The Milky Way's Galactic Center in the
night sky above Paranal Observatory (the laser creates a guide-star for the telescope). |
|
| Observation data | |
|---|---|
| Type | Sb, Sbc, or SB(rs)bc[1][2] (barred spiral galaxy) |
| Diameter | 100–180 kly (31–55 kpc)[3] |
| Thickness of thin stellar disk | ≈2 kly (0.6 kpc)[4][5] |
| Number of stars | 100–400 billion (2.5 × 1011 ± 1.5 × 1011)[6][7][8] |
| Oldest known star | ≥13.7 Gyr[9] |
| Mass | 0.8–1.5×1012 M☉[10][11][12][13] |
| Angular momentum | ≈1×1067 J s[14] |
| Sun's distance to Galactic Center | 26.4 ± 1.0 kly (8.09 ± 0.31 kpc)[15][16][17] |
| Sun's Galactic rotation period | 240 Myr[18] |
| Spiral pattern rotation period | 220–360 Myr[19] |
| Bar pattern rotation period | 100–120 Myr[19] |
| Speed relative to CMB rest frame | 631 ± 20 km/s[20] |
| Escape velocity at Sun's position | 550 km/s[13] |
| Dark matter density at Sun's position | |
| The Milky Way is the galaxy that contains our Solar System.The descriptive "milky" is derived from the appearance from Earth of the galaxy – a band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. The term Milky Way is a translation of the Latin via lactea, from the Greek γαλαξίας κύκλος (galaxías kýklos, "milky circle").[25][26][27] From Earth, the Milky Way appears as a band because its disk-shaped structure is viewed from within. Galileo Galilei first resolved the band of light into individual stars with his telescope in 1610. Until the early 1920s, most astronomers thought that the Milky Way contained all the stars in the Universe.[28] Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Curtis,[29] observations by Edwin Hubble showed that the Milky Way is just one of many galaxies. The Milky Way is a barred spiral galaxy with a diameter between 100,000[30] and 180,000 light-years.[31] The Milky Way is estimated to contain 100–400 billion stars.[32][33] There are probably at least 100 billion planets in the Milky Way.[34][35] The Solar System is located within the disk, about 26,000 light-years from the Galactic Center, on the inner edge of one of the spiral-shaped concentrations of gas and dust called the Orion Arm. The stars in the inner ≈10,000 light-years form a bulge and one or more bars that radiate from the bulge. The very center is marked by an intense radio source, named Sagittarius A*, which is likely to be a supermassive black hole. Stars and gases at a wide range of distances from the Galactic Center orbit at approximately 220 kilometers per second. The constant rotation speed contradicts the laws of Keplerian dynamics and suggests that much of the mass of the Milky Way does not emit or absorb electromagnetic radiation. This mass has been termed "dark matter".[36] The rotational period is about 240 million years at the position of the Sun.[18] The Milky Way as a whole is moving at a velocity of approximately 600 km per second with respect to extragalactic frames of reference. The oldest stars in the Milky Way are nearly as old as the Universe itself and thus probably formed shortly after the Dark Ages of the Big Bang.[9] The Milky Way has several satellite galaxies and is part of the Local Group of galaxies, which is a component of the Virgo Supercluster, which is itself a component of the Laniakea Supercluster. Appearance
A view of the Milky Way toward the constellation Sagittarius (including the Galactic Center) as seen from an area not polluted by light (the Black Rock Desert, Nevada). The bright object on the right is Jupiter, just above Antares.
This time-lapse video captures the Milky Way circling over ALMA.
The Milky Way has a relatively low surface brightness. Its visibility can be greatly reduced by background light such as light pollution or stray light from the Moon. The sky needs to be darker than about 20.2 magnitude per square arcsecond in order for the Milky Way to be seen.[42] It should be visible when the limiting magnitude is approximately +5.1 or better and shows a great deal of detail at +6.1.[43] This makes the Milky Way difficult to see from any brightly lit urban or suburban location, but very prominent when viewed from a rural area when the Moon is below the horizon.[nb 2] The new world atlas of artificial night sky brightness shows that more than one-third of Earth's population cannot see the Milky Way from their homes due to light pollution.[44] As viewed from Earth, the visible region of the Milky Way's Galactic plane occupies an area of the sky that includes 30 constellations.[45] The center of the Galaxy lies in the direction of the constellation Sagittarius; it is here that the Milky Way is brightest. From Sagittarius, the hazy band of white light appears to pass around to the Galactic anticenter in Auriga. The band then continues the rest of the way around the sky, back to Sagittarius. The band divides the night sky into two roughly equal hemispheres. The Galactic plane is inclined by about 60 degrees to the ecliptic (the plane of 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 β Comae Berenices, and the south Galactic pole is near α Sculptoris. Because of this high inclination, depending on the time of night and year, the arc of the Milky Way may appear relatively low or relatively high in the sky. For observers from approximately 65 degrees north to 65 degrees south on Earth's surface, the Milky Way passes directly overhead twice a day. 
The Milky Way arching at a high inclination across the night sky (fish-eye mosaic shot at Paranal, Chile). The bright object is Jupiter in the constellation Sagittarius, and the Magellanic Clouds can be seen on the left. Galactic north is downwards.
Size and mass
A photograph of galaxy UGC 12158, which is thought to resemble the Milky Way in appearance.
Schematic profile of the Milky Way.
Much of the mass of the Milky Way appears to be dark matter, an unknown and invisible form of matter that interacts gravitationally with ordinary matter. A dark matter halo is spread out relatively uniformly to a distance beyond one hundred kiloparsecs from the Galactic Center. Mathematical models of the Milky Way suggest that the mass of dark matter is 1–1.5×1012 M☉.[10][11][55] Recent studies indicate a range in mass, as large as 4.5×1012 M☉ [56] and as small as 8×1011 M☉.[57] The total mass of all the stars in the Milky Way is estimated to be between 4.6×1010 M☉[58] and 6.43×1010 M☉.[10] In addition to the stars, there is also interstellar gas, comprising 90% hydrogen and 10% helium by mass,[59] with two thirds of the hydrogen found in the atomic form and the remaining one-third as molecular hydrogen.[60] The mass of this gas is equal to between 10%[60] and 15%[59] of the total mass of the galaxy's stars. Interstellar dust accounts for an additional 1% of the total mass of the gas.[59] ContentsThe Milky Way contains between 200 and 400 billion stars[61][62] and at least 100 billion planets.[63] The exact figure depends on the number of very-low-mass stars, which are hard to detect, especially at distances of more than 300 ly (90 pc) from the Sun. As a comparison, the neighboring Andromeda Galaxy contains an estimated one trillion (1012) stars.[64] Filling the space between the stars is a disk of gas and dust called the interstellar medium. This disk has at least a comparable extent in radius to the stars,[65] whereas the thickness of the gas layer ranges from hundreds of light years for the colder gas to thousands of light years for warmer gas.[66][67]The disk of stars in the Milky Way does not have a sharp edge beyond which there are no stars. Rather, the concentration of stars decreases with distance from the center of the Milky Way. For reasons that are not understood, beyond a radius of roughly 40,000 ly (13 kpc) from the center, the number of stars per cubic parsec drops much faster with radius.[68] Surrounding the galactic disk is a spherical Galactic Halo of stars and globular clusters that extends further outward but is limited in size by the orbits of two Milky Way satellites, the Large and Small Magellanic Clouds, whose closest approach to the Galactic Center is about 180,000 ly (55 kpc).[69] At this distance or beyond, the orbits of most halo objects would be disrupted by the Magellanic Clouds. Hence, such objects would probably be ejected from the vicinity of the Milky Way. The integrated absolute visual magnitude of the Milky Way is estimated to be around −20.9.[70][71][a] Both gravitational microlensing and planetary transit observations indicate that there may be at least as many planets bound to stars as there are stars in the Milky Way,[34][72] and microlensing measurements indicate that there are more rogue planets not bound to host stars than there are stars.[73][74] The Milky Way contains at least one planet per star, resulting in 100–400 billion planets, according to a January 2013 study of the five-planet star system Kepler-32 with the Kepler space observatory.[35] A different January 2013 analysis of Kepler data estimated that at least 17 billion Earth-sized exoplanets reside in the Milky Way.[75] On November 4, 2013, astronomers reported, based on Kepler space mission data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarfs within the Milky Way.[76][77][78] 11 billion of these estimated planets may be orbiting Sun-like stars.[79] The nearest such planet may be 4.2 light-years away, according to a 2016 study.[80] Such Earth-sized planets may be more numerous than gas giants.[34] Besides exoplanets, "exocomets", comets beyond the Solar System, have also been detected and may be common in the Milky Way.[81]
Structure
An artist's impression that shows how the Milky Way would look from very
different perspectives than from Earth. From some angles, the central
bulge shows up as a peanut-shaped glowing ball of stars, and from above,
the central narrow bar appears clearly. The many spiral arms and their
associated dust clouds are also clearly seen.
Artist's conception of the spiral structure of the Milky Way with two major stellar arms and a bar[82]
 
Bright X-ray flares from Sagittarius A*, location of the supermassive black hole at the center of the Milky Way.[83]
Galactic quadrantsA galactic quadrant, or quadrant of the Milky Way, refers to one of four circular sectors in the division of the Milky Way. In actual astronomical practice, the delineation of the galactic quadrants is based upon the galactic coordinate system, which places the Sun as the origin of the mapping system.[86]Quadrants are described using ordinals—for example, "1st galactic quadrant",[87] "second galactic quadrant",[88] or "third quadrant of the Milky Way".[89] Viewing from the north galactic pole with 0 degrees (°) as the ray that runs starting from the Sun and through the Galactic Center, the quadrants are as follows:
Galactic CenterThe Sun is 25,000–28,000 ly (7.7–8.6 kpc) from the Galactic Center. This value is estimated using geometric-based methods or by measuring selected astronomical objects that serve as standard candles, with different techniques yielding various values within this approximate range.[15][16][17][91][92][93] In the inner few kpc (around 10,000 light-years radius) is a dense concentration of mostly old stars in a roughly spheroidal shape called the bulge.[94] It has been proposed that the Milky Way lacks a bulge formed due to a collision and merger between previous galaxies, and that instead it has a pseudobulge formed by its central bar.[95]The Galactic Center is marked by an intense radio source named Sagittarius A* (pronounced Sagittarius A-star). The motion of material around the center indicates that Sagittarius A* harbors a massive, compact object.[96] This concentration of mass is best explained as a supermassive black hole[nb 3][15][97] (SMBH) with an estimated mass of 4.1–4.5 million times the mass of the Sun.[97] The rate of accretion of the SMBH is consistent with an inactive galactic nucleus, being estimated at around 1×10−5 M☉ y−1.[98] Observations indicate that there are SMBH located near the center of most normal galaxies.[99][100] The nature of the Milky Way's bar is actively debated, with estimates for its half-length and orientation spanning from 1 to 5 kpc (3,000–16,000 ly) and 10–50 degrees relative to the line of sight from Earth to the Galactic Center.[92][93][101] Certain authors advocate that the Milky Way features two distinct bars, one nestled within the other.[102] However, RR Lyrae variables do not trace a prominent Galactic bar.[93][103][104] 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 Milky Way, as well as most of the Milky Way's star formation activity. Viewed from the Andromeda Galaxy, it would be the brightest feature of the Milky Way.[105] X-ray emission from the core is aligned with the massive stars surrounding the central bar[98] and the Galactic ridge.[106] 
Later, on January 5, 2015, NASA reported observing an X-ray flare 400 times brighter than usual, a record-breaker, from Sagittarius A*. The unusual event may have been caused by the breaking apart of an asteroid falling into the black hole or by the entanglement of magnetic field lines within gas flowing into Sagittarius A* |
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