Milky Way

Milky Way
This article is about the galaxy. For other uses, see Milky Way (disambiguation).
Milky Way galaxy
ESO-VLT-Laser-phot-33a-07 rsz.jpg
Image of the Milky Way's Galactic Center in the night sky above Paranal Observatory
Observation data
Type SBc (barred spiral galaxy)
Diameter 100,000-120,000 light years (30-37 kpc)[1] [2][unreliable source?]
Thickness 1,000 light years[1]
Volume 7.85 trillion cubic light years[citation needed]
Number of stars 400±200 billion (4±2×1011)[3][4] [5][unreliable source?]
Oldest known star 13.2 billion years[6]
Mass 1.0-1.5×1012 M☉ (2-3×1042 kg) [7]
Sun's distance to Galactic Center 27,200 ± 1,100 light years[8]
Sun's galactic rotation period 250 million years (negative rotation)[9]
Spiral pattern rotation period 50 million years[10]
Bar pattern rotation period 15 to 18 million years[10]
Speed relative to CMB rest frame 552 km/s[11]
See also: Galaxy, List of galaxies
galaxy that contains the Solar System.[12][a] This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky. The term "Milky Way" is a translation of the Latin for "milky road", Via Lactea, in turn derived from the Greek kyklos galaktikos or "milky circle", "milk" also being the root for the Greek word for galaxy, γαλαξίας (galaxias).[13][14]

The galaxy has this appearance because of the Earth's position within the galactic plane around two thirds of the way out from the center, on the inner edge of the Orion–Cygnus Arm, with the majority of the galaxy being seen edge on. The concept of this faint band of light being made up of stars was proven in 1610 when Galileo Galilei used his telescope to resolve it into individual stars. In the 1920s observations by astronomer Edwin Hubble showed that the Milky Way was just one of around 200 billion galaxies in the observable universe.

The Milky Way is a barred spiral galaxy 100,000-120,000 light years in diameter containing 200–600 billion stars. The galaxy is estimated to contain 50 billion planets, 500 million of which could be located in the habitable zone of their parent star. Depending on its structure the entire galaxy has a rotational rate of 1 per 15 to 50 million years. The galaxy is also moving at a rate of 552 to 630 km per second depending on the relative frame of reference. It is estimated to be about 13.2 billion years old, nearly as old as the Universe. The Milky Way is part of the Local Group of galaxies.

Contents

Appearance

A view of the Milky Way towards the Constellation Sagittarius (actually the Galactic Center) as seen from a non-light polluted area (the Black Rock Desert, Nevada).

All the stars that can be seen in the night sky are part of the Milky Way galaxy, but for the purposes of the astronomical activity of observing the celestial sphere the term "Milky Way" is limited to the hazy band of white light arching around the entire night sky.[15] The light originates from un-resolved stars and other material that lie within the galactic plane. Dark regions within the band, such as the Great Rift and the Coalsack, correspond to areas where light from distant stars is blocked by dark nebulae. The Milky Way has a relatively low surface brightness with an apparent magnitude of around +4.5 to +5.[citation needed] Its visibility is greatly affected by how bright the night sky is (limiting magnitude) due to light pollution, stray light from the moon, etc. It becomes visible at limiting magnitudes of +5.1 or higher, showing a great deal of detail above +6.1.[16] This makes the Milky Way difficult to see from any urban or suburban location due to light pollution, but very prominent when viewed from a rural area.[17]

The center of the galaxy lies in the direction of Sagittarius, and it is here that the Milky Way looks brightest. From Sagittarius, the hazy band of white light appears to pass westward through the constellations of Scorpius, Ara, Norma, Triangulum Australe, Circinus, Centaurus, Musca, Crux, Carina, Vela, Puppis, Canis Major, Monoceros, Orion and Gemini, Taurus, Auriga, Perseus, Andromeda, Cassiopeia, Cepheus and Lacerta, Cygnus, Vulpecula, Sagitta, Aquila, Ophiuchus, Scutum, and back to Sagittarius. The fact that the band divides the night sky into two roughly equal hemispheres indicates that the Solar System lies close to the galactic plane.

The galactic plane is inclined by about 60 degrees to the ecliptic (the plane of the Earth's orbit). 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 north galactic pole is situated at right ascension 12h 49m, declination +27.4° (B1950) near beta Comae Berenices, and the south galactic pole is near alpha Sculptoris.

Panoramic image of the Milky Way arching across the night sky shot from a dark sky location.

Size

The stellar disk of the Milky Way galaxy is approximately 100,000 light-years (30 kiloparsecs, 9×1017 km) in diameter, and is considered to be, on average, about 1,000 ly (0.3 kpc) thick.[1] It is estimated to contain at least 200 billion stars[18] and possibly up to 400 billion stars,[19] the exact figure depending on the number of very low-mass, or dwarf stars, which are hard to detect, especially more than 300 ly (90 pc) from the Sun, and so current estimates of the total number remain highly uncertain, though often speculated to be around 250 billion.[citation needed] This can be compared to the one trillion (1012) stars of the neighbouring Andromeda Galaxy.[20] The stellar disc does not have a sharp edge, a radius beyond which there are no stars. Rather, the number of stars drops smoothly with distance from the centre of the galaxy. Beyond a radius of roughly 40,000 ly (12 kpc), the number of stars drops much faster with radius,[21] for reasons that are not understood. Recent estimates give the galaxy a population of at least 50 billion planets, 500 million of which could be located in the habitable zone of their parent star.[22] New data suggests there may be up to twice as many free-floating planets in the Milky Way as there are stars.[23]

360-degree photographic panorama of the Milky Way showing our edge-on view of the galaxy.

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 (3.7 kpc)—twice the previously accepted value.[24]

As a guide to the relative physical scale of the Milky Way, if the Solar System out to the orbit of Pluto were reduced to the size of a US quarter (approximately 2.5 cm in diameter) the Milky Way would cover an area of 3.286 million square kilometers, an area equivalent to about a third of the United States (3.276 million square km), the total area of India (3.287 million square km), or the combined areas of Great Britain, France, Spain, Germany, Italy, Poland, Sweden, Finland and Norway (3.378 million square km).[25]

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 about 180,000 ly (55 kpc).[26] 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.

Age

Main articles: Galaxy formation and evolution and nucleocosmochronology
The Milky Way in August night sky

The ages of individual stars in the Milky Way can be estimated by measuring the abundance of long-lived radioactive elements such as thorium-232 and uranium-238, then comparing the results to estimates of their original abundance. These yield values of about 14.0 ± 2.4 Ga for CS 31082-001 and 13.8 ± 4 Ga for BD+17° 3248. Once a white dwarf star is formed, it begins to undergo radiative cooling and the surface temperature steadily drops. By measuring the temperatures of the coolest of these white dwarfs and comparing them to their expected initial temperature, an age estimate can be made. With this technique, the age of the globular cluster M4 was estimated as 12.7 ± 0.7 Ga. Globular clusters are among the oldest objects in the Milky Way galaxy, which thus set a lower limit on the galaxy age. Age estimates of the oldest of these clusters gives a best fit estimate of 12.6 Ga, and a 95% confidence upper limit of 16 Ga.[27]

In 2007, a star in the Galactic halo, HE 1523-0901, was estimated to be about 13.2 billion years old, nearly as old as the Universe. As the oldest known object in the Milky Way at that time, it placed a lower limit on the age of the Milky Way.[6][not in citation given][improper synthesis?] This estimate was determined using the UV-Visual Echelle Spectrograph of the Very Large Telescope to measure the relative strengths of spectral lines caused by the presence of Thorium and other elements created by the R-process. The line strengths yield abundances of different elemental isotopes, from which an estimate of the age of the star can be derived using nucleocosmochronology.[6]

The age of stars in the Galactic thin disk can be estimated in the same way as HE 1523-0901.[citation needed] Measurements of thin disk stars yield an estimate that the thin disk formed between 8.8 ± 1.7 billion years ago. These measurements suggest there was a hiatus of almost 5 billion years between the formation of the Galactic halo and the thin disk.[28]

Composition and structure

Artist's conception of the spiral structure of the Milky Way with two major stellar arms and a bar.[29]

The galaxy consists of a bar-shaped core region surrounded by a disc of gas, dust and stars forming four distinct arm structures spiraling 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.[30] Astronomers first began to suspect that the Milky Way is a barred spiral galaxy, rather than an ordinary spiral galaxy, in the 1990s.[31] Their suspicions were confirmed by the Spitzer Space Telescope observations in 2005[32] that showed the galaxy's central bar to be larger than previously suspected.

Estimates for the mass of the Milky Way vary, depending upon the method and data used. Recent estimates at the low end have placed the mass of the Milky Way at 5.8×1011 solar masses (M☉), somewhat smaller than the Andromeda Galaxy.[33][34][35] Other measurements by the Very Long Baseline Array (VLBA) have found velocities as large as 254 km/s for stars at the edge of the Milky Way, higher than the previously accepted value of 220 km/s.[36] As the orbital velocity depends on the mass enclosed, this implies that the Milky Way is more massive, roughly equaling the mass of Andromeda Galaxy at 7×1011 M☉ within 50 kiloparsecs (160,000 ly) of its center.[37] A recent measurement of the radial velocity of halo stars finds the mass enclosed within 80 kiloparsecs is 7×1011 solar masses.[38] Most of the mass of the galaxy is thought to be dark matter, which forms a dark matter halo that is spread out relatively uniformly to a distance beyond one hundred kiloparsecs from the Galactic Center. Modelling of the Milky Way suggests that the overall mass of the entire galaxy lies in the range 1-1.5×1012 M☉[7]

This mass in baryonic matter is estimated to include 200 to 400 billion stars.[39] Its integrated absolute visual magnitude has been estimated to be −20.9.[40]

Galactic Center

Main article: Galactic Center
A false-color infrared image of the core of the Milky Way galaxy taken by NASA's Spitzer Space Telescope. Older cool stars are blue, dust features lit up by large hot stars are shown in a reddish hue, and the bright white spot in the middle marks the site of a super-massive black hole at the center of the galaxy.

The galactic disc, which bulges outward at the Galactic Center, has a diameter of 70,000–100,000 light-years (20–30 kpc).[41] The exact distance from the Sun to the Galactic Center is actively debated. The latest estimates from geometric-based methods and standard candles yield distances to the Galactic Center of 8.0–8.7 kpc (26,000–28,000 ly). [8][42][43][44][45] The fact that the estimates span nearly 1 kpc only underscores the true uncertainty associated with the distance to the Galactic Center.

The Galactic Center harbors a compact object of very large mass as determined by the motion of material around the center.[46] The intense radio source named Sagittarius A*, thought to mark the center of the Milky Way, is confirmed to be a supermassive black hole.[47] Most galaxies are believed to have supermassive black holes at their centers.[48]

The nature of the galaxy's bar is also actively debated, with estimates for its half-length and orientation spanning from 1–5 kpc (3,300–16,000 ly) (short or a long bar) and 10–50 degrees.[44][45][49] Certain authors advocate that the galaxy features two distinct bars, one nestled within the other[50]. The bar is delineated by red clump stars, however, RR Lyr variables do not trace a prominent Galactic bar.[45][51][52] The bar may be 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.[53]

Notes

Spiral arms

Maps of the Milky Way's spiral structure are notoriously uncertain and exhibit striking differences.[54][55][29][56][57][58][59][60] Some 150 years after Alexander (1852)[61] first suggested that the Milky Way was a spiral, there is currently no consensus on the number or nature of the Galaxy's spiral arms. Perfect grand design logarithmic spiral patterns ineptly describe features near the Sun,[54][59] namely since galaxies commonly exhibit arms that branch, merge, twist unexpectedly, and feature a degree of irregularity.[59][45][60] The possible scenario of the Sun within a spur / Local arm[54] emphasizes that point and indicates that such features are likely not unique, and exist elsewhere in the galaxy.[59]

Each spiral arm describes a logarithmic spiral (as do the arms of all spiral galaxies) with a pitch of approximately 12 degrees. Until recently, there were believed to be four major spiral arms which all start near the galaxy's center. These are named as follows, according to the image at right:

Observed and extrapolated structure of the spiral arms.
Color Arm(s)
cyan 3-kpc and Perseus Arm
purple Norma and Outer arm (Along with a newly discovered extension)
green Scutum–Centaurus Arm
pink Carina–Sagittarius Arm
There are at least two smaller arms or spurs, including:
orange Orion–Cygnus Arm (which contains the Sun and Solar System)

Observations presented in 2008 by Robert Benjamin of the University of Wisconsin–Whitewater suggest that the Milky Way possesses only two major stellar arms: the Perseus arm and the Scutum-Centaurus arm. The rest of the arms are minor or adjunct arms.[29] This would mean that the Milky Way is similar in appearance to NGC 1365.

Outside of the major spiral arms is the Monoceros Ring (or Outer Ring), proposed by astronomers Brian Yanny and Heidi Jo Newberg, a ring of gas and stars torn from other galaxies billions of years ago.

As is typical for many galaxies, the distribution of mass in the Milky Way galaxy 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.[62] Hence the orbital period of the typical star is directly proportional only to the length of the path traveled. This is unlike the situation within the Solar System, where two-body gravitational dynamics dominate and different orbits are expected to have significantly different velocities associated with them. This difference is one of the major pieces of evidence for the existence of dark matter. Another interesting aspect is the so-called "wind-up problem" of the spiral arms. If the inner parts of the arms rotate faster than the outer part, then the galaxy will wind up so much that the spiral structure will be thinned out. But this is not what is observed in spiral galaxies; instead, astronomers propose that the spiral pattern is a density wave emanating from the Galactic Center. This can be likened to a moving traffic jam on a highway—the cars are all moving, but there is always a region of slow-moving cars. This model also agrees with enhanced star formation in or near spiral arms; the compressional waves increase the density of molecular hydrogen and protostars form as a result.

Halo

The galactic disk is surrounded by a spheroid halo of old stars and globular clusters, of which 90% lie within 100,000 light-years (30 kpc),[63] suggesting a stellar halo diameter of 200,000 light-years. However, a few globular clusters have been found farther, such as PAL 4 and AM1 at more than 200,000 light-years away from the Galactic Center. About 40% of these clusters are on retrograde orbits, which means they move in the opposite direction from the Milky Way rotation.[64] The globular clusters can follow rosette orbits about the galaxy, in contrast to the elliptical orbit of a planet.[65]

While the disk contains gas and dust which obscure 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.

Discoveries in the early 21st century have added dimension to the knowledge of the Milky Way's structure. With the discovery that the disk of the Andromeda Galaxy (M31) extends much further than previously thought,[66] the possibility of the disk of the Milky Way galaxy extending further is apparent, and this is supported by evidence from the discovery of the Outer Arm extension of the Cygnus Arm.[67] With the discovery of the Sagittarius Dwarf Elliptical Galaxy came the discovery of a ribbon of galactic debris as the polar orbit of the dwarf and its interaction with the Milky Way tears it apart. Similarly, with the discovery of the Canis Major Dwarf Galaxy, it was found that a ring of galactic debris from its interaction with the Milky Way encircles the galactic disk.

On January 9, 2006, Mario Jurić and others of Princeton University announced that the Sloan Digital Sky Survey of the northern sky 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 current models. The collection of stars rises close to perpendicular to the plane of the spiral arms of the galaxy. 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 (9 kpc) away.

Gamma-ray bubbles

Artistic diagram of the two gigantic X-ray/gamma-ray bubbles of Earth’s Galaxy, the Milky Way.

On November 9, 2010, Doug Finkbeiner of the Harvard–Smithsonian Center for Astrophysics announced that he had detected two gigantic spherical bubbles of energy erupting to the north and the south from the center of the Milky Way, using data of the Fermi Gamma-ray Space Telescope. The diameter of each of the bubbles is about 25,000 light-years (7.7 kpc); they stretch up to Grus and to Virgo on the night-sky of the southern hemisphere. Their origin remains unclear, so far.[68][69]

Sun's location and neighborhood

Diagram of the Sun location in the Milky Way Galaxy.

The Sun (and therefore the Earth and the Solar System) may be found close to the inner rim of the galaxy's Orion Arm, in the Local Fluff inside the Local Bubble, and in the Gould Belt, at a distance of 8.33 ± 0.35 kiloparsecs (27,200 ± 1,100 ly) from the Galactic Center.[8] [42] [70] The Sun is currently 5–30 parsecs (16–98 ly) from the central plane of the galactic disc.[71] The distance between the local arm and the next arm out, the Perseus Arm, is about 6,500 light-years (2.0 kpc).[72] The Sun, and thus the Solar System, is found in the galactic habitable zone.

There are about 208 stars brighter than absolute magnitude 8.5 within 15 parsecs (49 ly) of the Sun, giving a density of 0.0147 such stars per cubic parsec, or 0.000424 per cubic light-year (from List of nearest bright stars). On the other hand, there are 64 known stars (of any magnitude, not counting 4 brown dwarfs) within 5 parsecs (16 ly) of the Sun, giving a density of 0.122 stars per cubic parsec, or 0.00352 per cubic light-year (from List of nearest stars), illustrating the fact that most stars are less bright than absolute magnitude 8.5.

The Apex of the Sun's Way, or the solar apex, is 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 60 sky 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. In addition, the Sun oscillates up and down relative to the galactic plane approximately 2.7 times per orbit. This is very similar to how a simple harmonic oscillator works with no drag force (damping) term. These oscillations were until recently thought to coincide with mass extinction periods on Earth.[73] However, a reanalysis of the effects of the Sun's transit through the spiral structure based on CO data has failed to find these correlations.[74]

It takes the Solar System about 225–250 million years to complete one orbit of the galaxy (a galactic year),[75] so it is thought to have completed 20–25 orbits during the lifetime of the Sun and 1/1250 of a revolution since the origin of humans. The orbital speed of the Solar System about the center of the galaxy is approximately 220 km/s or 0.073% of the speed of light. At this speed, it takes around 1,400 years for the Solar System to travel a distance of 1 light-year, or 8 days to travel 1 AU (astronomical unit).[76]

A diagram of our location in the observable Universe. (Smaller image)

Environment

Main articles: Local Group and Andromeda–Milky Way collision
Broad infrared view of our Milky Way galaxy from the Spitzer Space Telescope created from more than 800,000 frames. This is the most detailed infrared picture of our galaxy to date.
Milky Way starscape taken from ESO's Paranal Observatory with enlarged region (bottom).

The Milky Way and the Andromeda Galaxy are a binary system of giant spiral galaxies belonging to a group of 50 closely bound galaxies known as the Local Group, itself being part of the Virgo Supercluster.

Two smaller galaxies and a number of dwarf galaxies in the Local Group orbit the Milky Way. 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, as well as some that have already been absorbed by the Milky Way, such as Omega Centauri. 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 galaxy, causing vibrations at certain frequencies when they pass through its edges.[77] 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 twentyfold increase in mass for the galaxy. 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 galactic disc with the known gas layer. As a result, the model predicts that the gravitational effect of the Magellanic Clouds is amplified as they pass through the galaxy.

Current measurements suggest the Andromeda Galaxy is approaching us at 100 to 140 kilometers per second. The Milky Way may collide with it in 3 to 4 billion years, depending on the importance of unknown lateral components to the galaxies' relative motion. If they collide, individual stars within the galaxies would not collide, but instead the two galaxies will merge to form a single elliptical galaxy over the course of about a billion years.[78]

Velocity

Galaxy rotation curve for the Milky Way. Vertical axis is speed of rotation about the Galactic Center. Horizontal axis is distance from the Galactic Center in kpcs. The sun is marked with a yellow ball. The observed curve of speed of rotation is blue. The predicted curve based upon stellar mass and gas in the Milky Way is red. Scatter in observations roughly indicated by gray bars. The difference is due to dark matter or perhaps a modification of the law of gravity.[79][80][81]

In the general sense, the absolute velocity 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 object's motion. (Motion must always be specified with respect to another object.) This must be kept in mind when discussing the galaxy's motion.

Astronomers believe the Milky Way is moving at approximately 630 km per second relative to the local co-moving frame of reference that moves with the Hubble flow.[82] If the galaxy is moving at 600 km/s, Earth travels 51.84 million km per day, or more than 18.9 billion km per year, about 4.5 times its closest distance from Pluto. The Milky Way is thought to be moving in the direction of the Great Attractor. The Local Group (a cluster of gravitationally bound galaxies containing, among others, the Milky Way and the Andromeda galaxy) is part of a supercluster called the Local Supercluster, centered near the Virgo Cluster: although they are moving away from each other at 967 km/s as part of the Hubble flow, the velocity is less than would be expected given the 16.8 million pc distance due to the gravitational attraction between the Local Group and the Virgo Cluster.[83]

Another reference frame is provided by the cosmic microwave background (CMB). The Milky Way is moving at around 552 km/s[11] with respect to the photons of the CMB, toward 10.5 right ascension, −24° declination (J2000 epoch, near the center of Hydra). This motion is observed by satellites such as the Cosmic Background Explorer (COBE) and the Wilkinson Microwave Anisotropy Probe (WMAP) as a dipole contribution to the CMB, as photons in equilibrium in the CMB frame get blue-shifted in the direction of the motion and red-shifted in the opposite direction.[citation needed]

The galaxy rotates about its center according to its galaxy rotation curve as shown in the figure. The discrepancy between the observed curve (relatively flat) and the curve based upon the known mass of the stars and gas in the Milky Way (decaying curve) is attributed to dark matter.[84]

History

Etymology

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 Ancient Greek Γαλαξίας, Galaxias, which is derived from the word for milk (γάλα, gala). This is also the origin of the word galaxy. In Greek myth, the Milky Way was caused by milk spilt by Hera when suckling Heracles.[85]

In Sanskrit and several other Indo-Aryan languages, the Milky Way is called Akash Ganga (आकाशगंगा, Ganges of the heavens).[86] The milky way is held to be sacred in the Hindu scriptures known as the Puranas, and the Ganges and the Milky Way are considered to be terrestrial-celestial analogs of each other.[86][87] However, the term Kshira (क्षीर, milk) is also used as an alternative name for the milky way in Hindu texts.[88]

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 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."[89] The Neoplatonist philosopher Olympiodorus the Younger (c. 495–570 A.D.) criticized this view, arguing that if the Milky Way were sublunary it should appear different at different times and places on the Earth, and that it should have parallax, which it does not. In his view, the Milky Way was celestial. This idea would be influential later in the Islamic world.[90]

The Arabian astronomer, Alhazen (965–1037 AD), refuted this by making the first attempt at observing and measuring the Milky Way's parallax,[91] 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."[92]

The Persian astronomer Abū Rayhān al-Bīrūnī (973–1048) proposed the Milky Way galaxy to be a collection of countless nebulous stars.[93] The Andalusian astronomer 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, citing his observation of a conjunction of Jupiter and Mars in 1106 or 1107 as evidence.[89] 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 these stars are larger than planets.[94]

The Persian astronomer Naṣīr al-Dīn al-Ṭūsī (1201,1274) in his Tadhkira writes: "The Milky Way, i.e. the galaxy, is made up of a very large number of small, tightly-clustered stars, which, on account of their concentration and smallness, seem to be cloudy patches. because of this, it was likened to milk in color."[95]

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.[96] 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. Kant referred to both our galaxy and the "extragalactic nebulae" as "island universes", a term still current up to the 1930s.[97]

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 visible sky. He produced a diagram of the shape of the galaxy with the Solar System close to the center.

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.[98]

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

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.[99] 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.[100]

The matter was conclusively settled by Edwin Hubble in the early 1920s using the Mount Wilson observatory 100 inch (2.5 m) Hooker telescope. With the light-gathering power of this new telescope he was able to produce astronomical photographs that resolved the outer parts of some spiral nebulae as collections of individual stars. He was also able to identify some Cepheid variables that he could use as a benchmark to estimate the distance to the nebulae: proving they were far too distant to be part of the Milky Way.[101] In 1936, Hubble produced a classification system for galaxies that is used to this day, the Hubble sequence.[102]

See also

Portal icon Astronomy portal
Portal icon Star portal
Portal icon Space portal

Notes

a. ^ 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 should receive the full name Milky Way Galaxy, or alternatively the Galaxy. However, it is unclear how widespread this convention is, and the term Milky Way is routinely used in either context.[103]

References

  1. ^ a b c Christian, Eric; Safi-Harb, Samar. "How large is the Milky Way?". NASA: Ask an Astrophysicist. http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980317b.html. Retrieved 2007-11-28. 
  2. ^ http://www.solstation.com/x-objects/pox186.htm
  3. ^ "NASA – Galaxy". Nasa.gov. 2007-11-29. http://www.nasa.gov/worldbook/galaxy_worldbook.html. Retrieved 2010-08-10. 
  4. ^ Dec16th2008. "How Many Stars are in the Milky Way?". Universe Today. http://www.universetoday.com/guide-to-space/milky-way/how-many-stars-are-in-the-milky-way/. Retrieved 2010-08-10. 
  5. ^ http://www.solstation.com/x-objects/pox186.htm
  6. ^ a b c Frebel, Anna; Christlieb, Norbert; Norris, John E.; Thom, Christopher; Beers, Timothy C.; Rhee, Jaehyon (2007). "Discovery of HE 1523-0901, a Strongly r-Process-enhanced Metal-poor Star with Detected Uranium". The Astrophysical Journal 660 (2): L117. arXiv:astro-ph/0703414. Bibcode 2007ApJ...660L.117F. doi:10.1086/518122. 
  7. ^ a b McMillan P.J. (2011) Mass models of the Milky Way, MNRAS
  8. ^ a b c Gillessen, S.; Eisenhauer, F.; Trippe, S.; Alexander, T.; Genzel, R.; Martins, F.; Ott, T. (2009). "Monitoring Stellar Orbits Around the Massive Black Hole in the Galactic Center". The Astrophysical Journal 692 (2): 1075. Bibcode 2009ApJ...692.1075G. doi:10.1088/0004-637X/692/2/1075. 
  9. ^ "The Sun". World Book at NASA. NASA. http://www.nasa.gov/worldbook/sun_worldbook.html. Retrieved 2010-03-22. "The sun is one of over 100 billion stars in the Milky Way galaxy. It is about 25,000 light-years from the center of the galaxy, and it revolves around the Galactic Center once about every 250 million years." 
  10. ^ a b Bissantz, Nicolai; Englmaier, Peter; Gerhard, Ortwin (2003). "Gas dynamics in the Milky Way: second pattern speed and large-scale morphology". Monthly Notices of the Royal Astronomical Society 340 (3): 949. arXiv:astro-ph/0212516. Bibcode 2003MNRAS.340..949B. doi:10.1046/j.1365-8711.2003.06358.x. 
  11. ^ a b Kogut, A.; Lineweaver, C.; Smoot, G. F.; Bennett, C. L.; Banday, A.; Boggess, N. W.; Cheng, E. S.; de Amici, G.; Fixsen, D. J.; Hinshaw, G.; Jackson, P. D.; Janssen, M.; Keegstra, P.; Loewenstein, K.; Lubin, P.; Mather, J. C.; Tenorio, L.; Weiss, R.; Wilkinson, D. T.; Wright, E. L. (1993). "Dipole Anisotropy in the COBE Differential Microwave Radiometers First-Year Sky Maps". Astrophysical Journal 419: 1. arXiv:astro-ph/9312056. Bibcode 1993ApJ...419....1K. doi:10.1086/173453. 
  12. ^ Collins Elementary English Dictionary – Complete and Unabridged 1991-2003 - Milky Way / The American Heritage Science Dictionary 2005 - Milky Way noted at thefreedictionary.com
  13. ^ Connie Jankowski (2010), Pioneers of Light and Sound, Compass Point Books, pp. 6, ISBN 0756543061, http://books.google.com/books?id=Wp6E3y-58LkC&pg=PT6&dq=%22milky+way%22+Via+Lactea+milk&hl=en&ei=pzNNToDeM6L30gGT6sDcBg&sa=X&oi=book_result&ct=result&resnum=8&ved=0CFYQ6AEwBzgK#v=onepage&q=%22milky%20way%22%20Via%20Lactea%20milk&f=false 
  14. ^ Jon Schiller (2010), Big Bang & Black Holes, CreateSpace, pp. 163, ISBN 1452865523, http://books.google.com/books?id=nbnEjck_N84C&pg=PA163&dq=kyklos+galaktikos+galaxias&hl=en&ei=BOJPTqLZAuS50AHEq5TnBg&sa=X&oi=book_result&ct=result&resnum=2&ved=0CDYQ6AEwAQ#v=onepage&q=kyklos%20galaktikos%20galaxias&f=false 
  15. ^ Pasachoff, Jay M. (1994). Astronomy: From the Earth to the Universe. Harcourt School. p. 500. ISBN 0-03-001667-3. 
  16. ^ magnitude estimates and quoted International Dark-Sky Association esimates from this post [1]
  17. ^ Note: see also Bortle Dark-Sky Scale
  18. ^ Sanders, Robert (January 9, 2006). "Milky Way Galaxy is warped and vibrating like a drum". UCBerkeley News. http://www.berkeley.edu/news/media/releases/2006/01/09_warp.shtml. Retrieved 2006-05-24. 
  19. ^ Frommert, H.; Kronberg, C. (August 25, 2005). "The Milky Way Galaxy". SEDS. http://www.seds.org/messier/more/mw.html. Retrieved 2007-05-09. 
  20. ^ Young, Kelly (2006-06-06). "Andromeda galaxy hosts a trillion stars". NewScientist. http://www.newscientist.com/article/dn9282-andromeda-galaxy-hosts-a-trillion-stars.html. Retrieved 2006-06-08. 
  21. ^ Sale; Drew; Knigge; Zijlstra; Irwin; Morris; Phillipps; Drake et al. (2009-09-21). "The Structure of the Outer Galactic Disc as revealed by IPHAS early A Stars". arXiv:0909.3857 [astro-ph.GA]. 
  22. ^ Borenstein, Seth (2011-02-19). "Cosmic census finds crowd of planets in our galaxy". The Washington Post. Associated Press. Archived from the original on 2011-02-21. http://www.webcitation.org/5wg3VVKg4. 
  23. ^ "Free-Floating Planets May be More Common Than Stars". Pasadena, CA: NASA's Jet Propulsion Laboratory. 2011-02-18. Archived from the original on 2011-05-25. http://www.webcitation.org/5ywyjNm12. "The team estimates there are about twice as many of them as stars." 
  24. ^ "Milky Way fatter than first thought". The Sydney Morning Herald. Australian Associated Press. 2008-02-20. http://news.smh.com.au/milky-way-fatter-than-first-thought/20080220-1tbv.html. Retrieved 2008-04-24. 
  25. ^ NASA How Big Is Our Universe? http://www.nasa.gov/audience/foreducators/5-8/features/F_How_Big_is_Our_Universe.html
  26. ^ Connors, et al.; Kawata, Daisuke; Gibson, Brad K. (2007). "N-body simulations of the Magellanic stream". Monthly Notices of the Royal Astronomical Society 371 (1): 108. arXiv:astro-ph/0508390. Bibcode 2006MNRAS.371..108C. doi:10.1111/j.1365-2966.2006.10659.x. 
  27. ^ Krauss, Lawrence M.; Chaboyer, Brian (January 2003), "Age Estimates of Globular Clusters in the Milky Way: Constraints on Cosmology Age Estimates of Globular Clusters in the Milky Way: Constraints on Cosmology", Science 299 (5603): 65–70, Bibcode 2003Sci...299...65K, doi:10.1126/science.1075631 
  28. ^ Del Peloso, E. F. (2005). "The age of the Galactic thin disk from Th/Eu nucleocosmochronology". Astronomy and Astrophysics 440 (3): 1153. arXiv:astro-ph/0506458. Bibcode 2005A&A...440.1153D. doi:10.1051/0004-6361:20053307. 
  29. ^ a b c Benjamin, R. A. (2008). "The Spiral Structure of the Galaxy: Something Old, Something New...". In Beuther, H.; Linz, H.; Henning, T. (ed.). Massive Star Formation: Observations Confront Theory. 387. Astronomical Society of the Pacific Conference Series. pp. 375. Bibcode 2008ASPC..387..375B. Lay summary (2008-06-03). 
    See also Bryner, Jeanna (2008-06-03). "New Images: Milky Way Loses Two Arms". Space.com. http://www.space.com/scienceastronomy/080603-aas-spiral-arms.html. Retrieved 2008-06-04. 
  30. ^ Ortwin, Gerhard (2002). "Mass distribution in our galaxy". Space Science Reviews 100 (1/4): 129–138. arXiv:astro-ph/0203110. Bibcode 2002astro.ph..3110G. doi:10.1023/A:1015818111633. 
  31. ^ Chen, W.; Gehrels, N.; Diehl, R.; Hartmann, D. (1996). "On the spiral arm interpretation of COMPTEL ^26^Al map features". Space Science Reviews 120: 315–316. Bibcode 1996A&AS..120C.315C. 
  32. ^ McKee, Maggie (August 16, 2005). "Bar at Milky Way's heart revealed". New Scientist. http://www.newscientist.com/article/dn7854--bar-at-milky-ways-heart-revealed.html. Retrieved 2009-06-17. 
  33. ^ Karachentsev, I. D.; Kashibadze, O. G. (2006). "Masses of the local group and of the M81 group estimated from distortions in the local velocity field". Astrophysics 49 (1): 3–18. Bibcode 2006Ap.....49....3K. doi:10.1007/s10511-006-0002-6. 
  34. ^ Vayntrub, Alina (2000). "Mass of the Milky Way". The Physics Factbook. http://hypertextbook.com/facts/2000/AlinaVayntrub.shtml. Retrieved 2007-05-09. 
  35. ^ Battaglia, G.; Helmi, A.; Morrison, H.; Harding, P.; Olszewski, E. W.; Mateo, M.; Freeman, K. C.; Norris, J.; Shectman, S. A. (2005). "The radial velocity dispersion profile of the Galactic halo: Constraining the density profile of the dark halo of the Milky Way". Monthly Notices of the Royal Astronomical Society 364: 433–442. arXiv:astro-ph/0506102. Bibcode 2005MNRAS.364..433B. doi:10.1111/j.1365-2966.2005.09367.x. 
  36. ^ "Milky Way a Swifter Spinner, More Massive, New Measurements Show". 2009-01-05. http://www.nrao.edu/pr/2009/mwrotate/. Retrieved 2009-01-20. 
  37. ^ Reid, M. J.; Men ten, K. M.; Zheng, X. W.; Brunthaler, A.; Moscadelli, L.; Xu, Y.; Zhang, B.; Sato, M.; Honma, M.; Hirota, T.; Hachisuka, K.; Choi, Y. K.; Moellenbrock, G. A.; Bartkiewicz, A. (2009). "Trigonometric Parallaxes of Massive Star-Forming Regions. VI. Galactic Structure, Fundamental Parameters, and Noncircular Motions". The Astrophysical Journal 700 (1): 137–148. arXiv:0902.3913. Bibcode 2009ApJ...700..137R. doi:10.1088/0004-637X/700/1/137. 
  38. ^ Gnedin, Oleg Y.; Brown, Warren R.; Geller, Margaret J.; Kenyon, Scott J. (2010). "The Mass Profile of the Galaxy to 80 kpc". The Astrophysical Journal 720 (1): L108. arXiv:1005.2619. Bibcode 2010ApJ...720L.108G. doi:10.1088/2041-8205/720/1/L108. 
  39. ^ Nicholos Wethington. "How Many Stars are in the Milky Way?". http://www.universetoday.com/guide-to-space/milky-way/how-many-stars-are-in-the-milky-way/. Retrieved 2010-04-09. 
  40. ^ Jerry Coffey. "Absolute Magnitude". http://www.universetoday.com/guide-to-space/the-universe/absolute-magnitude/. Retrieved 2010-04-09. 
  41. ^ Grant. J.; Lin, B. (2000). "The Stars of the Milky Way". Fairfax Public Access Corporation. http://members.fcac.org/~sol/chview/chv5.htm. Retrieved 2007-05-09. 
  42. ^ a b Ghez, A.M. et al. (2008) Measuring Distance and Properties of the Milky Way's Central Supermassive Black Hole with Stellar Orbits, MNRAS
  43. ^ Reid M.J. et al. (2009) A Trigonometric Parallax of Sgr B2, ApJ
  44. ^ a b Stellar populations in the Galactic bulge. Modelling the Galactic bulge with TRILEGAL. Bibcode 2009A&A...498...95V. doi:10.1051/0004-6361/20078472. 
  45. ^ a b c d Majaess, D. (2010). "Concerning the Distance to the Center of the Milky Way and Its Structure". Acta Astronomica 60: 55. Bibcode 2010AcA....60...55M. 
  46. ^ Mark H. Jones, Robert J. Lambourne, David John Adams (2004). An Introduction to Galaxies and Cosmology. Cambridge University Press. pp. 50–51. ISBN 0521546230. http://books.google.com/?id=36K1PfetZegC&pg=PA50&dq=Milky+Way+%22black+hole%22. 
  47. ^ For a photo see Chandra X-ray Observatory; January 6, 2003
  48. ^ Blandford, R.D. (1999). "Origin and evolution of massive black holes in galactic nuclei". Galaxy Dynamics, proceedings of a conference held at Rutgers University, 8–12 August 1998,ASP Conference Series vol. 182. Bibcode 1999ASPC..182...87B. 
  49. ^ . Bibcode 2008A&A...491..781C. doi:10.1051/0004-6361:200810720. 
  50. ^ Nishiyama, Shogo; Nagata, Tetsuya; Baba, Daisuke; Haba, Yasuaki; Kadowaki, Ryota; Kato, Daisuke; Kurita, Mikio; Nagashima, Chie et al. (2005). "A Distinct Structure inside the Galactic Bar". The Astrophysical Journal 621 (2): L105. arXiv:astro-ph/0502058. Bibcode 2005ApJ...621L.105N. doi:10.1086/429291. 
  51. ^ Alcock, C.; Allsman, R. A.; Alves, D. R.; Axelrod, T. S.; Becker, A. C.; Basu, A.; Baskett, L.; Bennett, D. P. et al. (1998). "The RR Lyrae Population of the Galactic Bulge from the MACHO Database: Mean Colors and Magnitudes". Astrophysical Journal v.492 492: 190. Bibcode 1998ApJ...492..190A. doi:10.1086/305017. 
  52. ^ Kunder, Andrea; Chaboyer, Brian (2008). "Metallicity Analysis of MACHO Galactic Bulge RR0 Lyrae Stars from their Light Curves". The Astronomical Journal 136 (6): 2441. Bibcode 2008AJ....136.2441K. doi:10.1088/0004-6256/136/6/2441. 
  53. ^ Staff (September 12, 2005). "Introduction: Galactic Ring Survey". Boston University. http://www.bu.edu/galacticring/new_introduction.htm. Retrieved 2007-05-10. 
  54. ^ a b c . Bibcode 2003A&A...397..133R. doi:10.1051/0004-6361:20021504. 
  55. ^ Nakanishi, Hiroyuki; Sofue, Yoshiaki (2003). "Three-Dimensional Distribution of the ISM in the Milky Way Galaxy: I. The H I Disk". Publications of the Astronomical Society of Japan 55: 191. arXiv:astro-ph/0304338. Bibcode 2003PASJ...55..191N. 
  56. ^ Vallée, Jacques P. (2008). "New Velocimetry and Revised Cartography of the Spiral Arms in the Milky WAY—A Consistent Symbiosis". The Astronomical Journal 135 (4): 1301. Bibcode 2008AJ....135.1301V. doi:10.1088/0004-6256/135/4/1301. 
  57. ^ . Bibcode 2009A&A...499..473H. doi:10.1051/0004-6361/200809692. 
  58. ^ Churchwell, Ed; Babler, Brian L.; Meade, Marilyn R.; Whitney, Barbara A.; Benjamin, Robert; Indebetouw, Remy; Cyganowski, Claudia; Robitaille, Thomas P. et al. (2009). "The Spitzer/GLIMPSE Surveys: A New View of the Milky Way". Publications of the Astronomical Society of the Pacific 121 (877): 213. Bibcode 2009PASP..121..213C. doi:10.1086/597811. 
  59. ^ a b c d Majaess; Turner; Lane (2009). "Searching Beyond the Obscuring Dust Between the Cygnus-Aquila Rifts for Cepheid Tracers of the Galaxy's Spiral Arms". arXiv:0909.0897 [astro-ph.GA]. 
  60. ^ a b Lepine; Roman-Lopes; Zulema Abraham; Junqueira; Mishurov (2010). "The spiral structure of the Galaxy revealed by CS sources and evidence for the 4:1 resonance". arXiv:1010.1790 [astro-ph.GA]. 
  61. ^ Alexander, Stephen (1852). "On the origin of the forms and the present condition of some of the clusters of stars, and several of the nebulae". Astronomical Journal 2: 97. Bibcode 1852AJ......2...97A. doi:10.1086/100231. 
  62. ^ Imamura, Jim (August 10, 2006). "Mass of the Milky Way Galaxy". University of Oregon. Archived from the original on 2007-03-01. http://web.archive.org/web/20070301055338/http://zebu.uoregon.edu/~imamura/123/lecture-2/mass.html. Retrieved 2007-05-10. 
  63. ^ Harris, William E. (February 2003). "Catalog of Parameters for Milky Way Globular Clusters: The Database" (text). SEDS. http://www.seds.org/messier/xtra/data/mwgc.dat.txt. Retrieved 2007-05-10. 
  64. ^ Dauphole, B.; Geffert, M.; Colin, J.; Ducourant, C.; Odenkirchen, M.; Tucholke, H.-J. (September 1996). "The kinematics of globular clusters, apocentric distances and a halo metallicity gradient.". Astronomy and Astrophysics 313: 119–128. Bibcode 1996A&A...313..119D. 
  65. ^ Gnedin, Oleg Y.; Lee, Hyung Mok; Ostriker, Jeremiah P. (September 1999). "Effects of Tidal Shocks on the Evolution of Globular Clusters". The Astrophysical Journal 522 (2): 935–949. arXiv:astro-ph/9806245. Bibcode 1999ApJ...522..935G. doi:10.1086/307659. 
  66. ^ Ibata, R.; Chapman, S.; Ferguson, A. M. N.; Lewis, G.; Irwin, M.; Tanvir, N. (2005). "On the accretion origin of a vast extended stellar disk around the Andromeda Galaxy". Astrophysical Journal 634 (1): 287–313. arXiv:astro-ph/0504164. Bibcode 2005ApJ...634..287I. doi:10.1086/491727. 
  67. ^ "Outer Disk Ring?". SolStation. http://www.solstation.com/x-objects/gal-ring.htm. Retrieved 2007-05-10. 
  68. ^ Dennis Overbye: Bubbles of Energy Are Found in Galaxy. The New York Times, November 9, 2010
  69. ^ Rätselhafte Blasen im All. Süddeutsche Zeitung, retrieved November 10, 2010
  70. ^ Reid, Mark J. (1993). "The distance to the center of the galaxy". Annual review of astronomy and astrophysics 31 (1): 345–372. Bibcode 1993ARA&A..31..345R. doi:10.1146/annurev.aa.31.090193.002021. 
  71. ^ Majaess, D. J.; Turner D. G., Lane D. J. (2009). "Characteristics of the Galaxy according to Cepheids". MNRAS 398 (1): 263–270. Bibcode 2009MNRAS.398..263M. doi:10.1111/j.1365-2966.2009.15096.x. 
  72. ^ English, Jayanne (1991-07-24). "Exposing the Stuff Between the Stars". Hubble News Desk. http://www.ras.ucalgary.ca/CGPS/press/aas00/pr/pr_14012000/pr_14012000map1.html. Retrieved 2007-05-10. 
  73. ^ Gillman, M. and Erenler, H. (2008). "The galactic cycle of extinction". International Journal of Astrobiology 7 (01). Bibcode 2008IJAsB...7...17G. doi:10.1017/S1473550408004047. 
  74. ^ Andrew C. Overholt, Adrian L. Melott, and Martin Pohl (2009). "Testing the link between terrestrial climate change and Galactic spiral arm transit". The Astrophysical Journal Letters 705 (2): L101. Bibcode 2009ApJ...705L.101O. doi:10.1088/0004-637X/705/2/L101. 
  75. ^ Leong, Stacy (2002). "Period of the Sun's Orbit around the Galaxy (Cosmic Year)". The Physics Factbook. http://hypertextbook.com/facts/2002/StacyLeong.shtml. Retrieved 2007-05-10. 
  76. ^ Garlick, Mark Antony (2002). The Story of the Solar System. Cambridge University. p. 46. ISBN 0521803365. 
  77. ^ "Milky Way galaxy is warped and vibrating like a drum" (Press release). University of California, Berkeley. 2006-01-09. http://www.berkeley.edu/news/media/releases/2006/01/09_warp.shtml. Retrieved 2007-10-18. 
  78. ^ Wong, Janet (April 14, 2000). "Astrophysicist maps out our own galaxy's end". University of Toronto. Archived from the original on 2007-01-08. http://web.archive.org/web/20070108183824/http://www.news.utoronto.ca/bin/000414b.asp. Retrieved 2007-01-11. 
  79. ^ Peter Schneider (2006). Extragalactic Astronomy and Cosmology. Springer. p. 4, Figure 1.4. ISBN 3540331743. http://books.google.com/?id=uP1Hz-6sHaMC&pg=PA100&dq=rotation+Milky+way. 
  80. ^ Theo Koupelis, Karl F Kuhn (2007). In Quest of the Universe. Jones & Bartlett Publishers. p. 492; Figure 16-13. ISBN 0763743879. http://books.google.com/?id=6rTttN4ZdyoC&pg=PA491&dq=Milky+Way+%22rotation+curve%22. 
  81. ^ Mark H. Jones, Robert J. Lambourne, David John Adams (2004). An Introduction to Galaxies and Cosmology. Cambridge University Press. p. 21; Figure 1.13. ISBN 0521546230. http://books.google.com/?id=36K1PfetZegC&pg=PA20&dq=Milky+Way+%22rotation+curve%22. 
  82. ^ Mark H. Jones, Robert J. Lambourne, David John Adams (2004). An Introduction to Galaxies and Cosmology. Cambridge University Press. p. 298. ISBN 0521546230. http://books.google.com/?id=36K1PfetZegC&pg=PA4&dq=movement+%22Milky+Way%22. 
  83. ^ Peirani, S; Defreitaspacheco, J (2006). "Mass determination of groups of galaxies: Effects of the cosmological constant". New Astronomy 11 (4): 325. arXiv:astro-ph/0508614. Bibcode 2006NewA...11..325P. doi:10.1016/j.newast.2005.08.008. 
  84. ^ Theo Koupelis, Karl F. Kuhn (2007). In Quest of the Universe. Jones & Bartlett Publishers. pp. 492, Figure 16–13. ISBN 0763743879. http://books.google.com/?id=6rTttN4ZdyoC&pg=PA491&dq=Milky+Way+%22rotation+curve%22+date:2004-2010. 
  85. ^ Waller, William H.; Paul W. Hodge (2003). "The Milky Way". Galaxies and the Cosmic Frontier. Harvard University Press. p. 91. ISBN 0674010795. 
  86. ^ a b A M T Jackson, R.E. Enthoven (1989). Folk Lore Notes. Asian Educational Services. ISBN 8120604857. http://books.google.com/?id=eRkuXFD93zsC. "... According to the Puranas, the milky way or akashganga is the celestial River Ganga which was brought down by Bhagirath ..." 
  87. ^ Hormusjee Shapoorjee Spencer (1965). The Aryan ecliptic cycle: glimpses into ancient Indo-Iranian religious history from 25628 B.C. to 292 A.D.. H.P. Vaswani. http://books.google.com/?id=jdMoAAAAYAAJ. "... There are two "Gangas"—one terrestrial and the other "akashic" or celestial ... bear reference only to the "Akash Ganga" which is the Milky Way ..." 
  88. ^ Edward C. Sachau (2001). Alberuni's India: an account of the religion, philosophy, literature, geography, chronology, astronomy, customs, laws and astrology of India about A.D. 1030. Routledge. ISBN 9780415244978. http://books.google.com/?id=a91-t4uw8A4C. "... revolves around Kshira, i.e. the Milky Way ..." 
  89. ^ a b Josep Puig Montada (September 28, 2007). "Ibn Bajja". Stanford Encyclopedia of Philosophy. http://plato.stanford.edu/entries/ibn-bajja. Retrieved 2008-07-11. 
  90. ^ Heidarzadeh, Tofigh (2008). A history of physical theories of comets, from Aristotle to Whipple. Springer. pp. 23–25. ISBN 140208322X. 
  91. ^ Mohamed, Mohaini (2000). Great Muslim Mathematicians. Penerbit UTM. pp. 49–50. ISBN 9835201579. 
  92. ^ Hamid-Eddine Bouali, Mourad Zghal, Zohra Ben Lakhdar (2005). "Popularisation of Optical Phenomena: Establishing the First Ibn Al-Haytham Workshop on Photography" (PDF). The Education and Training in Optics and Photonics Conference. http://spie.org/etop/ETOP2005_080.pdf. Retrieved 2008-07-08. 
  93. ^ O'Connor, John J.; Robertson, Edmund F., "Abu Rayhan Muhammad ibn Ahmad al-Biruni", MacTutor History of Mathematics archive, University of St Andrews, http://www-history.mcs.st-andrews.ac.uk/Biographies/Al-Biruni.html .
  94. ^ Livingston, John W. (1971). "Ibn Qayyim al-Jawziyyah: A Fourteenth Century Defense against Astrological Divination and Alchemical Transmutation". Journal of the American Oriental Society (American Oriental Society) 91 (1): 96–103 [99]. doi:10.2307/600445. JSTOR 600445. 
  95. ^ Ragep, Jamil, Nasir al-Din al-Tusi’s Memoir on Astronomy (al-Tadhkira fi `ilm al-hay’ a) Edition, Translation, Commentary and Introduction. 2 vols. Sources in the History of Mathematics and Physical Sciences. New York: Springer-Verlag, 1993. pp. 129
  96. ^ O'Connor, J. J.; Robertson, E. F. (November 2002). "Galileo Galilei". University of St Andrews. http://www-gap.dcs.st-and.ac.uk/~history/Biographies/Galileo.html. Retrieved 2007-01-08. 
  97. ^ Evans, J. C. (November 24, 1998). "Our Galaxy". George Mason University. http://physics.gmu.edu/~jevans/astr103/CourseNotes/ECText/ch20_txt.htm. Retrieved 2007-01-04. 
  98. ^ Abbey, Lenny. "The Earl of Rosse and the Leviathan of Parsontown". The Compleat Amateur Astronomer. http://labbey.com/Telescopes/Parsontown.html. Retrieved 2007-01-04. 
  99. ^ Heber D. Curtis (1988). "Novae in Spiral Nebulae and the Island Universe Theory". Publications of the Astronomical Society of the Pacific 100: 6. Bibcode 1988PASP..100....6C. doi:10.1086/132128. 
  100. ^ Weaver, Harold F.. "Robert Julius Trumpler". National Academy of Sciences. http://www.nap.edu/readingroom/books/biomems/rtrumpler.html. Retrieved 2007-01-05. 
  101. ^ Hubble, E. P. (1929). "A spiral nebula as a stellar system, Messier 31". Astrophysical Journal 69: 103–158. Bibcode 1929ApJ....69..103H. doi:10.1086/143167. 
  102. ^ Sandage, Allan (1989). "Edwin Hubble, 1889–1953". Journal of the Royal Astronomical Society of Canada 83 (6). http://antwrp.gsfc.nasa.gov/diamond_jubilee/1996/sandage_hubble.html. Retrieved 2007-01-08. 
  103. ^ Freedman, Roger A.; Kaufmann, William J. (2007). Universe. WH Freeman & Co.. p. 605. ISBN 0-7167-8584-6.  "Galaxies — Milky Way Galaxy". Encyclopedia Britannica. 19. Encyclopedia Britannica, Inc.. 1998. pp. 618.  Pasachoff, Jay M. (1994). Astronomy: From the Earth to the Universe. Harcourt School. p. 500. ISBN 0-03-001667-3. 

Further reading

External links

v · d · eThe Milky Way
Location
Milky Way subgroup → Local Group → Virgo Supercluster → Pisces-Cetus Supercluster Complex → Observable universe → Universe
The Milky Way Galaxy
Galactic core
Spiral arms
Scutum–Centaurus · Perseus · Norma–Cygnus · Carina–Sagittarius · Orion–Cygnus
Satellite galaxies
Sagittarius Stream  · Boötes II · Coma Berenices · Messier 54 · Palomar 12 · Segue 1 · Segue 2 · Terzan 7
Boötes I · Boötes III · Canes Venatici I · Canes Venatici II · Canis Major · Carina · Draco · Fornax · Hercules · Leo I · Leo II · Leo IV · Leo V · Phoenix · Pisces I · Pisces II · Sculptor · Sextans · Ursa Major I · Ursa Major II · Ursa Minor
Other
Monoceros Ring · Virgo Stream · Koposov I · Koposov II · Segue 3 · Willman 1
v · d · eEarth's location in the universe
Earth → Solar System → Local Interstellar Cloud → Local Bubble → Gould Belt → Orion–Cygnus Arm → Milky Way → Milky Way subgroup → Local Group → Virgo Supercluster → Pisces-Cetus Supercluster Complex → Observable universe → Universe
Each arrow should be read as "within" or "part of".


Wikimedia Foundation. 2010.

Look at other dictionaries:

  • Milky Way — Тип: Кондитерское изделие Владелец марки …   Википедия

  • Milky way — La Milky Way est une barre de chocolat produite par Mars Incorporated. La version américaine de la barre Milky Way est faite de chocolat malt nougat recouverte de caramel puis de chocolat au lait et est similaire à la barre Mars européenne. La… …   Wikipédia en Français

  • Milky Way — es una chocolatina elaborada y distribuida por Mars Incorporated. La barrita original, disponible en Estados Unidos, presenta un centro de nougat y chocolate o vainilla, con caramelo en la parte superior, y está cubierta por una capa de chocolate …   Wikipedia Español

  • Milky Way — Milky Milk y, a. 1. Consisting of, or containing, milk. [1913 Webster] Pails high foaming with a milky flood. Pope. [1913 Webster] 2. Like, or somewhat like, milk; whitish and turbid; as, the water is milky. Milky juice. Arbuthnot. [1913 Webster] …   The Collaborative International Dictionary of English

  • Milky way — Way Way, n. [OE. wey, way, AS. weg; akin to OS., D., OHG., & G. weg, Icel. vegr, Sw. v[ a]g, Dan. vei, Goth. wigs, L. via, and AS. wegan to move, L. vehere to carry, Skr. vah. [root]136. Cf. {Convex}, {Inveigh}, {Vehicle}, {Vex}, {Via}, {Voyage} …   The Collaborative International Dictionary of English

  • Milky Way — n the Milky Way the pale white band of stars that can be seen across the sky at night …   Dictionary of contemporary English

  • milky way — late 14c., loan translation of L. via lactea. See also GALAXY (Cf. galaxy) …   Etymology dictionary

  • Milky Way — ► NOUN ▪ the galaxy of which our solar system is a part, visible at night as a faint band of light crossing the sky …   English terms dictionary

  • Milky Way — the [transl. of L via lactea, transl. of Gr galaxias kyklos: see GALAXY] the spiral galaxy containing our sun: seen from the earth as a broad, faintly luminous band of stars and interstellar gas arching across the night sky, with the… …   English World dictionary

  • Milky Way —  Pour l’article homonyme, voir MilkyWay.  La Milky Way est une barre de chocolat produite par Mars Incorporated. La version américaine de la barre Milky Way est faite de chocolat malt nougat recouverte de caramel puis de chocolat au… …   Wikipédia en Français

Share the article and excerpts

Direct link
… Do a right-click on the link above
and select “Copy Link”