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For other uses, see Milky Way (disambiguation).
Milky Way Galaxy

Infrared image of the core of the Milky Way galaxy
Observation data
Type SBbc (barred spiral galaxy)
Diameter 100,000 light years
Thickness 12,000 light years (gas)[1]
1,000 light years (stars)
Number of stars 200 to 400 billion
Oldest star 13. Galaxy morphological classification is a system used by Astronomers to divide galaxies into groups based on their visual appearance A barred spiral galaxy is a Spiral galaxy with a central bar-shaped structure composed of Stars Bars are found in approximately half of all spiral galaxies 2 billion years
Mass 5. 8×1011 M☉
Sun's distance to galactic center 26,000 ± 1400 light-years
Sun's galactic rotation period 220 million years (negative rotation)
Spiral pattern rotation period 50 million years[2]
Bar pattern rotation period 15 to 18 million years[2]
Speed relative to CMB rest frame 552 km/s[3]
See also: Galaxy, List of galaxies
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The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias) sometimes referred to simply as "the Galaxy"), is a barred spiral galaxy that is part of the Local Group of galaxies. The solar mass is a standard way to express Mass in Astronomy, used to describe the masses of other Stars and galaxies. The Galactic Center is the rotational center of the Milky Way Galaxy. The Sun (Sol is the Star at the center of the Solar System. Density wave theory or the Lin-Shu density wave theory is a theory proposed by C A barred spiral galaxy is a Spiral galaxy with a central bar-shaped structure composed of Stars Bars are found in approximately half of all spiral galaxies A galaxy is a massive gravitationally bound system consisting of Stars an Interstellar medium of gas and dust, and Dark matter This is a list of notable galaxies. List 4C 3711 Abell 1835 IR1916 AM 0644-741 Latin ( lingua Latīna, laˈtiːna is an Italic language, historically spoken in Latium and Ancient Rome. Greek (el ελληνική γλώσσα or simply el ελληνικά — "Hellenic" is an Indo-European language, spoken today by 15-22 million people mainly A galaxy is a massive gravitationally bound system consisting of Stars an Interstellar medium of gas and dust, and Dark matter A barred spiral galaxy is a Spiral galaxy with a central bar-shaped structure composed of Stars Bars are found in approximately half of all spiral galaxies The Local Group is the group of galaxies that includes our galaxy the Milky Way. Although the Milky Way is one of billions of galaxies in the observable universe,[4] the Galaxy has special significance to humanity as it is the home galaxy of the planet Earth. In Big Bang Cosmology, the observable universe is the region of space bounded by a Sphere, centered on the observer that is small enough that EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 The plane of the Milky Way galaxy is visible from Earth as a band of light in the night sky, and it is the appearance of this band of light which has inspired the name for our galaxy.

Some sources hold that, strictly speaking, the term Milky Way should refer exclusively to the observation of the band of light, while the full name Milky Way Galaxy, or alternatively the Galaxy should be used to describe our galaxy as an astrophysical whole. [5][6][7] It is unclear how widespread the usage of this convention is, however, and the term Milky Way is routinely used in either context. Context is a notion used in the language sciences ( Linguistics, Sociolinguistics, Discourse analysis, Pragmatics, Semiotics, etc

Contents

View from Earth

The Milky Way galaxy, as viewed from the 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 originating from stars and other material which lie within the galactic plane. EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply White is a Color, the perception which is evoked by Light that stimulates all three types of color sensitive Cone cells in the Human eye Light, or visible light, is Electromagnetic radiation of a Wavelength that is visible to the Human eye (about 400–700 In Astronomy and Navigation, the celestial sphere is an imaginary rotating Sphere of "gigantic Radius " A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth

The Milky Way looks brightest in the direction of the constellation of Sagittarius, toward the galactic center. In common usage a constellation is a group of celestial bodies that are connected together in some arrangement typically stars to form a visible figure or picture The Galactic Center is the rotational center of the Milky Way Galaxy. 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 celestial equator is a Great circle on the imaginary Celestial sphere, in the same plane as the Earth 's Equator. Cassiopeia ( colloquially /ˌkæsiˈoʊpiə/ is a northern Constellation which Greek mythology considered to represent the vain queen Cassiopeia who boasted CRUX is a lightweight I686 -optimized Linux distribution targeted at experienced Linux users The equator (sometimes referred to colloquially as "the Line") is the intersection of the Earth 's surface with the plane perpendicular to the The ecliptic is the apparent path that the Sun traces out in the sky during the year 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. In Astronomy and Navigation, the celestial sphere is an imaginary rotating Sphere of "gigantic Radius " The galactic coordinate system is a Celestial coordinate system which is centered on the Sun and is aligned with the apparent center of the Milky Way galaxy The Milky Way has a relatively low surface brightness, making it difficult to see from any urban or suburban location suffering from light pollution. Surface brightness is a concept used in Astronomy when describing extended Astronomical objects such as galaxies and Nebulae General South San Jose (cropjpg||thumb|A suburban development in San Jose California. Light pollution, also known as photopollution or luminous pollution, is excess or obtrusive Light created mainly by Humans Among other effects

360-degree photographic panorama of the galaxy.

The Milky Way as seen from Death Valley, 2007. Death Valley is the lowest driest and hottest Valley in the United States. This is a panoramic picture. In its most general sense a panorama is any wide view of a physical space

Size

The stellar disk of the Milky Way galaxy is approximately 100,000 light years in diameter, and is believed to be, on average, about 1,000 light years thick. [8] It is estimated to contain at least 200 billion stars[9] and possibly up to 400 billion stars,[10] 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 light years - twice the previously accepted value. [11] As a guide to the relative physical scale of the Milky Way, if it were reduced to 130 km (80 mi) in diameter, the Solar System would be a mere 2 mm (0. The concept of scale is applicable if a system is represented proportionally by another system 08 inches) in width.

The Galactic Halo extends outward, but is limited in size by the orbits of the two Milky Way satellites, the Large and the Small Magellanic Clouds, whose perigalacticon is at ~180,000 light-years. History The Magellanic Clouds were certainly known since the earliest times by the ancient Middle Eastern peoples In Celestial mechanics, an apsis, plural apsides (ˈæpsɨdɪːz is the point of greatest or least distance of the Elliptical orbit of an object from A light-year or light year (symbol ly) is a unit of Length, equal to just under ten trillion Kilometres As defined by [12]

Age

A green and red Perseid meteor is striking the sky just below the Milky Way in August 2007.
A green and red Perseid meteor is striking the sky just below the Milky Way in August 2007. The Perseids (ˈpɝsiːɨdz pûr'sē-ĭdz is a prolific Meteor shower associated with the Comet Swift-Tuttle.
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 is estimated to be about 13. The study of galaxy formation and evolution is concerned with the processes that formed a heterogeneous universe from a homogeneous beginning the formation of the first galaxies the way Nucleocosmochronology, also known as cosmochronology, is a relatively new technique used to determine timescales for astrophysical objects and events HE 1523-0901 is the designation given to a Red giant Star located in the Milky Way galaxy 2 billion years, nearly as old as the Universe itself. The age of the Universe is the time elapsed between the theory of the Big Bang and the present day

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. The Very Large Telescope ( VLT) is a system of four separate Optical telescopes (the Antu telescope, the Kueyen telescope, the Melipal Measurement is the process of estimating the magnitude of some attribute of an object such as its length or weight relative to some standard ( unit of measurement) such as Beryllium (bəˈrɪliəm is a Chemical element with the symbol Be and Atomic number 4 A globular cluster is a spherical collection of Stars that orbits a galactic core as a Satellite. NGC 6397 is a Globular cluster in the Ara Constellation. It is located about 7200 Light-years from Earth, making it one of the [13]. 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.

Composition and structure

NGC 7331 is often referred to as "the Milky Way's twin." This is what observers from another galaxy might see when looking at our own. IC 342 is another galaxy that may resemble the Milky Way.
NGC 7331 is often referred to as "the Milky Way's twin. NGC 7331 is a Spiral galaxy about 40 million Light-years away in the Constellation Pegasus. " [14] This is what observers from another galaxy might see when looking at our own. IC 342 is another galaxy that may resemble the Milky Way. IC 342 is an Intermediate spiral galaxy in the Constellation Camelopardalis.

Observations by the Spitzer Space Telescope in 2005[15] confirmed the nature of the Milky Way as a barred spiral galaxy. The Spitzer Space Telescope (formerly the Space Infrared Telescope Facility, SIRTF) is an Infrared Space observatory. A barred spiral galaxy is a Spiral galaxy with a central bar-shaped structure composed of Stars Bars are found in approximately half of all spiral galaxies 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). Definition In Polar coordinates ( r, θ the curve can be written as r = ae^{b\theta}\ or \theta The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply The mass distribution within the Galaxy closely resembles the Sbc Hubble classification, which is a spiral galaxy with relatively loosely-wound arms. Galaxy morphological classification is a system used by Astronomers to divide galaxies into groups based on their visual appearance [16] Astronomers in the 1980s first began to suspect that the Milky Way is a barred spiral galaxy[17] rather than an ordinary spiral galaxy and their suspicions were confirmed by the Spitzer Space Telescope observations in 2005[15] which showed the Galaxy's central bar to be larger than previously suspected. A spiral galaxy is a Galaxy belonging to one of the three main classes of galaxy originally described by Edwin Hubble in his 1936 work “The Realm of the The Spitzer Space Telescope (formerly the Space Infrared Telescope Facility, SIRTF) is an Infrared Space observatory. As of 2006, the Milky Way's mass is thought to be about 5. Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object 8×1011 M☉[18][19][20] comprising 200 to 400 billion stars. The solar mass is a standard way to express Mass in Astronomy, used to describe the masses of other Stars and galaxies. 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 solar masses (M☉) which is spread out relatively evenly. In Physics and cosmology, dark matter is hypothetical Matter that does not interact with the electromagnetic force but whose presence can be inferred from Most of the mass of any galaxy is dominated by a component concentrated at the centre of the galaxy but dominating its dynamics throughout known as the Dark matter halo. The solar mass is a standard way to express Mass in Astronomy, used to describe the masses of other Stars and galaxies. [20]

Galactic center

Main article: Galactic Center
The galactic center in the direction of Sagittarius. The primary stars of Sagittarius are indicated in red.
The galactic center in the direction of Sagittarius. The Galactic Center is the rotational center of the Milky Way Galaxy. The primary stars of Sagittarius are indicated in red.

The galactic disc, which bulges outward at the galactic center, has a diameter of between 70,000 and 100,000 light-years. A light-year or light year (symbol ly) is a unit of Length, equal to just under ten trillion Kilometres As defined by [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 Sun (Sol is the Star at the center of the Solar System.

The galactic center harbors a compact object of very large mass (named Sagittarius A*), strongly suspected to be a supermassive black hole. Sagittarius A* (pronounced "A-star" standard abbreviation Sgr A*) is a bright and very compact source of Radio emission at the center of A supermassive black hole is a Black hole with a Mass of an order of magnitude between 105 and 1 Most galaxies are believed to have a supermassive black hole at their center. [22]

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). According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool Star, of the Main sequence, either late K A red giant is a luminous Giant star of low or intermediate mass (roughly 0 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. History The first direct measurements of an object at interstellar distances were undertaken by German Astronomer Friedrich Wilhelm Bessel in 1838 Star Formation is the process by which dense parts of Molecular clouds collapse into a ball of plasma to form a Star. Viewed from the Andromeda Galaxy, it would be the brightest feature of our own galaxy. The Andromeda Galaxy (ænˈdrɒmədə also known as Messier 31, M31, or NGC 224; often referred to as the Great Andromeda [23]

Spiral arms

Each spiral arm describes a logarithmic spiral (as do the arms of all spiral galaxies) with a pitch of approximately 12 degrees. Definition In Polar coordinates ( r, θ the curve can be written as r = ae^{b\theta}\ or \theta There are believed to be four major spiral arms which all start at the Galaxy's center. These are named as follows, according to the image at left:

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

Outside of the major spiral arms is the Outer Ring or Monoceros Ring, a ring of stars around the Milky Way proposed by astronomers Brian Yanny and Heidi Jo Newberg, which consists of gas and stars torn from other galaxies billions of years ago. History The first direct measurements of an object at interstellar distances were undertaken by German Astronomer Friedrich Wilhelm Bessel in 1838 The Perseus Spiral Arm is a major Spiral arm of the Milky Way galaxy The Milky Way Galaxy in which the Sun dwells in common with many other galaxies of similar type consists of a large mass of Stars drawn by gravitational The Milky Way Galaxy in which the Sun dwells in common with many other galaxies of similar type consists of a large mass of Stars drawn by gravitational The Scutum-Crux arm (also known as Centaurus Arm) is a secondary Spiral arm of the Milky Way. The Sagittarius Arm (also known as Sagittarius-Carina Arm; labeled "-I" is generally thought to be one of the Spiral arms of our home Galaxy The Orion Arm is a minor Spiral arm of the Milky Way galaxy The Solar System and Earth are within the Orion Arm The Sun (Sol is the Star at the center of the Solar System. The Monoceros Ring is a proposed ring of Stars around the Milky Way which consists of a Stellar stream torn from the Canis Major Dwarf Galaxy The Monoceros Ring is a proposed ring of Stars around the Milky Way which consists of a Stellar stream torn from the Canis Major Dwarf Galaxy

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. The orbital speed of a body generally a Planet, a Natural satellite, an artificial satellite, or a Multiple star, is the speed at which it Away from the central bulge or outer rim, the typical stellar velocity is between 210 and 240 km/s. [24] Hence the orbital period of the typical star is directly proportional only to the length of the path traveled. The orbital period is the time taken for a given object to make one complete Orbit about another object This is unlike in the Solar System where different orbits are also expected to have significantly different velocities associated with them, and is one of the major pieces of evidence for the existence of dark matter. In Physics and cosmology, dark matter is hypothetical Matter that does not interact with the electromagnetic force but whose presence can be inferred from Another interesting aspect is the so-called "wind-up problem" of the spiral arms. If one believes that 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 arms form as a result of a matter-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. Thus this results in several spiral arms where there are a lot of stars and gas. 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.

Artist's conception of the spiral structure of the Milky Way with two major, stellar arms and a bar.
Artist's conception of the spiral structure of the Milky Way with two major, stellar arms and a bar. [25]

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 University of Wisconsin&ndashWhitewater (also known as UW-Whitewater and abbreviated as UWW) is part of the University of Wisconsin System, located The rest of the arms are minor or adjunct arms. [25]

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,[26] suggesting a stellar halo diameter of 200,000 light-years. A spiral galaxy is a Galaxy belonging to one of the three main classes of galaxy originally described by Edwin Hubble in his 1936 work “The Realm of the A globular cluster is a spherical collection of Stars that orbits a galactic core as a Satellite. 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. While the disk contains gas and dust obscuring the view in some wavelengths, the spheroid component does not. Active star formation takes place in the disk (especially in the spiral arms, which represent areas of high density), but not in the halo. Star Formation is the process by which dense parts of Molecular clouds collapse into a ball of plasma to form a Star. Open clusters also occur primarily in the disk. An open cluster is a group of up to a few thousand Stars that were formed from the same Giant molecular cloud, and are still loosely gravitationally

Recent discoveries have added dimension to the knowledge of the Milky Way's structure. With the discovery that the disc of the Andromeda Galaxy (M31) extends much further than previously thought,[27] the possibility of the disk of the Galaxy extending further is apparent, and this is supported by evidence of the newly discovered Outer Arm extension of the Cygnus Arm. The Andromeda Galaxy (ænˈdrɒmədə also known as Messier 31, M31, or NGC 224; often referred to as the Great Andromeda The Milky Way Galaxy in which the Sun dwells in common with many other galaxies of similar type consists of a large mass of Stars drawn by gravitational [28] With the discovery of the Sagittarius Dwarf Elliptical Galaxy came the discovery of a ribbon of galactic debris as the polar orbit of Sagittarius and its interaction with the Milky Way tears it apart. The Sagittarius Dwarf Elliptical Galaxy ( SagDEG) is an elliptically looped shaped Satellite galaxy of the Milky Way Galaxy. 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. The Canis Major Dwarf galaxy is located in the same part of the sky as the Constellation of Canis Major.

On January 9, 2006, Mario Juric 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. Events 475 - Byzantine Emperor Zeno is forced to flee his capital at Constantinople. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Mario Jurić is a Croatian Astronomer. He is a prolific discoverer of Asteroids and has also discovered one Comet, 183P/Korlević-Jurić, Princeton University is a private Coeducational research university located in Princeton, New Jersey. The Sloan Digital Sky Survey or SDSS is a major multi-filter imaging and spectroscopic Redshift survey using a dedicated 2 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. A dwarf galaxy is a small Galaxy composed of up to several billion Stars a small number compared to our own Milky Way 's 200-400 billion stars This galaxy is tentatively named the Virgo Stellar Stream and is found in the direction of Virgo about 30,000 light-years away. The Virgo Stellar Stream (also known as Virgo Overdensity, Virgo Dwarf Galaxy, or Virgo super star cluster) is the proposed Virgo ( Virgin, symbol, Unicode ♍ is a Constellation of the Zodiac.

Sun's location

The Sun (and therefore the Earth and Solar System) may be found close to the inner rim of the Galaxy's Orion Arm, in the Local Fluff or the Gould Belt, at a hypothesized distance of 7. The Sun (Sol is the Star at the center of the Solar System. EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 The Solar System consists of the Sun and those celestial objects bound to it by Gravity. The Orion Arm is a minor Spiral arm of the Milky Way galaxy The Solar System and Earth are within the Orion Arm The Local Interstellar Cloud, casually called the Local Fluff, is the Interstellar cloud (roughly 30 light years across through which our Solar system The Gould Belt is a partial ring of Stars about 3000 Light years across tilted toward the Galactic plane by about 16 to 20 degrees 62±0. 32 kpc from the Galactic Center. History The first direct measurements of an object at interstellar distances were undertaken by German Astronomer Friedrich Wilhelm Bessel in 1838 The Galactic Center is the rotational center of the Milky Way Galaxy. [29][30][31][32] The distance between the local arm and the next arm out, the Perseus Arm, is about 6,500 light-years. The Perseus Spiral Arm is a major Spiral arm of the Milky Way galaxy [33] The Sun, and thus the Solar System, is found in what scientists call the galactic habitable zone. In Astronomy a habitable zone ( HZ) is a region of space where conditions are favorable for Life as it may be found on Earth.

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 solar apex, or the Apex of the Sun's Way refers to the direction that the Sun travels through space 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. Vega (α Lyr / α Lyrae / Alpha Lyrae ( or) is the brightest Star in the Constellation Lyra, the fifth brightest star in the night Hercules (ˈhɝkjəliːz ˈhɝkjuːliːz is the fifth largest of the 88 modern Constellations It was also one of Ptolemy 's 48 constellations The Galactic Center is the rotational center of the Milky Way Galaxy. 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 (dampening) term. This article is about the harmonic oscillator in classical mechanics Due to the relatively higher density of stars close to the galactic plane, these oscillations often coincide with mass extinction periods on earth, presumably due to increased impact events. An extinction event (also known as mass extinction; extinction-level event, ELE is a sharp decrease in the number of Species in a relatively short period See also Impact crater An impact event is the Collision of a large Meteoroid, Asteroid or Comet (generically [34]

It takes the Solar System about 225–250 million years to complete one orbit of the galaxy (a galactic year),[35] so it is thought to have completed 20–25 orbits during the lifetime of the Sun and 1/1250th of a revolution since the origin of humans. The galactic year, also known as a cosmic year, is the duration of time required for the Solar system to Orbit once around the center of the Milky Human evolution, or anthropogenesis, is the part of biological Evolution concerning the emergence of Homo sapiens as a distinct Species The orbital speed of the Solar System about the center of the Galaxy is approximately 220 km/s. The orbital speed of a body generally a Planet, a Natural satellite, an artificial satellite, or a Multiple star, is the speed at which it At this speed, it takes around 1400 years for the Solar System to travel a distance of 1 light-year, or 8 days to travel 1 AU. The astronomical unit ( AU or au or au or sometimes ua) is a unit of Length based on the distance from the Earth to the [36]

Environment

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.
Broad infrared view of our Milky Way Galaxy from the Spitzer Space Telescope created from more than 800,000 frames. The Spitzer Space Telescope (formerly the Space Infrared Telescope Facility, SIRTF) is an Infrared Space observatory. This is the most detailed infrared picture of our galaxy to date.
Main articles: Local group and Andromeda-Milky Way collision

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. The Local Group is the group of galaxies that includes our galaxy the Milky Way. The Andromeda-Milky Way collision is a predicted Galaxy collision, due to take place in approximately three billion years time between the two largest galaxies in the Local The Andromeda Galaxy (ænˈdrɒmədə also known as Messier 31, M31, or NGC 224; often referred to as the Great Andromeda A binary system is an astronomical term referring to two objects in space (usually Stars but also Planets galaxies or Asteroids which The Local Group is the group of galaxies that includes our galaxy the Milky Way. The Virgo Supercluster or Local Supercluster is the galactic Supercluster that contains the Local Group, the latter containing in its turn

Two smaller galaxies and a number of dwarf galaxies in the Local Group orbit the Milky Way. A dwarf galaxy is a small Galaxy composed of up to several billion Stars a small number compared to our own Milky Way 's 200-400 billion stars In Physics, an orbit is the gravitationally curved path of one object around a point or another body for example the gravitational orbit of a planet around a star The largest of these is the Large Magellanic Cloud with a diameter of 20,000 light-years. The Large Magellanic Cloud (LMC is a nearby Satellite galaxy of our own galaxy the Milky Way. It has a close companion, the Small Magellanic Cloud. The Small Magellanic Cloud (SMC is a Dwarf galaxy. It contains several hundred million stars The Magellanic Stream is a peculiar streamer of neutral hydrogen gas connecting these two small galaxies. Hierarchical Clustering tells us that galaxies are built up over time from collisions of smaller galaxies Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 The stream is thought to have been dragged from the Magellanic Clouds in tidal interactions with the Galaxy. 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 Canis Major Dwarf galaxy is located in the same part of the sky as the Constellation of Canis Major. The Sagittarius Dwarf Elliptical Galaxy ( SagDEG) is an elliptically looped shaped Satellite galaxy of the Milky Way Galaxy. The Ursa Minor Dwarf Dwarf elliptical galaxy was discovered by A The Sculptor Dwarf Galaxy (also called the Sculptor Dwarf Elliptical Galaxy or the Sculptor Dwarf Spheroidal Galaxy) is a Dwarf spheroidal galaxy that The Sextans Dwarf Spheroidal is a Dwarf spheroidal galaxy that was discovered in 1990 by Mike Irwin M The Fornax Dwarf Spheroidal is an elliptical Dwarf galaxy in the Constellation Fornax that was discovered in 1938 by Harlow Shapley. Leo I is a Dwarf spheroidal galaxy in the Constellation Leo. At about 820000 Light-years distant it is a member of the Local Group The smallest Milky Way dwarf galaxies are only 500 light-years in diameter. These include Carina Dwarf, Draco Dwarf, and Leo II Dwarf. The Carina Dwarf Spheroidal is a Dwarf galaxy in the Carina Constellation. The Draco Dwarf is a spheroidal galaxy which was discovered by Albert George Wilson of Lowell Observatory in 1954 on photographic plates of the Leo B (or Leo II) is an dwarf spheroidal Galaxy of the Local Group, part of the Milky Way subgroup in the Constellation of 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. The Zone of Avoidance (ZOA is the area of the night sky that is obscured by our own Galaxy, the Milky Way. 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. [37] 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. In Physics and cosmology, dark matter is hypothetical Matter that does not interact with the electromagnetic force but whose presence can be inferred from Taking dark matter into account results in an approximately twenty-fold increase in mass for the Galaxy. This calculation is according to a computer model made by Martin Weinberg of the University of Massachusetts, Amherst. The University of Massachusetts Amherst (otherwise known as UMass Amherst, Massachusetts, or UMass) is a research and Land-grant university in 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 Andromeda Galaxy (ænˈdrɒmədə also known as Messier 31, M31, or NGC 224; often referred to as the Great Andromeda 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, it is thought that the Sun and the other stars in the Milky Way will probably not collide with the stars of the Andromeda Galaxy, but that the two galaxies will merge to form a single elliptical galaxy over the course of about a billion years. An elliptical galaxy is a Galaxy belonging to one of the three main classes of galaxy originally described by Edwin Hubble (whose name was dedicated [38]

Velocity

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 Galaxy's motion. Albert Einstein ( German: ˈalbɐt ˈaɪ̯nʃtaɪ̯n; English: ˈælbɝt ˈaɪnstaɪn (14 March 1879 – 18 April 1955 was a German -born theoretical Special relativity (SR (also known as the special theory of relativity or STR) is the Physical theory of Measurement in Inertial In Physics, an inertial frame of reference is a Frame of reference which belongs to a set of frames in which Physical laws hold in the same and simplest (Motion must always be specified with respect to another object. )

Astronomers believe the Milky Way is moving at approximately 600 km 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 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 Galaxy is thought to be moving towards the constellation Hydra, and may someday become a close-knit member of the Virgo cluster of galaxies. Hydra (ˈhaɪdrə is the largest of the 88 modern Constellations and was also one of the 48 constellations listed by Ptolemy. The Virgo Cluster is a cluster of galaxies at a distance of approximately 59 ± 4 Mly (18

Another reference frame is provided by the cosmic microwave background (CMB). The Milky Way appears to be moving at around 552 km/s[3] with respect to the photons of the CMB. This motion is observed by satellites such as COBE and 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. The Doppler effect (or Doppler shift) named after Christian Doppler, is the change in Frequency and Wavelength of a Wave for In Physics and Astronomy, redshift occurs when Electromagnetic radiation – usually Visible light – emitted or reflected by

History

Etymology and beliefs

Jacopo Tintoretto's "The Origin of the Milky Way"
Jacopo Tintoretto's "The Origin of the Milky Way"
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. Tintoretto (real name Jacopo Comin; September 29, 1518 - May 31, 1594) was one of the greatest painters of the Venetian school and This is a list of the names for the Milky Way in various languages A creation myth is a supernatural mytho-[[religion religious]] story or explanation that describes the beginnings of humanity, Earth, life, and The English phrase is a translation from Greek Γαλαξίας, Galaxias, which is derived from the word for milk (γάλα, gala). English is a West Germanic language originating in England and is the First language for most people in the United Kingdom, the United States Greek (el ελληνική γλώσσα or simply el ελληνικά — "Hellenic" is an Indo-European language, spoken today by 15-22 million people mainly This is also the origin of the word galaxy. A galaxy is a massive gravitationally bound system consisting of Stars an Interstellar medium of gas and dust, and Dark matter In Greek myth, the Milky Way was caused by milk spilt by Hera when suckling Heracles. Greek mythology is the body of stories belonging to the ancient Greeks concerning their gods and Heroes the nature of the world and the origins and significance In the Olympian pantheon of classical Greek Mythology, Hera (ˈhɪərə or /ˈhɛrə/ Greek) or Here ( in Ionic and Homer In Greek mythology, Heracles or Herakles ("glory of Hera " or

The term Milky Way first appeared in English literature in a poem by Chaucer. The term English literature refers to Literature written in the English language, including literature composed in English by Writers not necessarily from Geoffrey Chaucer (c 1343 – 25 October 1400? was an English author poet Philosopher, bureaucrat, courtier and Diplomat.

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

—Geoffrey Chaucer, Geoffrey Chaucer The House of Fame, c. The House of Fame is a poem by Geoffrey Chaucer, it is one of his early works probably written between 1379 and 1380. 1380. [39]

In a large area from Central Asia to Africa, the name for the Milky Way is related to the word for straw. Central Asia is a region of Asia from the Caspian Sea in the west to central China in the east and from southern Russia in the north to northern Pakistan in the south Straw is an agricultural By-product, the dry stalk of a Cereal plant after the Grain or Seed has been removed It has been claimed that this was spread by Arabs who in turn borrowed the word from Armenian. The araB gene Promoter is a bacterial promoter activated by e L-arabinose binding The Armenian language (hy հայերեն լեզու hajɛɹɛn lɛzu —, conventional short form) is an Indo-European language spoken by the Armenian [40] In several Uralic, Turkic languages, Fenno-Ugric languages and in the Baltic languages the Milky Way is called the "Birds' Path". The Uralic languages (jʊˈrælɨk constitute a language family of 39 Languages spoken by approximately 20 million people The Turkic languages constitute a Language family of some thirty languages spoken by Turkic peoples across a vast area from Eastern Europe and the Finno-Ugric (ˌfɪnoʊˈjuːgɹɪk is a grouping of languages in the Uralic language family comprising Finnish, Estonian, Hungarian and The Baltic languages are a group of related languages belonging to the Indo-European language family and spoken mainly in areas extending east and southeast of the Baltic The Chinese name "Silver River" (銀河) is used throughout East Asia, including Korea and Japan. Korea is a geographic area composed of two sovereign countries a civilization and a former state situated on the Korean Peninsula in East Asia. For a topic outline on this subject see List of basic Japan topics. An alternative name for the Milky Way in ancient China, especially in poems, is "Heavenly River of Han"(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 ama no kawa). is a language spoken by over 130 million people in Japan and in Japanese emigrant communities In Swedish, it is called Vintergatan, or "Winter Street", because the stars in the belt were used to predict time of the approaching winter. Swedish ( is a North Germanic language spoken by more than nine million people predominantly in Sweden and parts of Finland, especially along the

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.
The shape of the Milky Way as deduced from star counts by William Herschel in 1785; the Solar System was assumed near center. A galaxy is a massive gravitationally bound system consisting of Stars an Interstellar medium of gas and dust, and Dark matter

As Aristotle informs us in Meteorologica (DK 59 A80), the Greek philosophers Anaxagoras (ca. Aristotle (Greek Aristotélēs) (384 BC – 322 BC was a Greek philosopher a student of Plato and teacher of Alexander the Great. Meteorology (or "Meteorologica" is a text by Aristotle which contains his theories about the earth sciences Ancient Greek philosophy focused on the role of Reason and Inquiry. Anaxagoras ( Greek: Ἀναξαγόρας c 500 BC &ndash 428 BC was a Pre-Socratic Greek philosopher famous for introducing the Cosmological 500 BC–428 BC) and Democritus (450–370 BC) proposed that the Milky Way might consist of distant stars. Democritus ( Greek:) was a pre-Socratic Greek Materialist Philosopher (born at Abdera in Thrace ca A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth The Persian astronomer Abū Rayhān al-Bīrūnī (973-1048 AD) further proposed the Milky Way galaxy to be a collection of countless nebulous stars. A galaxy is a massive gravitationally bound system consisting of Stars an Interstellar medium of gas and dust, and Dark matter A nebula (from Latin: "mist" pl nebulae or nebulæ, with ligature or nebulas) is an Interstellar cloud of [41] Actual proof of this 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. Galileo Galilei (15 February 1564 &ndash 8 January 1642 was a Tuscan ( Italian) Physicist, Mathematician, Astronomer, and Philosopher An optical telescope is a Telescope which is used to gather and focus light mainly from the visible part of the Electromagnetic spectrum [42] 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. Immanuel Kant (ɪmanuəl kant 22 April 1724 12 February 1804 was an 18th-century German Philosopher from the Prussian city of Königsberg Thomas Wright (1711 - 1786 was an English Astronomer, Mathematician, instrument maker Architect and garden designer Gravitation is a natural Phenomenon by which objects with Mass attract one another 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. A nebula (from Latin: "mist" pl nebulae or nebulæ, with ligature or nebulas) is an Interstellar cloud of [43]

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. The Sun (Sol is the Star at the center of the Solar System. Sir Frederick William Herschel FRS KH ( 15 November 1738 – 25 August 1822) was a German -born British 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
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. The Andromeda Galaxy (ænˈdrɒmədə also known as Messier 31, M31, or NGC 224; often referred to as the Great Andromeda William Parsons 3rd Earl of Rosse KP ( June 17, 1800 &ndash October 31, 1867) built the world's largest telescope in 1845 and it He also managed to make out individual point sources in some of these nebulae, lending credence to Kant's earlier conjecture. [44]

In 1917, Heber Curtis had observed the nova S Andromedae within the "Great Andromeda Nebula" (Messier object M31). Heber Doust Curtis ( June 27 1872 – January 9 1942) was an American Astronomer. |- style="background-color #A0B0FF" colspan="3"| Database References |- bgcolor="#FFFAFA" | Simbad || Data|- bgcolor="#FFFAFA" Andromeda ( is a Constellation named for the princess Andromeda ( Greek Ανδρομέδη = guardian of the men) a character in The Messier objects are a set of Astronomical objects first listed by French astronomer Charles Messier in his "Catalogue des Nébuleuses The Andromeda Galaxy (ænˈdrɒmədə also known as Messier 31, M31, or NGC 224; often referred to as the Great Andromeda Searching the photographic record, he found 11 more novae. A nova (pl novae or novas) is a Cataclysmic nuclear explosion caused by the accretion of hydrogen onto the surface of a White Curtis noticed that these novae were, on average, 10 magnitudes fainter than those that occurred within our galaxy. In Astronomy, magnitude refers to the Logarithmic measure of the brightness of an object measured in a specific Wavelength or Passband, usually 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. [45] 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. This is about the infamous discussion of astronomy For the Dream Theater song about Stem cell research, see The Great Debate (song. Harlow Shapley ( November 2 1885 &ndash October 20 1972) was an American Astronomer. 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. The Doppler effect (or Doppler shift) named after Christian Doppler, is the change in Frequency and Wavelength of a Wave for [46]

The matter was conclusively settled by Edwin Hubble in the early 1920s using a new telescope. Edwin Powell Hubble ( November 20, 1889 – September 28, 1953) was an American astronomer. 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. A Cepheid variable (pron ˈse-f(ē-id or ˈsē-f(ē-id or Cepheid is a member of a particular class of Variable stars notable for a fairly tight correlation [47] In 1936 Hubble produced a classification system for galaxies that is used to this day, the Hubble sequence. The Hubble sequence is a morphological classification scheme for Galaxies invented by Edwin Hubble in 1936 [48]


Further reading

See also

References

  1. ^ The Milky Way is twice the size we thought it was. The galactic coordinate system is a Celestial coordinate system which is centered on the Sun and is aligned with the apparent center of the Milky Way galaxy Smith's Cloud is a High velocity cloud of Hydrogen gas located in the constellation Aquila at Galactic coordinates l  = 39° University of Sydney News. University of Sydney (2008-02-20). 2008 ( MMVIII) is the current year in accordance with the Gregorian calendar, a Leap year that started on Tuesday of the Common Events 1472 - Orkney and Shetland are left by Norway to Scotland, due to a Dowry payment Retrieved on 2008-02-20. 2008 ( MMVIII) is the current year in accordance with the Gregorian calendar, a Leap year that started on Tuesday of the Common Events 1472 - Orkney and Shetland are left by Norway to Scotland, due to a Dowry payment
  2. ^ a b [astro-ph/0212516] Gas Dynamics in the Milky Way: Second Pattern Speed and Large-Scale Morphology
  3. ^ 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. doi:10.1086/173453. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.  
  4. ^ Between 1×1010 and 8×1010
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  43. ^ Evans, J. C. (November 24, 1998). Events 380 - Theodosius I makes his adventus, or formal Year 1998 ( MCMXCVIII) was a Common year starting on Thursday (link will display full 1998 Gregorian calendar) Our Galaxy. George Mason University. Retrieved on 2007-01-04. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 46 BC - Titus Labienus defeats Julius Caesar in the Battle of Ruspina.
  44. ^ Abbey, Lenny. The Earl of Rosse and the Leviathan of Parsontown. The Compleat Amateur Astronomer. Retrieved on 2007-01-04. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 46 BC - Titus Labienus defeats Julius Caesar in the Battle of Ruspina.
  45. ^ Heber D. Curtis (1988). Heber Doust Curtis ( June 27 1872 – January 9 1942) was an American Astronomer. "Novae in Spiral Nebulae and the Island Universe Theory". Publications of the Astronomical Society of the Pacific 100: 6.  
  46. ^ Weaver, Harold F. . Robert Julius Trumpler. National Academy of Sciences. Retrieved on 2007-01-05. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1477 - Battle of Nancy: Charles the Bold is killed and Burgundy becomes part of France.
  47. ^ E. P. Hubble (1929). Edwin Powell Hubble ( November 20, 1889 – September 28, 1953) was an American astronomer. "A spiral nebula as a stellar system, Messier 31". Astrophysical JournalEngl 69: 103–158. doi:10.1086/143167. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.  
  48. ^ Sandage, Allan (1989). "Edwin Hubble, 1889–1953". The Journal of the Royal Astronomical Society of Canada 83 (6).  

External links

Dictionary

Milky Way

-noun

  1. (astronomy) The galaxy in which Earth is located.
  2. (astronomy) A broad band of diffuse white light, visible in the night sky; our view of the Milky Way galaxy from inside.
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