The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Galaxia Kuklos), sometimes referred to simply as "the Galaxy", is a barred spiral galaxy which forms part of the Local Group. The Milky Way has special significance to humanity as the home of the solar system, which is located near the Orion Arm.
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 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 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 Galaxy is about 100,000 light years in diameter, about 3,000 light years in thickness, and about 250,000 light years in circumference. As a guide to the relative physical scale of the Milky Way, if the galaxy was reduced to 130 km (80 mi) in diameter, the solar system would be a mere 2 mm (0.08 in) in diameter.
Age
The age of the Galaxy is currently estimated to be about 13.6 billion (109) 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
As of 2005, the Milky Way is thought to comprise a large barred spiral galaxy of Hubble type SBbc (loosely wound barred spiral) with a total mass of about 1012 solar masses (M☉), comprising 200-400 billion stars [1].
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 [2].
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 27,700 light-years. The disk bulges outward at the center.
As with most galaxies, it is suspected that the galactic center harbours a supermassive black hole, with Sagittarius A* being thought to be the most plausible candidate for the ___location of this extreme concentration of mass.
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 210 and 240 km/s [3]. 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.
Each spiral arm describes a logarithmic spiral (as do the arms of all spiral galaxies) with a pitch of approximately 12 degrees (see Valle, below). 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 - 3kpc 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:
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. 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.
X-ray 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, 8.5±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 [4]).
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 8.5 kpc from the center of the galaxy and roughly 1/8 of an orbit before perigalacton (the sun's closest approach to the center, ~8.3 kpc).
It would take the solar system about 200-250 million years to complete one orbit ("galactic year"), and so is thought to have completed about 20-25 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 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 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.
Akashaganga is the Indian name for the milky way galaxy, which means Ganga(river) of the Sky.
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
- Foley, Ryan J. "Study Details Bar at Center of Milky Way". Retrieved August 17, 2005.
External links
- The Milky Way Galaxy, SEDS Messier pages
- MultiWavelength Milky Way NASA site with images and VRML models
- An Atlas of the Universe
- 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
