The Milky Way, sometimes called simply the Galaxy, is the galaxy in which our Solar System is located. It is a barred spiral galaxy that is part of the Local Group of galaxies. It is one of billions of galaxies in the observable universe.
Its name is a translation of the Latin Via Lactea, which derives from the Greek Γαλαξίας (Galaxias or Galaxiases), both of which refer to the pale band of light formed by the galactic plane as seen from Earth (see etymology of galaxy). Some sources hold that, strictly speaking, the term Milky Way should refer exclusively to the band of light that the galaxy forms in the night sky, while the galaxy as an astrophysical whole should receive the full name Milky Way Galaxy, or alternatively the Galaxy.[4][5][6] However, it is unclear how widespread this convention is, and the term Milky Way is routinely used in either context.
Appearance from Earth
The Milky Way galaxy, as viewed from Earth, itself situated on one of the spiral arms of the galaxy (see Sun's location), appears as a hazy band of white light in the night sky arching across the entire celestial sphere and originating from stars and other material that lie within the galactic plane. The plane of the Milky Way is inclined by about 60° to the ecliptic (the plane of the Earth's orbit), with the North Galactic Pole situated at right ascension 12h 49m, declination +27.4° (B1950) near beta Comae Berenices. The South Galactic Pole is near alpha Sculptoris.
The center of the galaxy is in the direction of Sagittarius, and the Milky Way then "passes" (going westward) through Scorpius, Ara, Norma, Triangulum Australe, Circinus, Centaurus, Musca, Crux, Carina, Vela, Puppis, Canis Major, Monoceros, Orion & Gemini, Taurus, Auriga, Perseus, Andromeda, Cassiopeia, Cepheus & Lacerta, Cygnus, Vulpecula, Sagitta, Aquila, Ophiuchus, Scutum, and back to Sagittarius.
The Milky Way looks brightest in the direction of the constellation of Sagittarius, toward the galactic center. Relative to the celestial equator, it 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 our Solar System lies close to the galactic plane. The Milky Way has a relatively low surface brightness, making it difficult to see from any urban or suburban location suffering from light pollution.
 360-degree photographic panorama of the galaxy. |  The Milky Way as seen from Death Valley, 2007. This is a panoramic picture. |
[edit] Size
The stellar disk of the Milky Way galaxy is approximately 100,000 light-years (9.5×1017 km) in diameter, and is believed to be, on average, about 1,000 ly (9.5×1015 km) thick.[7] It is estimated to contain at least 200 billion stars[8] and possibly up to 400 billion stars,[9] the exact figure depending on the number of very low-mass stars, which is highly uncertain. Extending beyond the stellar disk is a much thicker disk of gas. Recent observations indicate that the gaseous disk of the Milky Way has a thickness of around 12,000 ly (1.1×1017 km)—twice the previously accepted value.[10] As a guide to the relative physical scale of the Milky Way, if it were reduced to 100 m in diameter, the Solar System, including the Oort Cloud, would be no more than 2 mm in width.
The Galactic Halo extends outward, but is limited in size by the orbits of two Milky Way satellites, the Large and the Small Magellanic Clouds, whose perigalacticon is at ~180,000 ly (1.7×1018 km).[11] At this distance or beyond, the orbits of most halo objects would be disrupted by the Magellanic Clouds, and the objects would likely be ejected from the vicinity of the Milky way.
Recent measurements by the Very Long Baseline Array (VLBA) have revealed that the Milky Way is much larger than previously thought. The size of our home galaxy is now considered to be roughly similar to that of our largest local neighbour, the Andromeda galaxy. By using the VLBA to measure the apparent shift of far-flung star-forming regions when the Earth is on opposite sides of the Sun, the researchers were able to measure the distance to those regions using fewer assumptions than prior efforts. The results show that the Milky Way is about 15% wider than previously thought. The newer and more accurate estimate of the galaxy's rotational speed (and in turn the amount of dark matter contained by the galaxy) puts the figure at about 914,000km per hour, significantly higher than the widely accepted value of 792,000km per hour.[12]
[edit] Age
It is extremely difficult to define the age at which the Milky Way formed, but the age of the oldest star in the Galaxy yet discovered, HE 1523-0901, is estimated to be about 13.2 billion years, nearly as old as the Universe itself.[1]
This estimate is based on research by a team of astronomers in 2004 using 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.[13][citation needed] From this research, 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 was deduced to be 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 oldest stars in the Milky Way at 13.6 ± 0.8 billion years. Based upon this emerging science, the Galactic thin disk is estimated to have been formed between 6.5 and 10.1 billion years ago.
[edit] Composition and structure
The Galaxy consists of a bar-shaped core region surrounded by a disk of gas, dust and stars forming four distinct arm structures spiralling outward in a logarithmic spiral shape (see Spiral arms). The mass distribution within the Galaxy closely resembles the Sbc Hubble classification, which is a spiral galaxy with relatively loosely-wound arms.[15] Astronomers first began to suspect that the Milky Way is a barred spiral galaxy in the 1990s[16] rather than an ordinary spiral galaxy. Their suspicions were confirmed by the Spitzer Space Telescope observations in 2005[17] which showed the Galaxy's central bar to be larger than previously suspected. The Milky Way's mass is thought to be about 5.8×1011 solar masses (M☉)[18][19][20] comprising 200 to 400 billion stars. Its integrated absolute visual magnitude has been estimated to be −20.9. Most of the mass of the Galaxy is thought to be dark matter, forming a dark matter halo of an estimated 600–3000 billion M☉ which is spread out relatively uniformly.[20]
[edit] Galactic center
The galactic disc, which bulges outward at the galactic center, has a diameter of between 70,000 and 100,000 light-years.[21] The distance from the Sun to the galactic center is now estimated at 26,000 ± 1400 light-years, while older estimates could put the Sun as far as 35,000 light-years from the central bulge.
The galactic center harbors a compact object of very large mass as determined by the motion of material around the center.[22] The intense radio source named Sagittarius A*, thought to mark the center of the Milky Way, is newly confirmed to be a supermassive black hole. For a photo see Chandra X-ray Observatory; Jan. 6, 2003 Most galaxies are believed to have a supermassive black hole at their center.[23]
The Galaxy's bar is thought to be about 27,000 light-years long, running through its center at a 44 ± 10 degree angle to the line between the Sun and the center of the Galaxy. It is composed primarily of red stars, believed to be ancient (see red dwarf, red giant). The bar is surrounded by a ring called the "5-kpc ring" that contains a large fraction of the molecular hydrogen present in the Galaxy, as well as most of the Milky Way's star formation activity. Viewed from the Andromeda Galaxy, it would be the brightest feature of our own galaxy.
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