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Milky Way
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Milky Way
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Image:Milky Way 2005.jpg right|thumb|300px|A [[NASA artist's conception of what the Milky Way would look like if seen from its axis.]]
The '''Milky Way''' (a translation of the
Latin ''Via Lactea'', in turn derived from the
Greek language Greek ''Γαλαξίας'' (Galaxias), sometimes referred to simply as "the Galaxy"), is a
barred spiral galaxy which forms part of the
Local Group. Although the Milky Way is but
Hubble Deep Field 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 comprises
stars and other material lying within the
Galactic coordinate system galactic plane. The galaxy appears brightest in the direction of
Sagittarius, towards the
galactic center.
Relative to the
celestial equator, the Milky Way passes as far north as the
constellation of
Cassiopeia (constellation) Cassiopeia and as far south as the constellation of
Crux, indicating the high inclination of Earth's
equator 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
Sphere hemispheres indicates that the solar system lies close to the galactic plane.
The Milky Way Galaxy is about 80,000 to 100,000
light years in
diameter, about 3,000 light years in thickness, and about 250-300 thousand light years in
circumference. It is composed of 200 to 400 billion
star stars (exact number not yet known). As a guide to the relative physical
scale (ratio) 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 Milky Way's
absolute magnitude, which cannot be measured directly, is assumed by astronomical convention to be −20.5.
Age
The age of the Galaxy is currently estimated to be about 13.6
billion (10
9)
years, which is nearly as
Age of the universe old as the Universe itself.
[[http://www.eso.org/outreach/press-rel/pr-2004/pr-20-04.html 17 August 2004] - Press release, European Southern Observatory]
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
Image:Milky_Way_Spiral_Arms.png 250px|right|thumb|Observed structure of the Milky Way's spiral arms
As of 2005, the Milky Way is thought to comprise a large
barred spiral galaxy of
Hubble sequence Hubble type SBbc (loosely wound barred spiral) with a total
mass of about 600-3000
billion solar masses (M
☉)
[[http://hypertextbook.com/facts/2000/AlinaVayntrub.shtml The Physics Factbook] - entry citing references about the mass of the Milky Way. URL accessed March 16, 2006.][[http://arxiv.org/abs/astro-ph/0506102 The radial velocity dispersion profile of the Galactic halo: Constraining the density profile of the dark halo of the Milky Way], Battagli et al. 2005, MNRAS, 364 (2005) 433], comprising about 100 billion stars.
[[http://hypertextbook.com/facts/2000/MarissaWager..html The Physics Factbook] - entry citing references about the number of stars in the Milky Way. URL accessed March 16, 2006.]
It was only in the
1980s that astronomers began to suspect that the Milky Way is a barred spiral rather than an ordinary
spiral galaxy 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.
[[http://www.newscientistspace.com/article.ns?id=dn7854 16 August 2005] - New Scientist article]
The galactic disk has an estimated diameter of about 100,000
light-years (see
1 E20 m for a list of comparable distances). The distance from the
Sun to the galactic center is estimated at about 26,000 light-years. The disk bulges outward at the center.
The galactic center harbours a compact object of very large mass, strongly suspected to be a
supermassive black hole. Most galaxies are believed to have a supermassive black hole at their center.
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 center. Away from the central bulge or outer rim, the typical stellar velocity is between 210 and 240 km/s.
[[http://zebu.uoregon.edu/~imamura/123/lecture-2/mass.html http://zebu.uoregon.edu/~imamura/123/lecture-2/mass.html]] 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.
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 center of the galaxy. It is composed primarily of red stars, believed to be ancient.
Image:Milky Way Arms-Hypothetical.png 250px|right|thumb|Observed and extrapolated structure of the spiral arms 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 center. These are named as follows, according to the image at right:
*2 and 8 - 3k
parsec pc and
Perseus Arm
*3 and 7 -
Norma Arm Norma and
Cygnus Arm (Along with a newly discovered extension - 6)
*4 and 10 -
Scutum-Crux Arm Crux and
Scutum-Crux Arm Scutum Arm
*5 and 9 -
Sagittarius Arm Carina and
Sagittarius Arm 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
Galactic spheroid spheroid halo of old stars and
globular clusters. 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 in
dark matter, forming a
dark matter halo of an estimated 600-3000 billion
solar masses (M
☉) which is concentrated towards the Galactic Centre.
[filler]
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,
[[http://###.lanl.gov/abs/astro-ph/0504164 6 April 2005] - Ibata, R. et al, Astrophys. Journal, 634 (2005) 287-313] the possibility of the disk of the Milky Way extending further is a clear possibility and is supported by evidence of the newly discovered
Cygnus Arm Outer Arm extension of the
Cygnus Arm.
[[http://www.solstation.com/x-objects/gal-ring.htm http://www.solstation.com/x-objects/gal-ring.htm]] 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.
Image:Milky Way Galaxy center Chandra.jpg center|thumb|500px|X-ray mosaic image of Milky Way taken by [[Chandra X-ray Observatory]]
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
parsec kpc from the
Galactic Center.
[Reid, M. J. (1993), [http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1993ARA%26A..31..345R&db_key=AST&data_type=HTML&format= "The distance to the center of the Galaxy"]. Annual Review of Astronomy and Astrophysics, Vol. 31, p. 345-372.][Eisenhauer, F., et al (2003), [http://arxiv.org/abs/astro-ph/0306220 "A Geometric Determination of the Distance to the Galactic Center"] Astrophys.J. 597 L121-L124.][Horrobin, M. et al (2004), [http://www.mpe.mpg.de/SPIFFI/preprints/first_result_an1.pdf "First results from SPIFFI. I: The Galactic Center"] (PDF). Astronomische Nachrichten, Vol. 325, p. 120-123.] The distance between the local arm and the next arm out, the
Perseus Arm, is about
1 E19 m 6,500 light-years.
[[http://www.ras.ucalgary.ca/CGPS/press/aas00/pr/pr_14012000/pr_14012000map1.html 14 January 2000] - Press release, Canadian Galactic Plan Survey] 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 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. We are presently about 1/8 of an orbit before
perigalacticon (the sun's closest approach to the center) {{citeneeded}}.
It takes the
solar system about 225-250 million years to complete one orbit (a ''galactic year''),
[[http://hypertextbook.com/facts/2002/StacyLeong.shtml http://hypertextbook.com/facts/2002/StacyLeong.shtml]] and so is thought to have completed about 20-25 orbits during its lifetime. The
orbital speed of the solar system is 217 km/s, i.e. 1 light-year in ca. 1400 years, and 1
Astronomical unit AU in 8 days.
The
Hayden Planetarium uses 8.0 kpc in their interactive 3D Milky Way Atlas, which just includes the Galactic Center.
The galactic neighborhood
The Milky Way, the
Andromeda Galaxy and the
Triangulum Galaxy are the major members of the
Local Group, a group of some 35 closely bound galaxies; The Local Group is part of the
Virgo Supercluster.
The Milky Way is orbited by a number of
dwarf galaxy dwarf galaxies in the Local Group. The largest of these is the
Large Magellanic Cloud with a diameter of 20,000 light years. The smallest,
Carina Dwarf,
Draco Dwarf, and
Leo II (dwarf galaxy) Leo II Dwarf are only 500 light years in diameter. The other dwarfs orbiting our galaxy are the
Small Magellanic Cloud;
Canis Major Dwarf, the closest;
Sagittarius Dwarf Elliptical Galaxy, previously thought to be the closest;
Ursa Minor Dwarf;
Sculptor Dwarf,
Sextans Dwarf,
Fornax Dwarf, and
Leo I Dwarf.
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
Albert Einstein Einstein's
Special Theory of Relativity, which declares that there is no "preferred"
inertial frame of reference in space to compare the galaxy's motion with. (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 (constellation) Hydra.
Mythology
{{main|Milky Way (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 Galaxias (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
Chinese character 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 ''Linnunrata'' (pathway of the birds). The Finns observed 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
Finnish language
In Swedish, the Milky Way is known as ''Vintergatan'' (winter street), for apparent reasons; it is most visible
in the winter in Scandinavia.
Future of Milky Way
Current measurements suggest the
Andromeda Galaxy is approaching us at 300 kilometres per second, and that the Milky Way may collide with it in several (3-4) billion years. If they do collide, it is thought that our sun and other stars will probably not collide with other stars, but merge to form an elliptical-shaped galaxy over the course of about a billion years.
External links
-
The Milky Way Galaxy, SEDS Messier pages
-
MultiWavelength Milky Way NASA site with images and
VRML models
-
Composite image of the Milky Way
-
Widefield Image of the Summer Milky Way
-
The Milky Way Galaxy from ''An Atlas of the Universe''
-
Proposed Ring around the Milky Way
-
Milky Way spiral gets an extra arm New Scientist.com
*
-
http://www.solstation.com/x-objects/gal2arc.jpg
-
Possible New Milky Way Spiral Arm Sky and Telescope .com
*
-
http://skyandtelescope.com/mm_images/6829.jpg
-
The 1920 Shapley - Curtis Debate on the size of the Milky Way
-
Galactic center mosaic via sun-orbiting Spitzer infrared telescope
-
Milky Way Plan Views
-
Nearby Dust Clouds in the Milky Way
{{Milky Way Footer}}
Category:Milky Way Galaxy
Category:Milky Way's twins
Category:Galactic astronomy
Category:Barred spiral galaxies
af:Melkweg
ar:درب التبانة
bg:Млечен път
ca:Via LÃ ctia
cs:Galaxie MléÄ?ná dráha
da:Mælkevejen
de:Milchstraße
et:Linnutee
es:VÃa Láctea
eo:Lakta Vojo
fr:Voie lactée
ko:우리 �하
io:Lakto-Voyo
id:Bima Sakti
it:Via Lattea
he:שביל החלב
la:Orbis lacteus
lt:PaukÅ¡Ä?ių Takas
li:Mèlkweeg
hu:Tejút
nl:Melkweg (sterrenstelsel)
no:Melkeveien
ja:銀河系
pl:Droga Mleczna
pt:Via Láctea
ru:Галактика (Млечный Путь)
scn:Jolu di San JÃ bbucu
simple:Milky Way
sl:Naša Galaksija
fi:Linnunrata
sv:Vintergatan
th:ทางช้างเผืà¸à¸?
vi:Ngân HÃ
zh:银河系
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