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Milky Way

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

   The Milky Way (a translation of the Latin Via Lactea, in turn derived
   from the Greek Γαλαξίας (Galaxias), sometimes referred to simply as "
   the Galaxy"), is a barred spiral galaxy of the Local Group. Although
   the Milky Way is but one of billions of galaxies in the universe, the
   Galaxy has special significance to humanity as it is the home of the
   Solar System. Democritus (450 BC–370 BC) was the first known person to
   claim that the Milky Way consists of distant stars.

   The term "milky" originates from the hazy band of white light appearing
   across the celestial sphere visible from Earth, which is comprised of
   stars and other material lying within the galactic plane. The galaxy
   appears brightest in the direction of Sagittarius, towards the galactic
   centre. Relative to the celestial equator, the Milky Way 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 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 main disk of the Milky Way Galaxy is about 80,000 to 100,000
   light-years in diameter, about 250,000 to 300,000 light-years in
   circumference, and outside the Galactic core, about 1,000 light-years
   in thickness. It is composed of 200 to 400 billion stars . As a guide
   to the relative physical scale of the Milky Way, if the galaxy were
   reduced to 130 km (80 mi) in diameter, the solar system would be a mere
   2 mm (0.08 in) in width. The Galactic Halo extends out to 250,000 to
   400,000 light-years in diameter. As detailed in the Structure section
   below, new discoveries indicate that the disk extends much farther than
   previously thought.

   The Milky Way's absolute magnitude, which cannot be measured directly,
   is assumed by astronomical convention to be −20.5, although other
   authors give an absolute magnitude of -21.3.
   360-degree photographic panorama of the entire galaxy, from the
   viewpoint of our solar system.
   Enlarge
   360-degree photographic panorama of the entire galaxy, from the
   viewpoint of our solar system.
   Artist sketch of the Milky Way viewed from Earth on a dark, clear
   night.
   Enlarge
   Artist sketch of the Milky Way viewed from Earth on a dark, clear
   night.
   The galactic center in the direction of Sagittarius. The primary stars
   of Sagittarius are indicated in red.
   Enlarge
   The galactic centre in the direction of Sagittarius. The primary stars
   of Sagittarius are indicated in red.
   The Milky Way in Cygnus
   Enlarge
   The Milky Way in Cygnus

Age

   The age of the Galaxy is currently estimated to be about 13.6 billion
   years, which is nearly as old as the Universe itself.

   This estimate is based upon research performed in 2004 by a team of
   astronomers: Luca Pasquini, Piercarlo Bonifacio, Sofia Randich, Daniele
   Galli, and Raffaele G. Gratton. The team used the UV-Visual Echelle
   Spectrograph of the Very Large Telescope to measure, for the first
   time, the beryllium content of two stars in globular cluster NGC 6397.
   This allowed them to deduce the elapsed time between the rise of the
   first generation of stars in the entire Galaxy and the first generation
   of stars in the cluster, at 200 million to 300 million years. By
   including the estimated age of the stars in the globular cluster (13.4
   ± 0.8 billion years), they estimated the age of the Galaxy at 13.6 ±
   0.8 billion years.

Structure

   NASA artist's conception of the Milky Way Galaxy, viewed along its
   axis.
   Enlarge
   NASA artist's conception of the Milky Way Galaxy, viewed along its
   axis.
   Observed structure of the Milky Way's spiral arms
   Enlarge
   Observed structure of the Milky Way's spiral arms
   Observed and extrapolated structure of the spiral arms (click to see
   legend)
   Enlarge
   Observed and extrapolated structure of the spiral arms (click to see
   legend)

   As of 2005, the Milky Way is thought to comprise a large barred spiral
   galaxy of Hubble type SBbc (loosely wound barred spiral). As of 2006,
   the Milky Way's mass is thought to be about 5.8×10^11 M[☉] comprising
   200 to 400 billion stars. Its integrated absolute visual magnitude has
   been estimated to be -21.3.

   It was only in the 1980s that astronomers began to suspect that the
   Milky Way is a barred spiral rather than an ordinary spiral, which
   observations in 2005 with the Spitzer Space Telescope have since
   confirmed, showing that the galaxy's central bar is larger than
   previously suspected.

   The galactic disk, which bulges outward at the galactic centre, has an
   estimated diameter of about 100,000 light-years. The distance from the
   Sun to the galactic centre is now estimated at 26,000 ± 1400
   light-years while older estimates could put our parent star as far as
   35000 light-years from the central bulge.

   The galactic centre harbors a compact object of very large mass (named
   Sagittarius A*), strongly suspected to be a supermassive black hole.
   Most galaxies are believed to have a supermassive black hole at their
   centre.

   As is typical for many galaxies, the distribution of mass in the Milky
   Way is such that the orbital speed of most stars in the galaxy does not
   depend strongly on its distance from the centre. 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 travelled. This is unlike
   in the solar system where different orbits are also expected to have
   significantly different velocities associated with them, and is one of
   the major pieces of evidence for the existence of dark matter.

   The galaxy's bar is thought to be about 27,000 light-years long,
   running through the center of the galaxy at a 44 ± 10 degree angle to
   the line between our sun and the centre of the galaxy. It is composed
   primarily of red stars, believed to be ancient. 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 and most of the Milky
   Way's star formation activity. Viewed from the Andromeda Galaxy, it
   would be the brightest feature of the Milky Way

   Each spiral arm describes a logarithmic spiral (as do the arms of all
   spiral galaxies) with a pitch of approximately 12 degrees. There are
   believed to be four major spiral arms which all start at the Galaxy's
   centre. These are named as follows, according to the image at right:
     * 2 and 8 - 3k pc and Perseus Arm
     * 3 and 7 - Norma and Cygnus Arm (Along with a newly discovered
       extension - 6)
     * 4 and 10 - Crux and Scutum Arm
     * 5 and 9 - Carina and Sagittarius Arm

   There are at least two smaller arms or spurs, including:
     * 11 - Orion Arm (which contains the solar system and the Sun - 12)

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

   The galactic disk is surrounded by a spheroid halo of old stars and
   globular clusters approximately 250,000 to 400,000 light-years in
   diameter. While the disk contains gas and dust obscuring 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.

   Most of the mass of the Milky Way is thought to be dark matter, forming
   a dark matter halo of an estimated 600-3000 billion solar masses (M[☉])
   which is concentrated towards the Galactic Centre.

   Recent discoveries have given added dimension to our knowledge of the
   structure of the Milky Way. With the discovery that the disc 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 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 Sagittarius and its interaction
   with the Milky Way tears it apart. Similarly, with the discovery of the
   Canis Major Dwarf Galaxy, a ring of galactic debris from its
   interaction with the Milky Way encircles the galactic disk.

   On January 9, 2006 Mario Juric and others of Princeton University
   announced that the Sloan Digital Sky Survey of the northern sky has
   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 our 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 away.

   On May 9, 2006, Daniel Zucker and Vasily Belokurov announced that the
   Sloan Digital Sky Survey has discovered two dwarf galaxies towards the
   constellations Canes Venatici and Boötes.

The Sun's place in the Milky Way

   The Sun (and therefore the Earth and Solar System) may be found close
   to the inner rim of the Orion Arm, in the Local Fluff, at a distance of
   7.94±0.42 kpc from the Galactic Centre. The distance between the local
   arm and the next arm out, the Perseus Arm, is about 6,500 light-years.
   Our Sun, and thus the solar system, is found in what scientists call
   the galactic habitable zone.

   The Apex of the Sun's Way, or the solar apex, refers to 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 86 degrees to the
   direction of the Galactic Centre. 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.

   It takes the solar system about 225-250 million years to complete one
   orbit (a galactic year), and so is thought to have completed about
   20-25 orbits during its lifetime or 0.0008 orbit since the origin of
   humans. The orbital speed of the solar system is 217 km/s, i.e. 1
   light-year in ca. 1400 years, and 1 AU in 8 days.

The Milky Way environment

   NGC 7331 is often referred to as "the Milky Way's twin." This is what
   our galaxy might look like to someone from outside of our home galaxy.
   Enlarge
   NGC 7331 is often referred to as "the Milky Way's twin." This is what
   our galaxy might look like to someone from outside of our home galaxy.

   The Milky Way and the Andromeda Galaxy are a binary system of giant
   spiral galaxies. Together with their companion galaxies they form the
   Local Group, a group of some 50 closely bound galaxies. The Local Group
   is part of the Virgo Supercluster.

   The Milky Way is orbited by two smaller galaxies and a number of dwarf
   galaxies in the Local Group. 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. 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 Milky Way, causing vibrations at certain frequencies
   when they pass through the edges of our Galaxy. 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 Milky
   Way. 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 Milky Way disk with the known
   gas layer. As a result, the model predicts that the gravitational
   impact of the Magellanic Clouds is amplified as they pass through the
   Milky Way.

Speed through space

   In the general sense, the absolute speed 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 galaxy's motion.
   (Motion must always be specified with respect to another object.)

   With this in mind, many astronomers believe the galaxy is moving
   through space at approximately 600km per second relative to the
   observed locations of other nearby galaxies. Most recent estimates
   range from 130 km/s to 1,000 km/s. If indeed the Milky Way is moving at
   600 km per second, we are travelling 51.84 million km per day, or more
   than 18.9 billion km per year. For comparison, this would mean that
   each year, we are travelling about 4.5 times the distance that Pluto
   lies from the Earth (at its closest). The Milky Way is thought to be
   moving in the direction of the constellation Hydra, and may someday
   become a close-knit member of the Virgo cluster of galaxies. Our galaxy
   may also collide with the Andromeda galaxy in roughly 4 billion years.
   See below.

   Another reference frame is provided by the CMB. The Milky Way is moving
   at around 552 km/s with respect to the photons of the cosmic microwave
   background. This can be observed by satellites such as COBE and WMAP as
   a dipole contribution to the CMB, as photons in equilibrium at the CMB
   frame get blue-shifted in the direction of the motion and red-shifted
   in the opposite direction.

Mythology

   There are many creation myths around the world regarding the Milky Way.
   In particular, there are two similar ancient Greek stories that explain
   the etymology of the name Galaxies (Γαλαξίας) and its association with
   milk (γάλα). Some myths associate the constellation with a herd of
   cattle whose milk gives the sky its blue glow. In Eastern Asia, people
   believed that the hazy band of stars was the " Silvery River" of
   Heaven. This is written in hanzi as 銀河.

   Akashaganga is the Indian name for the Milky Way Galaxy, which means
   Ganges River of the Sky.

   According to Greek mythology, the Milky Way was formed by Hera, who
   spilled milk in the sky after discovering that Zeus had tricked her
   into feeding young Heracles. In another variant, Hermes snuck Heracles
   into Olympus to drink from the breasts of Hera who was asleep. Heracles
   bit Hera's nipple shooting her milk into the skies forming the Milky
   Way.

   In Finnish mythology the Milky Way was called pathway of the birds (
   Finnish: Linnunrata; Estonian: Linnutee). The Finns observed that the
   migratory birds used the galaxy as a guideline to travel south, where
   they believed Lintukoto (bird home) resided. Only later the scientists
   indeed confirmed the observation correct; the migratory birds use the
   Milky Way as a guide and dwell the winters in warm, southern lands. The
   Milky Way is even today called Linnunrata in the Finnish language. The
   name in the Baltic languages has the same meaning ( Lithuanian:
   Paukščių Takas; Latvian: Putnu Ceļš.

   In Swedish, the Milky Way is known as Vintergatan (winter street), for
   apparent reasons; it is most visible in the winter in Scandinavia.

   Ancient Armenian mythology called the Milky Way the "Straw Thief's
   Way", relating to one of the gods stealing straw and attempting to flee
   across the heavens in a wooden chariot, spilling some of the straw
   along the way.

   In Hindu mythology the galaxy is called the s'is'umara cakra: the
   dolphin disc. Looking in the night sky one recognized the band of stars
   as the belly of a dolphin.

   In Slovene, the galaxy is called Rimska cesta, literally "The Roman
   Road" (or "The Road to Rome", alluding to the ancient pilgrim path).
   Sometimes, infrequently, it is also called Mlečna cesta, which is a
   literal translation of its Greek/Latin name. i.e. "Milky Way".

   In the Iberian peninsula, the galaxy is known as Road to Santiago
   (Spanish: Camino de Santiago; Portuguese: Estrada de Santiago; Catalan:
   Camí de Santiago), alluding to the ancient pilgrim road to the Galician
   town of Santiago de Compostela, in Spain. Equivalents to Milky Way also
   exist in the three languages (Spanish and Portuguese: Via Láctea;
   Catalan: Via Làctia).

Future of the Milky Way

   Current measurements suggest the Andromeda Galaxy is approaching us at
   300 kilometers per second, and that the Milky Way might collide with it
   in several (3-4) billion years, depending on the importance of unknown
   lateral components to the galaxies' relative motion. If they do
   collide, it is thought that our sun and the other stars of the Milky
   Way will probably not collide with the stars of Andromeda, but that the
   two galaxies will merge to form a single elliptical galaxy over the
   course of about a billion years.
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