that contains a large fraction of the molecular hydrogen present
in the galaxy, as well as most of the Milky Way’s star
formation activity.
Viewed from the Andromeda
Galaxy, it would be the brightest feature of our own galaxy

Spiral arms

Each spiral arm describes a logarithmic
spiral (as do the
arms of all spiral galaxies) with a pitch of approximately 12
degrees. Until recently, there were believed to be four major
spiral arms which all start near the galaxy’s center. These are
named as follows, according to the image at right:

Observations presented in 2008 by Robert Benjamin of the University
of Wisconsin-Whitewater suggest
that the Milky Way possesses only two major stellar arms: the
Perseus arm and the Scutum-Centaurus arm. The rest of the arms
are minor or adjunct arms.^[33] This
would mean that the Milky Way is similar in appearance to NGC
1365.

Outside of the major spiral arms is the Monoceros
Ring (or Outer
Ring), proposed by astronomers Brian Yanny and Heidi
Jo Newberg, a ring of gas and stars torn from other galaxies
billions of years ago.

As is typical for many galaxies, the distribution of mass in the
Milky Way Galaxy is such that the orbital
speed of most
stars in the galaxy does not depend strongly on its distance
from the center. Away from the central bulge or outer rim, the
typical stellar velocity is between 210 and 240 km/s.^[34] Hence
the orbital
period of the
typical star is directly proportional only to the length of the
path traveled. This is unlike the situation within the Solar
System, where two-body gravitational dynamics dominate and
different orbits are expected to have significantly different
velocities associated with them. This difference is one of the
major pieces of evidence for the existence of dark
matter. Another interesting aspect is the so-called “wind-up
problem” of the spiral arms. If the inner parts of the arms
rotate faster than the outer part, then the galaxy will wind up
so much that the spiral structure will be thinned out. But this
is not what is observed in spiral galaxies; instead, astronomers
propose that the spiral pattern is a density wave emanating from
the galactic center. This can be likened to a moving traffic jam
on a highway—the cars are all moving, but there is always a
region of slow-moving cars. This model also agrees with enhanced
star formation in or near spiral arms; the compressional waves
increase the density of molecular hydrogen and protostars form
as a result.

Halo

The galactic disk is surrounded by a spheroid
halo of old stars
and globular
clusters, of which 90% lie within 100,000 light-years,^[35] suggesting
a stellar halo diameter of 200,000 light-years. However, a few
globular clusters have been found farther, such as PAL 4 and AM1
at more than 200,000 light-years away from the galactic center.
About 40% of these clusters are on retrograde
orbits, which means they move in the opposite direction from
the Milky Way rotation.^[36] The
globular clusters can follow rosette
orbits about the
galaxy, in contrast to theelliptical
orbit of a
planet.^[37]

While the disk contains gas and dust which obscure the view in
some wavelengths, the spheroid component does not. Active star
formation takes
place in the disk (especially in the spiral arms, which
represent areas of high density), but not in the halo. Open
clusters also
occur primarily in the disk.

Discoveries in the early 21st century have added dimension to
the knowledge of the Milky Way’s structure. With the discovery
that the disk of the Andromeda
Galaxy (M31)
extends much further than previously thought,^[38] the
possibility of the disk of the Milky Way galaxy extending
further is apparent, and this is supported by evidence from the
discoverey of the Outer Arm extension of the Cygnus
Arm.^[39] With
the discovery of the Sagittarius
Dwarf Elliptical Galaxy came
the discovery of a ribbon of galactic debris as the polar orbit
of the dwarf and its interaction with the Milky Way tears it
apart. Similarly, with the discovery of the Canis
Major Dwarf Galaxy, it was found that a ring of galactic
debris from its interaction with the Milky Way encircles the
galactic disk.

On January 9, 2006, Mario
Jurić and others
of Princeton
University announced
that the Sloan
Digital Sky Survey of
the northern sky found a huge and diffuse structure (spread out
across an area around 5,000 times the size of a full moon)
within the Milky Way that does not seem to fit within current
models. The collection of stars rises close to perpendicular to
the plane of the spiral arms of the galaxy. The proposed likely
interpretation is that a dwarf
galaxy is merging
with the Milky Way. This galaxy is tentatively named the Virgo
Stellar Stream and
is found in the direction of Virgo about
30,000 light-years away.

Sun’s location and neighborhood

The Sun (and
therefore the Earth and
the Solar
System) may be found close to the inner rim of the galaxy’s Orion
Arm, in the Local
Fluff inside the Local
Bubble, and in the Gould
Belt, at a distance of 7.62±0.32 kpc (~25,000±1,000 ly)
from the Galactic
Center.^{[40][41][42][43][44]} The
Sun is currently 5–30 parsecs from the central plane of the
galactic disc.^[44] The
distance between the local arm and the next arm out, the Perseus
Arm, is about 6,500 light-years.^[45] The
Sun, and thus the Solar System, is found in the galactic
habitable zone.

There are about 208 stars brighter than absolute
magnitude 8.5
within 15 parsecs of
the Sun, giving a density of 0.0147 such stars per cubic parsec,
or 0.000424 per cubic light-year (from List
of nearest bright stars). On the other hand, there are 64
known stars (of any magnitude, not counting 4 brown
dwarfs) within 5 parsecs of the Sun, giving a density of
0.122 stars per cubic parsec, or 0.00352 per cubic light-year
(from List
of nearest stars), illustrating the fact that most stars are
less bright than absolute magnitude 8.5.

The Apex of the Sun’s Way, or the solar
apex, is the direction that the Sun travels through space in
the Milky Way. The general direction of the Sun’s galactic
motion is towards the starVega near
the constellation of Hercules,
at an angle of roughly 60 sky degrees to the direction of the Galactic
Center. The Sun’s orbit around the Galaxy is expected to be
roughly elliptical with the addition of perturbations due to the
galactic spiral arms and non-uniform mass distributions. In
addition, the Sun oscillates up and down relative to the
galactic plane approximately 2.7 times per orbit. This is very
similar to how a simple
harmonic oscillator works
with no drag force (damping) term. These oscillations often
coincide with mass
extinction periods
on Earth; presumably the higher density of stars close to the
galactic plane leads to more impact
events.^[46]

It takes the Solar System about 225–250 million years to
complete one orbit of the galaxy (a galactic
year),^[47] so
it is thought to have completed 20–25 orbits during the lifetime
of the Sun and 1/1250 of a revolution since the origin
of humans. The orbital
speed of the
Solar System about the center of the Galaxy is approximately
220 km/s. At this speed, it takes around 1,400 years for the
Solar System to travel a distance of 1 light-year, or 8 days to
travel 1 AU (astronomical
unit).^[48]

Environment

The Milky Way and the Andromeda
Galaxy are a binary
system of giant
spiral galaxies belonging to a group of 50 closely bound
galaxies known as the Local
Group, itself being part of the Virgo
Supercluster.

Two smaller galaxies and a number of dwarf
galaxies in the
Local Group orbit the
Milky Way. The largest of these is the Large
Magellanic Cloud with
a diameter of 20,000 light-years. It has a close companion, the Small
Magellanic Cloud. The Magellanic
Stream is a
peculiar streamer of neutral hydrogen gas
connecting these two small galaxies. The stream is thought to
have been dragged from the Magellanic Clouds in tidal
interactions with the Milky Way. Some of the dwarf
galaxies orbiting the Milky Way are Canis
Major Dwarf (the
closest),Sagittarius
Dwarf Elliptical Galaxy, Ursa
Minor Dwarf, Sculptor
Dwarf, Sextans
Dwarf, Fornax
Dwarf, and Leo
I Dwarf. The smallest Milky Way dwarf galaxies are only 500
light-years in diameter. These include Carina
Dwarf, Draco
Dwarf, and Leo
II Dwarf. There may still be undetected dwarf galaxies,
which are dynamically bound to the Milky Way, as well as some
that have already been absorbed by the Milky Way, such asOmega
Centauri. Observations through the zone
of avoidance are
frequently detecting new distant and nearby galaxies. Some
galaxies consisting mostly of gas and dust may also have evaded
detection so far.

In January 2006, researchers reported that the heretofore
unexplained warp in the disk of the Milky Way has now been
mapped and found to be a ripple or vibration set up by the Large
and Small Magellanic Clouds as they circle the Galaxy, causing
vibrations at certain frequencies when they pass through its
edges.^[49] Previously,
these two galaxies, at around 2% of the mass of the Milky Way,
were considered too small to influence the Milky Way. However,
by taking into account dark
matter, the movement of these two galaxies creates a wake
that influences the larger Milky Way. Taking dark matter into
account results in an approximately twenty-fold increase in mass
for the Galaxy. This calculation is according to a computer
model made by Martin Weinberg of the University
of Massachusetts, Amherst. In this model, the dark matter is
spreading out from the galactic disc with the known gas layer.
As a result, the model predicts that the gravitational effect of
the Magellanic Clouds is amplified as they pass through the
Galaxy.

Current measurements suggest the Andromeda
Galaxy is
approaching us at 100 to 140 kilometers per second. The Milky
Way may collide with it in 3 to 4 billion years, depending on
the importance of unknown lateral components to the galaxies’
relative motion. If they collide, individual stars within the
galaxies would not collide, but instead the two galaxies will
merge to form a single elliptical
galaxy over the
course of about a billion years.^[50]

Velocity

In the general sense, the absolute velocity of any object
through space is not a meaningful question according to Einstein‘s special
theory of relativity, which declares that there is no
“preferred” inertial
frame of reference in
space with which to compare the object’s motion. (Motion must
always be specified with respect to another object.) This must
be kept in mind when discussing the Galaxy’s motion.

Astronomers believe the Milky Way is moving at approximately
630 km per second relative to the local co-moving frame of
reference that moves with the Hubble
flow.^[54] If
the Galaxy is moving at 600 km/s, Earth travels 51.84 million km
per day, or more than 18.9 billion km per year, about 4.5 times
its closest distance from Pluto.
The Milky Way is thought to be moving in the direction of the Great
Attractor. The Local
Group (a cluster
of gravitationally bound galaxies containing, among others, the
Milky Way and the Andromeda
galaxy) is part of asupercluster called
the Local
Supercluster, centered near the Virgo
Cluster: although they are moving away from each other at
967 km/s as part of the Hubble
flow, the velocity is less than would be expected given the
16.8 million pc distance due to the gravitational attraction
between the Local Group and the Virgo Cluster.^[55]

Another reference frame is provided by the cosmic
microwave background (CMB).
The Milky Way is moving at around 552 km/s^[8] with
respect to the photons of the CMB, toward 10.5 right
ascension, -24° declination (J2000 epoch,
near the center of Hydra).
This motion is observed by satellites such as the Cosmic
Background Explorer (COBE)
and the Wilkinson
Microwave Anisotropy Probe (WMAP)
as a dipole contribution to the CMB, as photons in equilibrium
in the CMB frame get blue-shifted in
the direction of the motion and red-shifted in
the opposite direction.^{[citation
needed]}

The galaxy rotates about its center according to its galaxy
rotation curve as
shown in the figure. The discrepancy between the observed curve
(relatively flat) and the curve based upon the known mass of the
stars and gas in the Milky Way (decaying curve) is attributed to dark
matter.^[56]