From Wikipedia, the free encyclopedia
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 the Latin Via Lactea
, in turn translated from the Greek
), 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
However, it is unclear how widespread this convention is, and the term Milky Way
is routinely used in either context.
Appearance from Earth
of the galaxy lies in the direction of Sagittarius
, and it is here that Milky Way looks brightest. 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. From Sagittarius, the Milky Way appears to pass westward through the constellations of Scorpius
, Triangulum Australe
, Canis Major
, 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 Milky Way has a relatively low surface brightness
due to the interstellar medium
that fills the galactic disk, and preventing us from seeing the bright galactic center. It is thus difficult to see from any urban
location suffering from light pollution
360-degree photographic panorama of the galaxy.
A panorama of the Milky Way, as seen from Death Valley
The plane of our Milky Way Galaxy, which we see edge-on from our perspective on Earth, cuts a luminous swath across the image. Credit: ESO
The Milky Way arches across this rare 360-degree panorama of the night sky above the Paranal platform
, home of ESO
’s Very Large Telescope
. The image was made from 37 individual frames with a total exposure time of about 30 minutes, taken in the early morning hours. The Moon
is just rising and the zodiacal light shines above it, while the Milky Way stretches across the sky opposite the observatory. Credit: ESO
The Galactic Plane seen by the ATLASGAL survey, divided into sections. Credit ESO
The stellar disk of the Milky Way Galaxy is approximately 100,000 light-years
mi) in diameter, and is considered to be, on average, about 1,000 ly (9×1015
It is estimated to contain at least 200 billion stars
and possibly up to 400 billion stars,
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
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
) the number of stars drops much faster with radius
, 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.
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
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.
Recent measurements by the Very Long Baseline Array
(VLBA) have revealed that the Milky Way is much more massive than some previously thought. The mass of our home galaxy is now considered to be roughly similar to that of our largest local neighbour, the Andromeda Galaxy
. By using the VLBA to measure the apparent shift or parallax
of far-flung star-forming regions when the Earth is on opposite sides of the Sun, the researchers were able to measure the distance to those regions using fewer assumptions than prior efforts. The newer and more accurate estimate of the galaxy's rotational speed (and in turn the amount of dark matter
contained by the galaxy) is about 254 km/s, significantly higher than the widely accepted value of 220 km/s.
This in turn implies that the Milky Way has a total mass of approximately 3 trillion solar masses, about 50% more massive than previously thought.
A green and red Perseid
meteor streaks across the sky just below the Milky Way in August 2007.
As of 2004, the age of the oldest star in the galaxy yet discovered, HE 1523-0901
, is estimated to be about 13.2 billion years, nearly as old as the Universe
This estimate was determined using the UV-Visual Echelle Spectrograph of the Very Large Telescope
content of two stars in globular cluster NGC 6397
The elapsed time between the rise of the first generation of stars in the Milky Way and the first generation of stars in the cluster was deduced to be 200 million to 300 million years. By including the estimated age of the stars in the globular cluster, 13.4 ± 0.8 billion years
, the estimated age of the oldest stars in the Milky Way is 13.6 ± 0.8 billion years
. The Galactic thin disk is estimated to have been formed between 6.5 and 10.1 billion years ago.
Composition and structure
The galactic center in the direction of Sagittarius
. The primary stars of Sagittarius are indicated in red.
The 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.
Astronomers first 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
observations in 2005
which showed the galaxy's central bar to be larger than previously suspected.
Observed structure of the Milky Way's spiral arms. Our Sun
is in the Local Spur.
The galactic disc, which bulges outward at the galactic center, has a diameter of between 70,000 and 100,000 light-years.
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 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.
Observed and extrapolated structure of the spiral arms.
Artist's conception of the spiral structure of the Milky Way with two major stellar arms and a bar.
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.
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.
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.
The galactic disk is surrounded by a spheroid halo
of old stars and globular clusters
, of which 90% lie within 100,000 light-years,
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. 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.
Recent discoveries 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 our own galaxy extending further is apparent, and this is supported by evidence of the newly discovered Outer Arm extension of the Cygnus 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.
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
(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
) from the Galactic Center
The Sun is currently 5–30 parsecs from the central plane of the galactic disc.
The distance between the local arm and the next arm out, the Perseus Arm
, is about 6,500 light-years.
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 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 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
It takes the Solar System about 225–250 million years to complete one orbit of the galaxy (a galactic year
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
Broad infrared view of our Milky Way Galaxy from the Spitzer Space Telescope
created from more than 800,000 frames. This is the most detailed infrared picture of our galaxy to date.
Milky way starscape taken from Paranal
Location of the starscape in relation to the rest of the galaxy.
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 as Omega 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.
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.
Galaxy rotation curve
for the Milky Way. Vertical axis is speed of rotation about the galactic center
. Horizontal axis is distance from the 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 dark matter
or perhaps a modification of the law of gravity
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
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 a supercluster
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.
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
Etymology and beliefs
There are many creation myths
around the world which explain the origin of the Milky Way and give it its name. The English
phrase is a translation from Ancient Greek
, which is derived from the word for milk (γάλα, gala
). This is also the origin of the word galaxy
. In Greek myth
, the Milky Way was caused by milk spilt by Hera
when suckling Heracles
and several other Indo-Aryan languages
, the Milky Way is called Akash Ganga
(आकाशगंगा, Ganges of the heavens
The milky way is held to be sacred in the Hindu scriptures known as the Puranas
, and the Ganges and the milky way are considered to be terrestrial-celestial analogs of each other.
However, the term Kshira
) is also used as an alternative name for the milky way in Hindu texts.
In a large area from Central Asia
, the name for the Milky Way is related to the word for "straw
". This may have originated in ancient Armenian mythology
, (Յարդ զողի Ճանապարհ hard goghi chanaparh
, or "Trail of the Straw Thief"), and been carried abroad by Arabs
In several Uralic
, Turkic languages
, Fenno-Ugric languages
and in the Baltic languages
the Milky Way is called the "Birds' Path" (Linnunrata
), since the route of the migratory birds
appear to follow the Milky Way. (The Qi Xi
legend celebrated in many Asian cultures
references a seasonal bridge across
the Milky Way formed by birds, usually magpies or crows.) The Chinese
name "Silver River" (銀河) is used throughout East Asia
, including Korea
. An alternative name for the Milky Way in ancient China, especially in poems, is "Heavenly Han River
"(天汉). In Japanese
, "Silver River" (銀河 ginga
) means galaxies in general and the Milky Way is called the "Silver River System" (銀河系 gingakei
) or the "River of Heaven" (天の川 Amanokawa or Amanogawa
). In Swedish
, it is called Vintergatan
, or "Winter Avenue", because the stars in the belt were used to predict when winter would arrive.
In some of the Iberian languages
, the Milky Way's name translates as the "Road of Saint James
" (e.g., in Spanish it is sometimes called "El camino de Santiago
The shape of the Milky Way as deduced from star counts by William Herschel in 1785; the Solar System was assumed near center.
Photograph of the "Great Andromeda Nebula" from 1899, later identified as the Andromeda Galaxy
Actual proof of the Milky Way consisting of many stars came in 1610 when Galileo Galilei
used a telescope
to study the Milky Way and discovered that it was 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.
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 Galaxy with the 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. .^ I don't care if it's a Chronicle reward or what, I'd just like to have some planks to make things out of.
- Star Wars Galaxies - Your Star Wars Galaxies Community Resource 6 February 2010 11:33 UTC swg.stratics.com [Source type: General]
In 1917, Heber Curtis
had observed the nova S Andromedae
within the "Great Andromeda
Nebula" (Messier object M31
). 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 our galaxy. 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 place between 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 was an external galaxy, Curtis noted the appearance of dark lanes resembling the dust clouds in the Milky Way, as well as the significant Doppler shift
The matter was conclusively settled by Edwin Hubble
in the early 1920s using a new telescope. He was able to resolve the outer parts of some spiral nebulae as collections of individual stars and identified some Cepheid variables
, thus allowing him to estimate the distance to the nebulae: they were far too distant to be part of the Milky Way.
In 1936, Hubble produced a classification system for galaxies that is used to this day, the Hubble sequence
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- ^ Eisenhauer, F.; Schödel, R.; Genzel, R.; Ott, T.; Tecza, M.; Abuter, R.; Eckart, A.; Alexander, T. (2003). "A Geometric Determination of the Distance to the Galactic Center". The Astrophysical Journal 597: L121–L124. doi:10.1086/380188. http://adsabs.harvard.edu/abs/2003astro.ph..6220E. Retrieved 2007-05-10.
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- The Milky Way Galaxy from An Atlas of the Universe
- A 3D map of the Milky Way Galaxy
- Chromoscope Tools to Explore the known Milky Way
- Milky Way – IRAS (infrared) survey wikisky.org
- Milky Way – H-Alpha survey wikisky.org
- Interactive full screen Silverlight map of the Milky Way
- Running Rings Around the Galaxy Spitzer Space Telescope News
- The Milky Way Galaxy, SEDS Messier pages
- MultiWavelength Milky Way, NASA site with images and VRML models
- Milky Way Explorer, detailed images in infrared with radio, microwave and hydrogen-alpha as well
- Face-on Milky Way maps, within about 10 thousand parsecs
- The Milky Way at the Astro-Photography Site Of Mister T. Yoshida.
- Widefield Image of the Summer Milky Way
- Proposed Ring around the Milky Way
- Milky Way spiral gets an extra arm, New Scientist.com
- Possible New Milky Way Spiral Arm, Sky and Telescope.com
- The Milky Way spiral arms and a possible climate connection
- Galactic center mosaic via sun-orbiting Spitzer infrared telescope
- Milky Way Plan Views, The University of Calgary Radio Astronomy Laboratory
- Our Growing, Breathing Galaxy, Scientific American Magazine (January 2004 Issue)
- Deriving The Shape Of The Galactic Stellar Disc, SkyNightly (March 17, 2006)
- Digital Sky LLC, Digital Sky's Milky Way Panorama and other images
- A new view of the Milky Way galaxy obtained by the Diffuse Infrared Background Experiment (DIRBE) on NASA's Cosmic Background Explorer satellite (COBE).
- Image of Milky Way galaxy arms, Chandra X-ray Observatory Center
- The 1920 Shapley – Curtis Debate on the size of the Milky Way
- Milky Way Voyage – India's First & Largest Star Party
- Astronomy Picture of the Day:
- Moving Milkyway seen from Teneriffe without any lightpollution
- Multi-Gigapixel Infrared Milky Way A zoomable, annotated version of the Spitzer Space Telescope GLIMPSE survey.
- Animated tour of the Milky Way, University of Glamorgan