History

Etymology and beliefs

Main articles: List of names for the Milky Way and Milky Way (mythology)

There are many creation myths around the world which explain the origin of the Milky Way and give it its name. The English phrase is a translation from Greek Γαλαξίας, Galaxias, which is derived from the word for milk (γάλα, gala). This is also the origin of the word galaxy. Indians call it the Akashganga or a celestial form of the holy river, Ganga.^{[citation needed]} In Greek myth, the Milky Way was caused by milk spilt by Hera when suckling Heracles.

The term Milky Way first appeared in English literature in a poem by Chaucer.

"See yonder, lo, the Galaxyë
Which men clepeth the Milky Wey,
For hit is whyt."

—Geoffrey Chaucer, Geoffrey Chaucer The House of Fame, c. 1380.^[47]

In a large area from Central Asia to Africa, the name for the Milky Way is related to the word for straw. It has been claimed that this was spread by Arabs who in turn borrowed the word from Armenian.^[48] In several Uralic, Turkic languages, Fenno-Ugric languages and in the Baltic languages the Milky Way is called the "Birds' Path". The Chinese name "Silver River" (銀河) is used throughout East Asia, including Korea and Japan. An alternative name for the Milky Way in ancient China, especially in poems, is "Heavenly Han River"(天汉). In Japanese, "Silver River" (銀河 ginga) means galaxies in general and the Milky Way is called the "Silver River System" (銀河系 gingakei) or the "River of Heaven" (天の川 Amanokawa or Amanokawa). In Swedish, it is called Vintergatan, or "Winter Street", because the stars in the belt were used to predict time of the approaching winter.^{[citation needed]}

Discovery

See also: Galaxy—Observation history

The shape of the Milky Way as deduced from star counts by William Herschel in 1785; the Solar System was assumed near center.

As Aristotle (384-322 BC) informs us in Meteorologica (DK 59 A80), the Greek philosophers Anaxagoras (ca. 500–428 BC) and Democritus (450–370 BC) proposed that the Milky Way might consist of distant stars. However, Aristotle himself believed the Milky Way to be caused by "the ignition of the fiery exhalation of some stars which were large, numerous and close together" and that the "ignition takes place in the upper part of the atmosphere, in the region of the world which is continuous with the heavenly motions."^[49] The Arabian astronomer, Alhazen (965-1037 AD), refuted this by making the first attempt at observing and measuring the Milky Way's parallax,^[50] and he thus "determined that because the Milky Way had no parallax, it was very remote from the earth and did not belong to the atmosphere."^[51]

The Persian astronomer, Abū Rayhān al-Bīrūnī (973-1048), proposed the Milky Way galaxy to be a collection of countless nebulous stars.^[52] Avempace (d. 1138) proposed the Milky Way to be made up of many stars but appears to be a continuous image due to the effect of refraction in the Earth's atmosphere.^[49] Ibn Qayyim Al-Jawziyya (1292-1350) proposed the Milky Way galaxy to be "a myriad of tiny stars packed together in the sphere of the fixed stars" and that that these stars are larger than planets.^[53]

Actual proof of the Milky Way consisting of many stars came in 1610 when Galileo Galilei used a telescope to study the Milky Way and discovered that it was composed of a huge number of faint stars.^[54] In a treatise in 1755, Immanuel Kant, drawing on earlier work by Thomas Wright, speculated (correctly) that the Milky Way might be a rotating body of a huge number of stars, held together by gravitational forces akin to the Solar System but on much larger scales. The resulting disk of stars would be seen as a band on the sky from our perspective inside the disk. Kant also conjectured that some of the nebulae visible in the night sky might be separate "galaxies" themselves, similar to our own.^[55]

The first attempt to describe the shape of the Milky Way and the position of the Sun within it was carried out by William Herschel in 1785 by carefully counting the number of stars in different regions of the sky. He produced a diagram of the shape of the Galaxy with the Solar System close to the center.

Photograph of the "Great Andromeda Nebula" from 1899, later identified as the Andromeda Galaxy

In 1845, Lord Rosse constructed a new telescope and was able to distinguish between elliptical and spiral-shaped nebulae. He also managed to make out individual point sources in some of these nebulae, lending credence to Kant's earlier conjecture.^[56]

In 1917, Heber Curtis had observed the nova S Andromedae within the "Great Andromeda Nebula" (Messier object M31). Searching the photographic record, he found 11 more novae. Curtis noticed that these novae were, on average, 10 magnitudes fainter than those that occurred within our galaxy. As a result he was able to come up with a distance estimate of 150,000 parsecs. He became a proponent of the "island universes" hypothesis, which held that the spiral nebulae were actually independent galaxies.^[57] In 1920 the Great Debate took place between Harlow Shapley and Heber Curtis, concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the universe. To support his claim that the Great Andromeda Nebula was an external galaxy, Curtis noted the appearance of dark lanes resembling the dust clouds in the Milky Way, as well as the significant Doppler shift.^[58]

The matter was conclusively settled by Edwin Hubble in the early 1920s using a new telescope. He was able to resolve the outer parts of some spiral nebulae as collections of individual stars and identified some Cepheid variables, thus allowing him to estimate the distance to the nebulae: they were far too distant to be part of the Milky Way.^[59] In 1936 Hubble produced a classification system for galaxies that is used to this day, the Hubble sequence.

milky way

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

A green and red Perseid meteor streaks across the sky just below the Milky Way in August 2007.

Main articles: Galaxy formation and evolution and nucleocosmochronology

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 Milky Way is thought to be a barred spiral galaxy. Messier 109 is one possible analog.^[14]

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]

The galactic center in the direction of Sagittarius. The primary stars of Sagittarius are indicated in red.

[edit] Galactic center

Main article: 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.