The
Milky Way (a translation of the
Latin Via Lactea, in turn derived from the
Greek Galaxia Kuklos) is the
galaxy in which the
Earth is found. When viewed from the Earth and its environs, it appears in the night sky as a hazy band of
white light (hence "milky") across the
celestial sphere, formed by
stars within the disc of its namesake
galaxy. It is also simply known as
the Galaxy, as the Earth's
solar system is a part of it. The Milky Way appears brightest in the direction of
Sagittarius, where the
galactic center lies.
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. This reflects the fact that the Earth's
equatorial plane is highly inclined to the galactic plane, as is the
ecliptic (the plane in which the Earth and the other significant planets orbit the
Sun). The fact that the Milky Way divides the night sky into two roughly equal hemispheres reflects the fact that the
solar system lies close to the galactic plane.
To put the Milky Way into perspective, if you made it 130 km (80 mi) in diameter, our solar system would only be 2 mm (0.08 in) in diameter. Also, if a beam of light were to be shot around the Milky Way, it would take almost 250,000 years to complete its journey.
Age
In 2004, a team of astronomers estimated the age of the Galaxy. The team consisted of Luca Pasquini, Piercarlo Bonifacio, Sofia Randich, Daniele Galli, and Raffaele G. Gratton. They 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 time elapsed 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. They added in the estimated age of the stars in the globular cluster: 13,400 ± 800 million years. The sum is their estimated age of the Milky Way Galaxy: 13,600 ± 800 million years.
Structure
Observed structure of the Milky Way's spiral arms
The Milky Way galaxy is a large
spiral galaxy of
Hubble type SBbc (loosely wound barred spiral) with a total
mass of about
solar masses (M
☉), comprising 200-400
billion stars (see [3]). The galactic disk has a 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 about 27,700 light-years.
The stars in the Galaxy's disk rotate around the Galaxy's
center, which is suspected to harbour a
supermassive black hole.
Sagittarius A
- is thought to be the most plausible candidate for the location of this supermassive black hole. It takes the solar system about 226 million years to complete one orbit ("galactic year"), and so has completed about 25 orbits during its lifetime. The orbital speed is 217 km/s, i.e. 1 light-year in ca. 1400 years, and 1 AU in 8 days. The orbital speed of stars in the Milky Way does not depend much on the distance to the center: it is always between 200 and 250 km/s for the Sun's neighbours http://zebu.uoregon.edu/~imamura/123/lecture-2/mass.html. Hence the orbital period is approximately proportional to the distance from the star to the Galaxy's center (without the power 1.5 which applies in the case of a central mass). The disk has a bulge at the center.
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 (see [1]). There are believed to be four major spiral arms and which all start at the Galaxy's center. These are named as follows, according to the image at right:
There are at least two smaller arms or spurs, one of which is:
The
Earth's
solar system may be found close to the inner rim of this Arm, in the
Local Fluff, 8.0±0.5 kpc from the
galactic center. The distance between the local arm and the next arm out, the Perseus Arm, is about
6,500 light-years (see [2]).
Outside of these is the Outer Ring or Monoceros Ring, a ring of stars around the Milky Way proposed by astronomers Brian Yanny and Heidi Jo Newberg. This ring consists of gas and stars torn from other galaxies as they merged with our own billions of years ago.
The galactic disk is surrounded by a
spheroid halo of old stars and
globular clusters.
While the disk contains gas and dust obscuring the view in some wavelengths, the halo 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.
Milky Way
X-ray image of Milky Way taken by Chandra X-ray Observatory
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 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 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.
Mythology
There are numerous
legends in many traditions around the world regarding the creation of 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). One legend describes the Milky Way as a smear of milk, created when the baby Herakles suckled from the goddess
Hera. When Hera realized that the suckling infant was not her own but the illegitimate son of
Zeus and another woman, she pushed it away and the spurting milk became the Milky Way.
Another story tells that the milk came from the goddess
Rhea, the wife of
Cronus, and the suckling infant was Zeus himself. Cronus swallowed his children to ensure his position as head of the
Pantheon and sky god, and so Rhea conceived a plan to save her newborn son Zeus: She wrapped a stone in infant's clothes and gave it to Cronus to swallow. Cronus asked her to nurse the child once more before he swallowed it, and the milk that spurted when she pressed her nipple against the rock eventually became the Milky Way.
Older mythology associates the constellation with a herd of
dairy cows/cattle, whose milk gives the blue glow, and where each
cow is a star. As such, it is intimately associated with legends concerning the constellation of
Gemini, which it is in contact with. Firstly, with Gemini, it may form the origin of the myth of
Castor and Polydeuces, concerning cattle raiding. Secondly, again with Gemini, but also with other features of the
Zodiac sign of
Gemini (i.e.
Canis Major,
Orion,
Auriga, and the deserted area now regarded as Camelopardalis), it may form the origin of the myth of the
Cattle of Geryon, one of
The Twelve Labours of Herakles.
Peoples in Eastern Asia believed that the hazy band of stars were "Silvery River" of the Heaven. Also,
Altair and
Vega were thought to be two lovers, who were bound to meet only once a year on the seventh day of the seventh month, when the
magpies and
crows form the bridge over the galactic river. That day is celebrated as 七夕, (literally the
Seventh Night,) called
Qi Xi in
Chinese,
Tanabata in
Japanese, and Chilseok in
Korean.
References
# J. P. Vallée, "The Milky Way's Spiral Arms traced by Magnetic Fields, Dust, Gas and Stars",
The Astrophysical Journal, volume 454, pp. 119-124, 1995. Available online through
NASA's Astrophysics Data System
#
Press release, Canadian Galactic Plane Survey
#
Press release, European Southern Observatory
# Sandage, A. & Fouts, G.,
The Astrophysical Journal, volume 97, p. 74, 1987
External links
Category:Milky Way Galaxy
Category:Barred spiral galaxies
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