Milky Way is the galaxy[nb 1] that contains our Solar
System. The descriptive "milky" is derived from the appearance
Earth of the galaxy – a band of light seen in the night sky
formed from stars that cannot be individually distinguished by the
naked eye. The term
Milky Way is a translation of the
lactea, from the Greek γαλαξίας κύκλος (galaxías
kýklos, "milky circle"). From Earth, the Milky Way
appears as a band because its disk-shaped structure is viewed from
Galileo Galilei first resolved the band of light into
individual stars with his telescope in 1610. Until the early 1920s,
most astronomers thought that the
Milky Way contained all the stars in
the Universe. Following the 1920 Great Debate between the
Harlow Shapley and Heber Curtis, observations by Edwin
Hubble showed that the
Milky Way is just one of many galaxies.
Milky Way is a barred spiral galaxy with a diameter between
100,000 and 180,000 light-years (ly). The
Milky Way is
estimated to contain 100–400 billion stars. There are
probably at least 100 billion planets in the Milky Way. The
Solar System is located within the disk, about 26,000 light-years from
the Galactic Center, on the inner edge of the Orion Arm, one of the
spiral-shaped concentrations of gas and dust. The stars in the inner
≈10,000 light-years form a bulge and one or more bars that radiate
from the bulge. The very center is marked by an intense radio source,
named Sagittarius A*, which is likely to be a supermassive black hole.
Stars and gases at a wide range of distances from the Galactic Center
orbit at approximately 220 kilometers per second. The constant
rotation speed contradicts the laws of Keplerian dynamics and suggests
that much of the mass of the
Milky Way does not emit or absorb
electromagnetic radiation. This mass has been termed "dark
matter". The rotational period is about 240 million years at the
position of the Sun. The
Milky Way as a whole is moving at a
velocity of approximately 600 km per second with respect to
extragalactic frames of reference. The oldest stars in the Milky Way
are nearly as old as the
Universe itself and thus probably formed
shortly after the Dark Ages of the Big Bang.
Milky Way has several satellite galaxies and is part of the Local
Group of galaxies, which is a component of the Virgo Supercluster,
which is itself a component of the Laniakea Supercluster.
2 Size and mass
4.1 Galactic quadrants
4.2 Galactic Center
4.3 Spiral arms
4.4.1 Gaseous halo
4.5 Sun’s location and neighborhood
4.6 Galactic rotation
5.1 Age and cosmological history
8 Etymology and mythology
9 Astronomical history
10 See also
13 Further reading
14 External links
A view of the
Milky Way toward the constellation Sagittarius
(including the Galactic Center) as seen from an area not polluted by
light (the Black Rock Desert, Nevada). The bright object on the right
is Jupiter, just above Antares.
This time-lapse video captures the
Milky Way circling over ALMA.
Milky Way is visible from
Earth as a hazy band of white light some
30 degrees wide arcing across the sky. Although all the individual
naked-eye stars in the entire sky are part of the Milky Way,
the light in this band originates from the accumulation of unresolved
stars and other material located in the direction of the galactic
plane. Dark regions within the band, such as the Great Rift and the
Coalsack, are areas where light from distant stars is blocked by
interstellar dust. The area of the sky obscured by the
Milky Way is
called the Zone of Avoidance.
Milky Way has a relatively low surface brightness. Its visibility
can be greatly reduced by background light such as light pollution or
stray light from the Moon. The sky needs to be darker than about 20.2
magnitude per square arcsecond in order for the
Milky Way to be
seen. It should be visible when the limiting magnitude is
approximately +5.1 or better and shows a great deal of detail at
+6.1. This makes the
Milky Way difficult to see from any brightly
lit urban or suburban location, but very prominent when viewed from a
rural area when the
Moon is below the horizon.[nb 2] "The new world
atlas of artificial night sky brightness" shows that more than
one-third of Earth's population cannot see the
Milky Way from their
homes due to light pollution.
As viewed from Earth, the visible region of the Milky Way's Galactic
plane occupies an area of the sky that includes 30 constellations.
The center of the
Galaxy lies in the direction of the constellation
Sagittarius; it is here that the
Milky Way is brightest. From
Sagittarius, the hazy band of white light appears to pass around to
Galactic anticenter in Auriga. The band then continues the rest of
the way around the sky, back to Sagittarius. The band divides the
night sky into two roughly equal hemispheres.
Galactic plane is inclined by about 60 degrees to the ecliptic
(the plane of Earth's orbit). Relative to the celestial equator, it
passes as far north as the constellation of Cassiopeia and 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. The north Galactic pole is situated at right
ascension 12h 49m, declination +27.4° (B1950) near β Comae
Berenices, and the south Galactic pole is near α Sculptoris. Because
of this high inclination, depending on the time of night and year, the
arc of the
Milky Way may appear relatively low or relatively high in
the sky. For observers from approximately 65 degrees north to 65
degrees south on Earth's surface, the
Milky Way passes directly
overhead twice a day.
Milky Way arching at a high inclination across the night sky
(fish-eye mosaic shot at Paranal, Chile). The bright object is Jupiter
in the constellation Sagittarius, and the
Magellanic Clouds can be
seen on the left. Galactic north is downwards.
Size and mass
A photograph of galaxy UGC 12158, which is thought to resemble the
Milky Way in appearance.
Milky Way is the second-largest galaxy in the Local Group, with
its stellar disk approximately 100,000 ly (30 kpc) in
diameter, and, on average, approximately 1,000 ly (0.3 kpc)
thick. As a guide to the relative physical scale of the Milky
Way, if the
Solar System out to
Neptune were the size of a US quarter
(24.3 mm (0.955 in)), the
Milky Way would be approximately
the size of the continental United States. A ring-like filament of
stars wrapping around the
Milky Way may belong to the Milky Way
itself, rippling above and below the relatively flat galactic
plane. If so, that would mean a diameter of 150,000–180,000
light-years (46–55 kpc).
Schematic profile of the Milky Way.
Estimates of the mass of the
Milky Way vary, depending upon the method
and data used. At the low end of the estimate range, the mass of the
Milky Way is 5.8×1011 solar masses (M☉), somewhat less than
that of the Andromeda Galaxy. Measurements using the Very
Long Baseline Array in 2009 found velocities as large as 254 km/s
(570,000 mph) for stars at the outer edge of the Milky Way.
Because the orbital velocity depends on the total mass inside the
orbital radius, this suggests that the
Milky Way is more massive,
roughly equaling the mass of Andromeda
Galaxy at 7×1011 M☉
within 160,000 ly (49 kpc) of its center. In 2010, a
measurement of the radial velocity of halo stars found that the mass
enclosed within 80 kiloparsecs is 7×1011 M☉. According to
a study published in 2014, the mass of the entire
Milky Way is
estimated to be 8.5×1011 M☉, which is about half the mass
of the Andromeda Galaxy.
Much of the mass of the
Milky Way appears to be dark matter, an
unknown and invisible form of matter that interacts gravitationally
with ordinary matter. A dark matter halo is spread out relatively
uniformly to a distance beyond one hundred kiloparsecs (kpc) from the
Galactic Center. Mathematical models of the
Milky Way suggest that the
mass of dark matter is 1–1.5×1012 M☉. Recent
studies indicate a range in mass, as large as 4.5×1012 M☉ 
and as small as 8×1011 M☉.
The total mass of all the stars in the
Milky Way is estimated to be
between 4.6×1010 M☉ and 6.43×1010 M☉. In
addition to the stars, there is also interstellar gas, comprising 90%
hydrogen and 10% helium by mass, with two thirds of the hydrogen
found in the atomic form and the remaining one-third as molecular
hydrogen. The mass of this gas is equal to between 10% and
15% of the total mass of the galaxy's stars. Interstellar dust
accounts for an additional 1% of the total mass of the gas.
This article or section appears to contradict itself. Please see the
talk page for more information. (April 2018)
Further information: Exoplanet
Milky Way contains between 200 and 400 billion stars
and at least 100 billion planets. The exact figure depends on
the number of very-low-mass stars, which are hard to detect,
especially at distances of more than 300 ly (90 pc) from the
Sun. As a comparison, the neighboring Andromeda
Galaxy contains an
estimated one trillion (1012) stars. The
Milky Way may also
contain perhaps ten billion white dwarfs, a billion neutron stars, and
a hundred million black holes.[nb 3] Filling the space
between the stars is a disk of gas and dust called the interstellar
medium. This disk has at least a comparable extent in radius to the
stars, whereas the thickness of the gas layer ranges from hundreds
of light years for the colder gas to thousands of light years for
The disk of stars in the
Milky Way does not have a sharp edge beyond
which there are no stars. Rather, the concentration of stars decreases
with distance from the center of the Milky Way. For reasons that are
not understood, beyond a radius of roughly 40,000 ly (13 kpc) from the
center, the number of stars per cubic parsec drops much faster with
radius. Surrounding the galactic disk is a spherical Galactic Halo
of stars and globular clusters that extends further outward but is
limited in size by the orbits of two
Milky Way satellites, the Large
and Small Magellanic Clouds, whose closest approach to the Galactic
Center is about 180,000 ly (55 kpc). At this distance or
beyond, the orbits of most halo objects would be disrupted by the
Magellanic Clouds. Hence, such objects would probably be ejected from
the vicinity of the Milky Way. The integrated absolute visual
magnitude of the
Milky Way is estimated to be around
Both gravitational microlensing and planetary transit observations
indicate that there may be at least as many planets bound to stars as
there are stars in the Milky Way, and microlensing
measurements indicate that there are more rogue planets not bound to
host stars than there are stars. The
Milky Way contains at
least one planet per star, resulting in 100–400 billion planets,
according to a January 2013 study of the five-planet star system
Kepler-32 with the Kepler space observatory. A different January
2013 analysis of Kepler data estimated that at least 17 billion
Earth-sized exoplanets reside in the Milky Way. On November 4,
2013, astronomers reported, based on Kepler space mission data, that
there could be as many as 40 billion Earth-sized planets orbiting in
the habitable zones of Sun-like stars and red dwarfs within the Milky
Way. 11 billion of these estimated planets may be orbiting
Sun-like stars. The nearest such planet may be 4.2 light-years
away, according to a 2016 study. Such Earth-sized planets may be
more numerous than gas giants. Besides exoplanets, "exocomets",
comets beyond the Solar System, have also been detected and may be
common in the Milky Way.
360-degree panorama view of the
Milky Way (an assembled mosaic of
photographs) by ESO. The galactic centre is in the middle of the view,
with galactic north up.
An artist's impression that shows how the
Milky Way would look from
very different perspectives than from Earth. From some angles, the
central bulge shows up as a peanut-shaped glowing ball of stars, and
from above, the central narrow bar appears clearly. The many spiral
arms and their associated dust clouds are also clearly seen.
Artist's conception of the spiral structure of the
Milky Way with two
major stellar arms and a bar
Spitzer reveals what cannot be seen in visible light: cooler stars
(blue), heated dust (reddish hue), and Sgr A* as bright white spot in
X-ray flares from Sagittarius A*, location of the supermassive
black hole at the center of the Milky Way.
Milky Way consists of a bar-shaped core region surrounded by a
disk of gas, dust and stars. The mass distribution within the Milky
Way closely resembles the type Sbc in the Hubble classification, which
represents spiral galaxies with relatively loosely wound arms.
Astronomers began to suspect that the
Milky Way is a barred spiral
galaxy, rather than an ordinary spiral galaxy, in the 1990s. Their
suspicions were confirmed by the
Spitzer Space Telescope
Spitzer Space Telescope observations
in 2005 that showed the Milky Way's central bar to be larger than
Main article: Galactic quadrant
A galactic quadrant, or quadrant of the Milky Way, refers to one of
four circular sectors in the division of the Milky Way. In actual
astronomical practice, the delineation of the galactic quadrants is
based upon the galactic coordinate system, which places the
Sun as the
origin of the mapping system.
Quadrants are described using ordinals—for example, "1st galactic
quadrant", "second galactic quadrant", or "third quadrant of
the Milky Way". Viewing from the north galactic pole with 0
degrees (°) as the ray that runs starting from the
Sun and through
the Galactic Center, the quadrants are as follows:
1st galactic quadrant – 0° ≤ longitude (ℓ) ≤ 90°
2nd galactic quadrant – 90° ≤ ℓ ≤ 180°
3rd galactic quadrant – 180° ≤ ℓ ≤ 270°
4th galactic quadrant – 270° ≤ ℓ ≤ 360° (0°)
Main article: Galactic Center
Sun is 25,000–28,000 ly (7.7–8.6 kpc) from the
Galactic Center. This value is estimated using geometric-based methods
or by measuring selected astronomical objects that serve as standard
candles, with different techniques yielding various values within this
approximate range. In the inner few kpc
(around 10,000 light-years radius) is a dense concentration of mostly
old stars in a roughly spheroidal shape called the bulge. It has
been proposed that the
Milky Way lacks a bulge formed due to a
collision and merger between previous galaxies, and that instead it
has a pseudobulge formed by its central bar.
Galactic Center is marked by an intense radio source named
Sagittarius A* (pronounced Sagittarius A-star). The motion of material
around the center indicates that
Sagittarius A* harbors a massive,
compact object. This concentration of mass is best explained as a
supermassive black hole[nb 4] (SMBH) with an estimated mass of
4.1–4.5 million times the mass of the Sun. The rate of accretion
of the SMBH is consistent with an inactive galactic nucleus, being
estimated at around
6995100000000000000♠1×10−5 M☉ y−1. Observations
indicate that there are SMBH located near the center of most normal
The nature of the Milky Way's bar is actively debated, with estimates
for its half-length and orientation spanning from 1 to 5 kpc
(3,000–16,000 ly) and 10–50 degrees relative to the line of
Earth to the Galactic Center. Certain authors
advocate that the
Milky Way features two distinct bars, one nestled
within the other. However, RR Lyrae variables do not trace a
prominent Galactic bar. The bar may be surrounded by a
ring called the "5-kpc ring" that contains a large fraction of the
molecular hydrogen present in the Milky Way, as well as most of the
Milky Way's star formation activity. Viewed from the Andromeda Galaxy,
it would be the brightest feature of the Milky Way. X-ray
emission from the core is aligned with the massive stars surrounding
the central bar and the Galactic ridge.
Illustration of the two gigantic X-ray/gamma-ray bubbles (blue-violet)
Milky Way (center)
In 2010, two gigantic spherical bubbles of high energy emission were
detected to the north and the south of the
Milky Way core, using data
from the Fermi
Gamma-ray Space Telescope. The diameter of each of the
bubbles is about 25,000 light-years (7.7 kpc); they stretch up to
Grus and to Virgo on the night-sky of the southern
hemisphere. Subsequently, observations with the Parkes
Telescope at radio frequencies identified polarized emission that is
associated with the Fermi bubbles. These observations are best
interpreted as a magnetized outflow driven by star formation in the
central 640 ly (200 pc) of the Milky Way.
Later, on January 5, 2015,
NASA reported observing an
X-ray flare 400
times brighter than usual, a record-breaker, from Sagittarius A*. The
unusual event may have been caused by the breaking apart of an
asteroid falling into the black hole or by the entanglement of
magnetic field lines within gas flowing into Sagittarius A*.
Further information: Spiral galaxy
Outside the gravitational influence of the Galactic bars, the
structure of the interstellar medium and stars in the disk of the
Milky Way is organized into four spiral arms. Spiral arms
typically contain a higher density of interstellar gas and dust than
the Galactic average as well as a greater concentration of star
formation, as traced by H II regions and molecular
The Milky Way's spiral structure is uncertain, and there is currently
no consensus on the nature of the Milky Way's spiral arms. Perfect
logarithmic spiral patterns only crudely describe features near the
Sun, because galaxies commonly have arms that branch, merge,
twist unexpectedly, and feature a degree of
irregularity. The possible scenario of the
Sun within a
spur / Local arm emphasizes that point and indicates that such
features are probably not unique, and exist elsewhere in the Milky
Way. Estimates of the pitch angle of the arms range from about
7° to 25°. There are thought to be four spiral arms that
all start near the Milky Way's center. These are named as
follows, with the positions of the arms shown in the image at right:
Observed (normal lines) and extrapolated (dotted lines) structure of
the spiral arms. The gray lines radiating from the Sun's position
(upper center) list the three-letter abbreviations of the
3-kpc Arm (
Near 3 kpc Arm
Near 3 kpc Arm and Far 3 kpc Arm) and Perseus Arm
Outer arm (Along with extension discovered in 2004)
There are at least two smaller arms or spurs, including:
Orion–Cygnus Arm (which contains the
Sun and Solar System)
Two spiral arms, the Scutum–
Centaurus arm and the
Carina–Sagittarius arm, have tangent points inside the Sun's orbit
about the center of the Milky Way. If these arms contain an
overdensity of stars compared to the average density of stars in the
Galactic disk, it would be detectable by counting the stars near the
tangent point. Two surveys of near-infrared light, which is sensitive
primarily to red giants and not affected by dust extinction, detected
the predicted overabundance in the Scutum–
Centaurus arm but not in
the Carina–Sagittarius arm: the
Scutum-Centaurus Arm contains
approximately 30% more red giants than would be expected in the
absence of a spiral arm. This observation suggests that the
Milky Way possesses only two major stellar arms: the Perseus arm and
Centaurus arm. The rest of the arms contain excess gas
but not excess old stars. In December 2013, astronomers found that
the distribution of young stars and star-forming regions matches the
four-arm spiral description of the Milky Way. Thus, the
Milky Way appears to have two spiral arms as traced by old stars and
four spiral arms as traced by gas and young stars. The explanation for
this apparent discrepancy is unclear.
Clusters detected by WISE used to trace the Milky Way's spiral arms
Near 3 kpc Arm
Near 3 kpc Arm (also called Expanding 3 kpc Arm or simply 3 kpc
Arm) was discovered in the 1950s by astronomer van Woerden and
collaborators through 21-centimeter radio measurements of HI (atomic
hydrogen). It was found to be expanding away from the
central bulge at more than 50 km/s. It is located in the fourth
galactic quadrant at a distance of about 5.2 kpc from the
Sun and 3.3
kpc from the Galactic Center. The
Far 3 kpc Arm
Far 3 kpc Arm was discovered in 2008
by astronomer Tom Dame (Harvard-Smithsonian CfA). It is located in the
first galactic quadrant at a distance of 3 kpc (about 10,000 ly) from
the Galactic Center.
A simulation published in 2011 suggested that the
Milky Way may have
obtained its spiral arm structure as a result of repeated collisions
with the Sagittarius Dwarf Elliptical Galaxy.
It has been suggested that the
Milky Way contains two different spiral
patterns: an inner one, formed by the Sagittarius arm, that rotates
fast and an outer one, formed by the Carina and Perseus arms, whose
rotation velocity is slower and whose arms are tightly wound. In this
scenario, suggested by numerical simulations of the dynamics of the
different spiral arms, the outer pattern would form an outer
pseudoring, and the two patterns would be connected by the Cygnus
The long filamentary molecular cloud dubbed "Nessie" probably forms a
dense "spine" of the Scutum–Centarus Arm
Outside of the major spiral arms is the
Monoceros Ring (or Outer
Ring), a ring of gas and stars torn from other galaxies billions of
years ago. However, several members of the scientific community
recently restated their position affirming the
Monoceros structure is
nothing more than an over-density produced by the flared and warped
thick disk of the Milky Way.
The Galactic disk is surrounded by a spheroidal halo of old stars and
globular clusters, of which 90% lie within 100,000 light-years
(30 kpc) of the Galactic Center. However, a few globular
clusters have been found farther, such as PAL 4 and AM1 at more than
200,000 light-years from the Galactic Center. About 40% of the Milky
Way's clusters are on retrograde orbits, which means they move in the
opposite direction from the
Milky Way rotation. The globular
clusters can follow rosette orbits about the Milky Way, in contrast to
the elliptical orbit of a planet around a star.
Although the disk contains dust that obscures the view in some
wavelengths, the halo component does not. Active star formation takes
place in the disk (especially in the spiral arms, which represent
areas of high density), but does not take place in the halo, as there
is little gas cool enough to collapse into stars. Open clusters
are also located 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, the possibility of the disk of the Milky Way
extending further is apparent, and this is supported by evidence from
the discovery of the Outer Arm extension of the Cygnus Arm
and of a similar extension of the Scutum-
Centaurus Arm. 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.
Sloan Digital Sky Survey
Sloan Digital Sky Survey of the northern sky shows 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 Milky Way. 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
(9 kpc) away.
In addition to the stellar halo, the Chandra
XMM-Newton, and Suzaku have provided evidence that there is a gaseous
halo with a large amount of hot gas. The halo extends for hundreds of
thousand of light years, much further than the stellar halo and close
to the distance of the Large and Small Magellanic Clouds. The mass of
this hot halo is nearly equivalent to the mass of the Milky Way
itself. The temperature of this halo gas is between 1
and 2.5 million K (1.8 and 4.5 million oF).
Observations of distant galaxies indicate that the
Universe had about
one-sixth as much baryonic (ordinary) matter as dark matter when it
was just a few billion years old. However, only about half of those
baryons are accounted for in the modern
Universe based on observations
of nearby galaxies like the Milky Way. If the finding that the
mass of the halo is comparable to the mass of the
Milky Way is
confirmed, it could be the identity of the missing baryons around the
Sun’s location and neighborhood
Diagram of the Sun’s location in the Milky Way. The angles represent
longitudes in the galactic coordinate system.
Diagram of the stars in the Solar neighborhood
Sun is near the inner rim of the Orion Arm, within the Local Fluff
of the Local Bubble, and in the Gould Belt, at a distance of
26.4 ± 1.0 kly
(8.09 ± 0.31 kpc) from the Galactic
Sun is currently 5–30 parsecs (16–98 ly) from the
central plane of the Galactic disk. The distance between the
local arm and the next arm out, the Perseus Arm, is about 2,000
parsecs (6,500 ly). The Sun, and thus the Solar System, is
located in the Milky Way's galactic habitable zone.
There are about 208 stars brighter than absolute magnitude 8.5 within
a sphere with a radius of 15 parsecs (49 ly) from the Sun, giving
a density of one star per 69 cubic parsecs, or one star per 2,360
cubic light-years (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 (16 ly) of the Sun, giving a density of
about one star per 8.2 cubic parsecs, or one per 284 cubic light-years
(from List of nearest stars). This illustrates the fact that there are
far more faint stars than bright stars: in the entire sky, there are
about 500 stars brighter than apparent magnitude 4 but 15.5 million
stars brighter than apparent magnitude 14.
The apex of the Sun's way, or the solar apex, is the direction that
Sun travels through space in the Milky Way. The general direction
of the Sun's Galactic motion is towards the star
Vega near the
constellation of Hercules, at an angle of roughly 60 sky degrees to
the direction of the Galactic Center. The Sun's orbit about the Milky
Way 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 passes through 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 were until recently thought to coincide with mass
lifeform extinction periods on Earth. However, a reanalysis of
the effects of the Sun's transit through the spiral structure based on
CO data has failed to find a correlation.
It takes the
Solar System about 240 million years to complete one
orbit of the
Milky Way (a galactic year), so the
Sun is thought to
have completed 18–20 orbits during its lifetime and 1/1250 of a
revolution since the origin of humans. The orbital speed of the Solar
System about the center of the
Milky Way is approximately
220 km/s (490,000 mph) or 0.073% of the speed of light. The
Sun moves through the heliosphere at 84,000 km/h
(52,000 mph). 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). The
Solar System is headed in the
direction of the zodiacal constellation Scorpius, which follows the
Galaxy rotation curve for the Milky Way. Vertical axis is speed of
rotation about the Galactic Center. Horizontal axis is distance from
Galactic Center in kpcs. The
Sun is marked with a yellow ball. The
observed curve of speed of rotation is blue. The predicted curve based
upon stellar mass and gas in the
Milky Way is red. Scatter in
observations roughly indicated by gray bars. The difference is due to
The stars and gas in the
Milky Way rotate about its center
differentially, meaning that the rotation period varies with location.
As is typical for spiral galaxies, the orbital speed of most stars in
Milky Way does not depend strongly on their distance from the
center. Away from the central bulge or outer rim, the typical stellar
orbital speed is between 210 and 240 km/s (470,000 and
540,000 mph). 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 have
significantly different velocities associated with them. The rotation
curve (shown in the figure) describes this rotation. Toward the center
Milky Way the orbit speeds are too low, whereas beyond 7 kpcs
the speeds are too high to match what would be expected from the
universal law of gravitation.
Milky Way contained only the mass observed in stars, gas, and
other baryonic (ordinary) matter, the rotation speed would decrease
with distance from the center. However, the observed curve is
relatively flat, indicating that there is additional mass that cannot
be detected directly with electromagnetic radiation. This
inconsistency is attributed to dark matter. The rotation curve of
Milky Way agrees with the universal rotation curve of spiral
galaxies, the best evidence for the existence of dark matter in
galaxies. Alternatively, a minority of astronomers propose that a
modification of the law of gravity may explain the observed rotation
view • discuss • edit
Earliest universe (−13.80)
Omega Centauri forms
Milky Way Galaxy
spiral arms form
Alpha Centauri forms
Earliest sexual reproduction
Axis scale: billion years
Human timeline and Life timeline
Galaxy formation and evolution
Milky Way began as one or several small overdensities in the mass
distribution in the
Universe shortly after the Big Bang. Some of
these overdensities were the seeds of globular clusters in which the
oldest remaining stars in what is now the
Milky Way formed. Nearly
half the matter in the
Milky Way may have come from other distant
galaxies. Nonetheless, these stars and clusters now comprise the
stellar halo of the Milky Way. Within a few billion years of the birth
of the first stars, the mass of the
Milky Way was large enough so that
it was spinning relatively quickly. Due to conservation of angular
momentum, this led the gaseous interstellar medium to collapse from a
roughly spheroidal shape to a disk. Therefore, later generations of
stars formed in this spiral disk. Most younger stars, including the
Sun, are observed to be in the disk.
Since the first stars began to form, the
Milky Way has grown through
both galaxy mergers (particularly early in the Milky Way's growth) and
accretion of gas directly from the Galactic halo. The Milky Way
is currently accreting material from two of its nearest satellite
galaxies, the Large and Small Magellanic Clouds, through the
Magellanic Stream. Direct accretion of gas is observed in
high-velocity clouds like the Smith Cloud. However,
properties of the
Milky Way such as stellar mass, angular momentum,
and metallicity in its outermost regions suggest it has undergone no
mergers with large galaxies in the last 10 billion years. This lack of
recent major mergers is unusual among similar spiral galaxies; its
neighbour the Andromeda
Galaxy appears to have a more typical history
shaped by more recent mergers with relatively large
According to recent studies, the
Milky Way as well as the Andromeda
Galaxy lie in what in the galaxy color–magnitude diagram is known as
the "green valley", a region populated by galaxies 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, star formation will typically have been extinguished
within about five billion years from now, even accounting for the
expected, short-term increase in the rate of star formation due to the
collision between both the
Milky Way and the Andromeda Galaxy. In
fact, measurements of other galaxies similar to the
Milky Way suggest
it is among the reddest and brightest spiral galaxies that are still
forming new stars and it is just slightly bluer than the bluest red
Age and cosmological history
Night sky from a hypothetical planet within the
Milky Way 10 billion
Globular clusters are among the oldest objects in the Milky Way, which
thus set a lower limit on the age of the Milky Way. The ages of
individual stars in the
Milky Way can be estimated by measuring the
abundance of long-lived radioactive elements such as thorium-232 and
uranium-238, then comparing the results to estimates of their original
abundance, a technique called nucleocosmochronology. These yield
values of about 12.5 ± 3 billion years for CS 31082-001 and 13.8
± 4 billion years for BD +17° 3248. Once a white dwarf is
formed, it begins to undergo radiative cooling and the surface
temperature steadily drops. By measuring the temperatures of the
coolest of these white dwarfs and comparing them to their expected
initial temperature, an age estimate can be made. With this technique,
the age of the globular cluster M4 was estimated as 12.7 ± 0.7
billion years. Age estimates of the oldest of these clusters gives a
best fit estimate of 12.6 billion years, and a 95% confidence
upper limit of 16 billion years.
Several individual stars have been found in the Milky Way's halo with
measured ages very close to the 13.80-billion-year age of the
Universe. In 2007, a star in the galactic halo, HE 1523-0901, was
estimated to be about 13.2 billion years old. As the oldest known
object in the
Milky Way at that time, this measurement placed a lower
limit on the age of the Milky Way. This estimate was made using
the UV-Visual Echelle Spectrograph of the
Very Large Telescope
Very Large Telescope to
measure the relative strengths of spectral lines caused by the
presence of thorium 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. Another star, HD 140283, is 14.5 ± 0.7
billion years old.
The age of stars in the galactic thin disk has also been estimated
using nucleocosmochronology. Measurements of thin disk stars yield an
estimate that the thin disk formed 8.8 ± 1.7 billion years ago. These
measurements suggest there was a hiatus of almost 5 billion years
between the formation of the galactic halo and the thin disk.
Recent analysis of the chemical signatures of thousands of stars
suggests that stellar formation might have dropped by an order of
magnitude at the time of disk formation, 10 to 8 billion years ago,
when interstellar gas was too hot to form new stars at the same rate
The satellite galaxies surrounding the Milky way are not randomly
distributed, but seemed to be the result of a break-up of some larger
system producing a ring structure 500,000 light years in diameter and
50,000 light years wide. Close encounters between galaxies, like
that expected in 4 billion years with the Andromeda
Galaxy rips off
huge tails of gas, which, over time can coalesce to form dwarf
galaxies in a ring at right angles to the main disc.
Diagram of the galaxies in the
Local Group relative to the Milky Way
The position of the
Local Group within the Virgo Supercluster
Main article: Local Group
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, surrounded by a Local Void, itself being
part of the Virgo Supercluster. Surrounding the
Virgo Supercluster are
a number of voids, devoid of many galaxies, the Microscopium Void to
the "north", the Sculptor Void to the "left", the
Bootes Void to the
"right" and the Canes-Major Void to the South. These voids change
shape over time, creating filamentous structures of galaxies. The
Virgo Supercluster, for instance, is being drawn towards the Great
Attractor, which in turn forms part of a greater structure,
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 14,000 light-years. It has a close companion,
the Small Magellanic Cloud. The
Magellanic Stream is a stream of
neutral hydrogen gas extending from these two small galaxies across
100° of the sky. 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 dwarf galaxies of the
Milky Way are only 500 light-years in
diameter. These include Carina Dwarf, Draco Dwarf, and Leo II Dwarf.
There may still be undetected dwarf galaxies that are dynamically
bound to the Milky Way, which is supported by the detection of nine
new satellites of the
Milky Way in a relatively small patch of the
night sky in 2015. There are also some dwarf galaxies that have
already been absorbed by the Milky Way, such as Omega Centauri.
In 2014 researchers reported that most satellite galaxies of the Milky
Way actually lie in a very large disk and orbit in the same
direction. This came as a surprise: according to standard
cosmology, the satellite galaxies should form in dark matter halos,
and they should be widely distributed and moving in random directions.
This discrepancy is still not fully explained.
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 orbit the Milky Way, causing vibrations when they pass through
its edges. Previously, these two galaxies, at around 2% of the mass of
the Milky Way, were considered too small to influence the Milky Way.
However, in a computer model, the movement of these two galaxies
creates a dark matter wake that amplifies their influence on the
larger Milky Way.
Current measurements suggest the Andromeda
Galaxy is approaching us at
100 to 140 km/s (220,000 to 310,000 mph). In 3 to 4 billion
years, there may be an Andromeda–
Milky Way collision, depending on
the importance of unknown lateral components to the galaxies' relative
motion. If they collide, the chance of individual stars colliding with
each other is extremely low, but instead the two galaxies will merge
to form a single elliptical galaxy or perhaps a large disk galaxy
over the course of about a billion years.
Although special relativity states that there is no "preferred"
inertial frame of reference in space with which to compare the Milky
Milky Way does have a velocity with respect to cosmological
frames of reference.
One such frame of reference is the Hubble flow, the apparent motions
of galaxy clusters due to the expansion of space. Individual galaxies,
including the Milky Way, have peculiar velocities relative to the
average flow. Thus, to compare the
Milky Way to the Hubble flow, one
must consider a volume large enough so that the expansion of the
Universe dominates over local, random motions. A large enough volume
means that the mean motion of galaxies within this volume is equal to
the Hubble flow. Astronomers believe the
Milky Way is moving at
approximately 630 km/s (1,400,000 mph) with respect to this
local co-moving frame of reference. The
Milky Way is moving in
the general direction of the
Great Attractor and other galaxy
clusters, including the Shapley supercluster, behind it. The
Local Group (a cluster of gravitationally bound galaxies containing,
among others, the
Milky Way and the Andromeda Galaxy) is part of a
supercluster called the Local Supercluster, centered near the Virgo
Cluster: although they are moving away from each other at
967 km/s (2,160,000 mph) as part of the Hubble flow, this
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.
Another reference frame is provided by the cosmic microwave background
Milky Way is moving at 552 ± 6 km/s
(1,235,000 ± 13,000 mph) 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
Cosmic Background Explorer (COBE) and the
Wilkinson Microwave Anisotropy Probe
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
Etymology and mythology
The Origin of the
Milky Way (c. 1575–1580) by Tintoretto
List of names for the Milky Way and Milky Way
In the Babylonian epic poem Enûma Eliš, the
Milky Way is created
from the severed tail of the primeval salt water dragoness Tiamat, set
in the sky by Marduk, the Babylonian national god, after slaying
her. This story was once thought to have been based on an
older Sumerian version in which
Tiamat is instead slain by
Nippur, but is now though to be purely an invention of
Babylonian propagandists with the intention to show
Marduk as superior
to the Sumerian deities.
Dôn (literally "The Court of Dôn") is the traditional Welsh
name for the constellation Cassiopeia. At least three of Dôn's
children also have astronomical associations: Caer
fortress of Gwydion") is the traditional Welsh name for the Milky Way,
Arianrhod ("The Fortress of Arianrhod") being the
constellation of Corona Borealis.
In western culture, the name "Milky Way" is derived from its
appearance as a dim un-resolved "milky" glowing band arching across
the night sky. The term is a translation of the Classical
lactea, in turn derived from the
Hellenistic Greek γαλαξίας,
short for γαλαξίας κύκλος (galaxías kýklos, "milky
Ancient Greek γαλαξίας (galaxias) – from root
γαλακτ-, γάλα ("milk") + -ίας (forming adjectives) – is
also the root of "galaxy", the name for our, and later all such,
collections of stars.
In Greek mythology, the
Milky Way was formed after the trickster god
Hermes suckled the infant
Heracles at the breast of Hera, the queen of
the gods, while she was asleep. When
Hera awoke, she tore
Heracles away from her breast and splattered her breast milk across
the heavens. In another version of the story, Athena, the
patron goddess of heroes, tricked
Hera into suckling Heracles
voluntarily, but he bit her nipple so hard that she flung
him away, spraying milk everywhere.
The Milky Way, or "milk circle", was just one of 11 "circles" the
Greeks identified in the sky, others being the zodiac, the meridian,
the horizon, the equator, the tropics of Cancer and Capricorn, Arctic
and Antarctic circles, and two colure circles passing through both
Galaxy § Observation history
The shape of the
Milky Way as deduced from star counts by William
Herschel in 1785; the
Solar System was assumed near center
Meteorologica (DK 59 A80),
Aristotle (384–322 BC) wrote that the
Anaxagoras (c. 500–428 BC) and Democritus
(460–370 BC) proposed that the
Milky Way might consist of distant
Aristotle himself believed the
Milky Way to be
caused by "the ignition of the fiery exhalation of some stars which
were large, numerous and close together" and that the "ignition
takes place in the upper part of the atmosphere, in the region of the
world which is continuous with the heavenly motions." The
Olympiodorus the Younger (c. 495–570
A.D.) criticized this view, arguing that if the
Milky Way were
sublunary, it should appear different at different times and places on
Earth, and that it should have parallax, which it does not. In his
Milky Way is celestial. This idea would be influential later
in the Islamic world.
The Persian astronomer
Abū Rayhān al-Bīrūnī
Abū Rayhān al-Bīrūnī (973–1048) proposed
Milky Way is "a collection of countless fragments of the
nature of nebulous stars". The Andalusian astronomer Avempace (d
1138) proposed the
Milky Way to be made up of many stars but appears
to be a continuous image due to the effect of refraction in Earth's
atmosphere, citing his observation of a conjunction of Jupiter and
Mars in 1106 or 1107 as evidence. Ibn Qayyim Al-Jawziyya
(1292–1350) proposed that the
Milky Way is "a myriad of tiny stars
packed together in the sphere of the fixed stars" and that these stars
are larger than planets.
According to Jamil Ragep, the Persian astronomer Naṣīr al-Dīn
al-Ṭūsī (1201–1274) in his Tadhkira writes: "The Milky Way, i.e.
the Galaxy, is made up of a very large number of small, tightly
clustered stars, which, on account of their concentration and
smallness, seem to be cloudy patches. Because of this, it was likened
to milk in color."
Actual proof of the
Milky Way consisting of many stars came in 1610
Galileo Galilei used a telescope to study the
Milky Way and
discovered that it is composed of a huge number of faint
stars. In a treatise in 1755, Immanuel Kant, drawing on
earlier work by Thomas Wright, speculated (correctly) that the
Milky Way might be a rotating body of a huge number of stars, held
together by gravitational forces akin to the
Solar System but on much
larger scales. The resulting disk of stars would be seen as a
band on the sky from our perspective inside the disk. Kant also
conjectured that some of the nebulae visible in the night sky might be
separate "galaxies" themselves, similar to our own. Kant referred to
Milky Way and the "extragalactic nebulae" as "island
universes", a term still current up to the 1930s.
The first attempt to describe the shape of the
Milky Way and the
position of the
Sun within it was carried out by
William Herschel in
1785 by carefully counting the number of stars in different regions of
the visible sky. He produced a diagram of the shape of the Milky Way
Solar System close to the center.
In 1845, Lord Rosse constructed a new telescope and was able to
distinguish between elliptical and spiral-shaped nebulae. He also
managed to make out individual point sources in some of these nebulae,
lending credence to Kant's earlier conjecture.
Photograph of the "Great Andromeda Nebula" from 1899, later identified
as the Andromeda Galaxy
Heber Curtis had observed the nova
S Andromedae within the
Messier object 31). Searching the photographic
record, he found 11 more novae. Curtis noticed that these novae were,
on average, 10 magnitudes fainter than those that occurred within the
Milky Way. As a result, he was able to come up with a distance
estimate of 150,000 parsecs. He became a proponent of the "island
universes" hypothesis, which held that the spiral nebulae were
actually independent galaxies. In 1920 the Great Debate took
Harlow Shapley and Heber Curtis, concerning the nature
of the Milky Way, spiral nebulae, and the dimensions of the Universe.
To support his claim that the Great Andromeda
Nebula is an external
galaxy, Curtis noted the appearance of dark lanes resembling the dust
clouds in the Milky Way, as well as the significant Doppler
The controversy was conclusively settled by
Edwin Hubble in the early
1920s using the Mount Wilson observatory 2.5 m (100 in)
Hooker telescope. With the light-gathering power of this new
telescope, he was able to produce astronomical photographs that
resolved the outer parts of some spiral nebulae as collections of
individual stars. He was also able to identify some Cepheid variables
that he could use as a benchmark to estimate the distance to the
nebulae. He found that the Andromeda
Nebula is 275,000 parsecs from
the Sun, far too distant to be part of the Milky Way.
The ESA spacecraft Gaia provides distance estimates by determining the
parallax of a billion stars and is mapping the
Milky Way with four
planned releases of maps in 2022.
List of galaxies
^ Jay M. Pasachoff in his textbook Astronomy: From the
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Milky Way Galaxy. See:
Pasachoff, Jay M. (1994). Astronomy: From the
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Milky Way is very
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Webster, trans., Book 1, Part 8, pp. 39–40 Archived April 11, 2016,
at the Wayback Machine. : "(2) Anaxagoras, Democritus, and their
schools say that the milky way is the light of certain stars."
Aristotle with Ross, 1931), p. 41: Archived April 11, 2016, at the
Wayback Machine. "For it is natural to suppose that, if the motion of
a single star excites a flame, that of all the stars should have a
similar result, and especially in that region in which the stars are
biggest and most numerous and nearest to one another."
Aristotle with Ross, 1931), p. 43 : Archived April 11, 2016,
at the Wayback Machine. "We have now explained the phenomena that
occur in that part of the terrestrial world which is continuous with
the motions of the heavens, namely, shooting-stars and the burning
flame, comets and the milky way, these being the chief affections that
appear in that region."
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^ Ragep, Jamil (1993). Nasir al-Din al-Tusi’s Memoir on Astronomy
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^ Thomas Wright, An Original Theory or New Hypothesis of the Universe
… (London, England: H. Chapelle, 1750).
On page 57 Archived November 20, 2016, at the Wayback Machine., Wright
stated that despite their mutual gravitational attraction, the stars
in the constellations don't collide because they are in orbit, so
centrifugal force keeps them separated: " … centrifugal force, which
not only preserves them in their orbits, but prevents them from
rushing all together, by the common universal law of gravity, … "
On page 48 Archived November 20, 2016, at the Wayback Machine., Wright
stated that the form of the
Milky Way is a ring: " … the stars are
not infinitely dispersed and distributed in a promiscuous manner
throughout all the mundane space, without order or design, … this
phænomenon [is] no other than a certain effect arising from the
observer's situation, … To a spectator placed in an indefinite
space, … it [i.e. the
Milky Way (Via Lactea)] [is] a vast ring of
stars … "
On page 65 Archived November 20, 2016, at the Wayback Machine., Wright
speculated that the central body of the Milky Way, around which the
rest of the galaxy revolves, might not be visible to us: " ... the
central body A, being supposed as incognitum [i.e. an unknown],
without [i.e. outside of] the finite view; ... "
On page 73 Archived November 20, 2016, at the Wayback Machine., Wright
Milky Way the Vortex Magnus (the great whirlpool) and
estimated its diameter to be 8.64×1012 miles (13.9×1012 km).
On page 33 Archived November 20, 2016, at the Wayback Machine., Wright
speculated that there are a vast number of inhabited planets in the
galaxy: " … ; therefore we may justly suppose, that so many
radiant bodies [i.e. stars] were not created barely to enlighten an
infinite void, but to … display an infinite shapeless universe,
crowded with myriads of glorious worlds, all variously revolving round
them; and … with an inconceivable variety of beings and states,
animate … "
^ Immanuel Kant, Allgemeine Naturgeschichte und Theorie des Himmels
… Archived November 20, 2016, at the Wayback Machine. [Universal
Natural History and Theory of Heaven … ], (Koenigsberg and Leipzig,
(Germany): Johann Friederich Petersen, 1755). On pages 2–3, Kant
acknowledged his debt to Thomas Wright: "Dem Herrn Wright von Durham,
einen Engeländer, war es vorbehalten, einen glücklichen Schritt zu
einer Bemerkung zu thun, welche von ihm selber zu keiner gar zu
tüchtigen Absicht gebraucht zu seyn scheinet, und deren nützliche
Anwendung er nicht genugsam beobachtet hat. Er betrachtete die
Fixsterne nicht als ein ungeordnetes und ohne Absicht zerstreutes
Gewimmel, sondern er fand eine systematische Verfassung im Ganzen, und
eine allgemeine Beziehung dieser Gestirne gegen einen Hauptplan der
Raume, die sie einnehmen." (To Mr. Wright of Durham, an Englishman, it
was reserved to take a happy step towards an observation, which
seemed, to him and to no one else, to be needed for a clever idea, the
exploitation of which he hasn't studied sufficiently. He regarded the
fixed stars not as a disorganized swarm that was scattered without a
design; rather, he found a systematic shape in the whole, and a
general relation between these stars and the principal plane of the
space that they occupy.)
^ Kant (1755), pages xxxiii–xxxvi of the Preface (Vorrede): Archived
November 20, 2016, at the Wayback Machine. "Ich betrachtete die Art
neblichter Sterne, deren Herr von Maupertuis in der Abhandlung von der
Figur der Gestirne gedenket, und die die Figur von mehr oder weniger
offenen Ellipsen vorstellen, und versicherte mich leicht, daß sie
nichts anders als eine Häufung vieler Fixsterne seyn können. Die
jederzeit abgemessene Rundung dieser Figuren belehrte mich, daß hier
ein unbegreiflich zahlreiches Sternenheer, und zwar um einen
gemeinschaftlichen Mittelpunkt, müste geordnet seyn, weil sonst ihre
freye Stellungen gegen einander, wohl irreguläre Gestalten, aber
nicht abgemessene Figuren vorstellen würden. Ich sahe auch ein: daß
sie in dem System, darinn sie sich vereinigt befinden, vornemlich auf
eine Fläche beschränkt seyn müßten, weil sie nicht zirkelrunde,
sondern elliptische Figuren abbilden, und daß sie wegen ihres blassen
Lichts unbegreiflich weit von uns abstehen." (I considered the type of
nebulous stars, which Mr. de Maupertuis considered in his treatise on
the shape of stars, and which present the figures of more or less open
ellipses, and I readily assured myself, that they could be nothing
else than a cluster of fixed stars. That these figures always measured
round informed me that here an inconceivably numerous host of stars,
[which were clustered] around a common center, must be orderly,
because otherwise their free positions among each other would probably
present irregular forms, not measurable figures. I also realized: that
in the system in which they find themselves bound, they must be
restricted primarily to a plane, because they display not circular,
but elliptical figures, and that on account of their faint light, they
are located inconceivably far from us.)
^ Evans, J. C. (November 24, 1998). "Our Galaxy". George Mason
University. Archived from the original on June 30, 2012. Retrieved
January 4, 2007.
^ The term Weltinsel (island universe) appears nowhere in Kant's book
of 1755. The term first appeared in 1850, in the third volume of von
Humboldt's Kosmos: Alexander von Humboldt, Kosmos, vol. 3 (Stuttgart
& Tübingen, (Germany): J.G. Cotta, 1850), pages 187, 189. From
page 187: Archived November 20, 2016, at the Wayback Machine. "Thomas
Wright von Durham, Kant, Lambert und zuerst auch William Herschel
waren geneigt die Gestalt der Milchstraße und die scheinbare
Anhäufung der Sterne in derselben als eine Folge der abgeplatteten
Gestalt und ungleichen Dimensionen der Weltinsel (Sternschict) zu
betrachten, in welche unser Sonnensystem eingeschlossen ist." (Thomas
Wright of Durham, Kant, Lambert and first of all also William Herschel
were inclined to regard the shape of the
Milky Way and the apparent
clustering of stars in it as a consequence of the oblate shape and
unequal dimensions of the world island (star stratum), in which our
solar system is included.)
In the English translation—Alexander von Humboldt with E.C. Otté,
trans., Cosmos … (New York City: Harper & Brothers, 1897), vols.
3–5—see page 147.
William Herschel (1785) "On the Construction of the Heavens,"
Philosophical Transactions of the Royal Society of London, 75 :
213–266. Herschel's diagram of the
Milky Way appears immediately
after the article's last page. See:
Google Books Archived November 20, 2016, at the Wayback Machine.
The Royal Society of London Archived April 6, 2016, at the Wayback
^ Abbey, Lenny. "The Earl of Rosse and the Leviathan of Parsontown".
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^ Weaver, Harold F. "Robert Julius Trumpler". National Academy of
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^ Sandage, Allan (1989). "Edwin Hubble, 1889–1953". Journal of the
Royal Astronomical Society of Canada. 83 (6).
^ Hubble, E. P. (1929). "A spiral nebula as a stellar system, Messier
31". The Astrophysical Journal. 69: 103–158.
Milky Way Map Is a Spectacular Billion-
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^ "Gaia > Gaia DR1". www.cosmos.esa.int. Archived from the original
on September 15, 2016. Retrieved September 15, 2016.
Thorsten Dambeck in Sky and Telescope, "Gaia's Mission to the Milky
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Cristina Chiappini, The Formation and Evolution of the Milky Way,
American Scientist, November/December 2001, pp. 506–515
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The Milky Way
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"Cosmic View" (1957 essay)
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