|
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
The Suns 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. 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 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. 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 Hayden Planetarium uses 8.0 kpc in their interactive 3D Milky Way Atlas, which just includes the Galactic Center. The Milky Way environment 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 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 the 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. 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 Spanish, the galaxy is sometimes called "Camino de Santiago", "The Road to Santiago 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 they will merge to form an elliptical galaxy over the course of about a billion years. | ||||||||||||
|
| |||||||||||||
 |
|
| |