MILKY WAY
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 which forms part 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 (460 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 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 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 stars (exact number not yet known). 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 Milky Way's absolute magnitude, which cannot be measured directly, is assumed by astronomical convention to be −20.5.
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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 [1] .
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 600-3000 billion solar masses (M☉) [2] [19] , comprising about 100 billion stars [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 [4] .
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 [5] . 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. 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 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. [19]
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 [6] , the possibility of the disk of the Milky Way extending further is a clear possibility and is supported by evidence of the newly discovered Outer Arm extention of the Cygnus Arm [7] . 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.
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 Center[8][9][10] . The distance between the local arm and the next arm out, the Perseus Arm, is about 6,500 light-years [11] . 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) [citation needed].
It takes the solar system about 225-250 million years to complete one orbit (a galactic year) [12] , 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 AU in 8 days.
The Hayden Planetarium uses 8.0 kpc in their interactive 3D Milky Way Atlas, which justs 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 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 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 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.
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 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 Hercules 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.
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.
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