This time-lapse video captures the Milky Way circling over. The 'Milky Way' can be seen as a hazy band of white light some 30 degrees wide arcing across the sky. Although all the individual naked-eye in the entire sky are part of the Milky Way, the light in this band originates from the accumulation of stars and other material located in the direction of the.
Dark regions within the band, such as the and the, are areas where light from distant stars is blocked. The area of the sky obscured by the Milky Way is called the.
The Milky Way has a relatively low. Its visibility can be greatly reduced by background light such as or stray light from the. The sky needs to be darker than about 20.2 per square arcsecond in order for the Milky Way to be seen. It should be visible when the is approximately +5.1 or better and shows a great deal of detail at +6.1. 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.
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. As viewed from Earth, the visible region of the Milky Way's Galactic plane occupies an area of the sky that includes 30. The lies in the direction of the constellation; it is here that the Milky Way is brightest. From Sagittarius, the hazy band of white light appears to pass around to the in.
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. The Galactic plane is inclined by about 60 degrees to the (the plane of Earth's orbit). Relative to the, it passes as far north as the constellation of and as far south as the constellation of, indicating the high inclination of Earth’s and the plane of the ecliptic, relative to the Galactic plane. The north Galactic pole is situated at 12 h 49 m, +27.4° near, and the south Galactic pole is near.
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. A photograph of galaxy, which is thought to resemble the Milky Way in appearance.
The Milky Way is the second-largest galaxy in the, with its stellar disk approximately 100,000 ly (30 kpc) in diameter, and, on average, approximately 1,000 ly (0.3 kpc) thick. As a guide to the relative physical scale of the Milky Way, if the Solar System out to were the size of a US quarter (24.3 mm (0.955 in)), the Milky Way would be approximately the size of the continental United States. A ring-like filament of stars wrapping around the Milky Way may belong to the Milky Way itself, rippling above and below the relatively flat galactic plane. If so, that would mean a diameter of 150,000–180,000 light-years (46–55 kpc). Schematic profile of the Milky Way. Estimates of the mass of the Milky Way vary, depending upon the method and data used.
At the low end of the estimate range, the mass of the Milky Way is 5.8 ×10 11 ( M ☉), somewhat less than that of the. Measurements using the in 2009 found velocities as large as 254 km/s (570,000 mph) for stars at the outer edge of the Milky Way. Because the orbital velocity depends on the total mass inside the orbital radius, this suggests that the Milky Way is more massive, roughly equaling the mass of Andromeda Galaxy at 7 ×10 11 M ☉ within 160,000 ly (49 kpc) of its center.
In 2010, a measurement of the radial velocity of halo stars found that the mass enclosed within 80 kilo is 7 ×10 11 M ☉. According to a study published in 2014, the mass of the entire Milky Way is estimated to be 8.5 ×10 11 M ☉, which is about half the mass of the Andromeda Galaxy. Much of the mass of the Milky Way appears to be, an unknown and invisible form of matter that interacts gravitationally with ordinary matter. A 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 ×10 12 M ☉. Recent studies indicate a range in mass, as large as 4.5 ×10 12 M ☉ and as small as 8 ×10 11 M ☉. The total mass of all the stars in the Milky Way is estimated to be between 4.6 ×10 10 M ☉ and 6.43 ×10 10 M ☉.
In addition to the stars, there is also interstellar gas, comprising 90% and 10% by mass, with two thirds of the hydrogen found in the and the remaining one-third as. The mass of this gas is equal to between 10% and 15% of the total mass of the galaxy's stars. Accounts for an additional 1% of the total mass of the gas. Further information: The Milky Way contains between 200 and 400 billion stars and at least 100 billion planets. 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 (10 12) stars. Filling the space between the stars is a disk of gas and dust called the. This disk has at least a comparable extent in radius to the stars, 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. 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 drops much faster with radius.
Surrounding the galactic disk is a spherical of stars and that extends further outward but is limited in size by the orbits of two Milky Way satellites, the Large and Small, whose to the Galactic Center is about 180,000 ly (55 kpc). 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 of the Milky Way is estimated to be around −20.9. Both 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, and microlensing measurements indicate that there are more not bound to host stars than there are stars.
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 with the space observatory. A different January 2013 analysis of Kepler data estimated that at least 17 billion reside in the Milky Way. On November 4, 2013, astronomers reported, based on data, that there could be as many as 40 billion Earth-sized orbiting in the of and within the Milky Way. 11 billion of these estimated planets may be orbiting Sun-like stars.
The nearest such planet may be 4.2 light-years away, according to a 2016 study. Such Earth-sized planets may be more numerous than gas giants.
Besides exoplanets, ', beyond the Solar System, have also been detected and may be common in the Milky Way. Bright flares from, location of the at the center of the Milky Way. The Milky Way consists of a bar-shaped core region surrounded by a disk of and stars. The mass distribution within the Milky Way closely resembles the type Sbc in the, which represents spiral galaxies with relatively loosely wound arms.
Astronomers began to suspect that the Milky Way is a, rather than an ordinary, in the 1990s. Their suspicions were confirmed by the observations in 2005 that showed the Milky Way's central bar to be larger than previously thought. Galactic quadrants.
Main article: A 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, which places the as the. Quadrants are described using —for example, '1st galactic quadrant', 'second galactic quadrant', or 'third quadrant of the Milky Way'. Viewing from the with 0 (°) as the that runs starting from the Sun and through the Galactic Center, the quadrants are as follows:.
1st galactic quadrant – 0° ≤ (ℓ) ≤ 90°. 2nd galactic quadrant – 90° ≤ ℓ ≤ 180°. 3rd galactic quadrant – 180° ≤ ℓ ≤ 270°. 4th galactic quadrant – 270° ≤ ℓ ≤ 360° (0°) Galactic Center. Main article: The Sun is 25,000–28,000 ly (7.7–8.6 kpc) from the Galactic Center. This value is estimated using -based methods or by measuring selected astronomical objects that serve as, with different techniques yielding various values within this approximate range. 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.
It has been proposed that the Milky Way lacks a formed due to a, and that instead it has a formed by its central bar. The Galactic Center is marked by an intense named (pronounced Sagittarius A-star). The motion of material around the center indicates that Sagittarius A. harbors a massive, compact object. This concentration of mass is best explained as a (SMBH) with an estimated mass of 4.1–4.5 million times the. The rate of accretion of the SMBH is consistent with an, being estimated at around 000000000♠1 ×10 −5 M ☉ y −1.
Observations indicate that there are SMBH located near the center of most normal galaxies. 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. Certain authors advocate that the Milky Way features two distinct bars, one nestled within the other. However, do not trace a prominent Galactic bar. 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 activity.
Viewed from the Andromeda Galaxy, it would be the brightest feature of the Milky Way. X-ray emission from the core is aligned with the massive stars surrounding the central bar and the. Illustration of the two gigantic / bubbles (blue-violet) of the Milky Way (center) In 2010, two gigantic spherical bubbles of high energy emission were detected to the north and the south of the Milky Way core, using data from the. The diameter of each of the bubbles is about 25,000 light-years (7.7 kpc); they stretch up to and to on the night-sky of the southern hemisphere.
Subsequently, observations with the at radio frequencies identified polarized emission that is associated with the Fermi bubbles. These observations are best interpreted as a magnetized outflow driven by star formation in the central 640 ly (200 pc) of the Milky Way. Later, on January 5, 2015, reported observing an 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 falling into the black hole or by the entanglement of within gas flowing into Sagittarius A. For more details on this topic, see. Outside the gravitational influence of the Galactic bars, the structure of the interstellar medium and stars in the disk of the Milky Way is organized into four spiral arms.
Spiral arms typically contain a higher density of interstellar gas and dust than the Galactic average as well as a greater concentration of star formation, as traced by and. The Milky Way's spiral structure is uncertain, and there is currently no consensus on the nature of the Milky Way's spiral arms.
Perfect logarithmic spiral patterns only crudely describe features near the Sun, because galaxies commonly have arms that branch, merge, twist unexpectedly, and feature a degree of irregularity. The possible scenario of the Sun within a spur / Local arm emphasizes that point and indicates that such features are probably not unique, and exist elsewhere in the Milky Way. Estimates of the pitch angle of the arms range from about 7° to 25°. There are thought to be four spiral arms that all start near the Milky Way's center. These are named as follows, with the positions of the arms shown in the image at right. Observed (normal lines) and extrapolated (dotted lines) structure of the spiral arms.
The gray lines radiating from the Sun's position (upper center) list the three-letter abbreviations of the corresponding constellations. Color Arm(s) cyan 3-kpc Arm ( and ) and purple and (Along with extension discovered in 2004 ) green pink There are at least two smaller arms or spurs, including: orange (which contains the Sun and Solar System) Two spiral arms, the Scutum–Centaurus arm and the Carina–Sagittarius arm, have tangent points inside the Sun's orbit about the center of the Milky Way. If these arms contain an overdensity of stars compared to the average density of stars in the Galactic disk, it would be detectable by counting the stars near the tangent point. Two surveys of near-infrared light, which is sensitive primarily to red giants and not affected by dust extinction, detected the predicted overabundance in the Scutum–Centaurus arm but not in the Carina–Sagittarius arm: the Scutum-Centaurus Arm contains approximately 30% more than would be expected in the absence of a spiral arm. This observation suggests that the Milky Way possesses only two major stellar arms: the Perseus arm and the Scutum–Centaurus arm. The rest of the arms contain excess gas but not excess old stars. In December 2013, astronomers found that the distribution of young stars and star-forming regions matches the four-arm spiral description of the Milky Way.
Thus, the Milky Way appears to have two spiral arms as traced by old stars and four spiral arms as traced by gas and young stars. The explanation for this apparent discrepancy is unclear. Clusters detected by used to trace the Milky Way's spiral arms The Near 3 kpc Arm (also called Expanding 3 kpc Arm or simply 3 kpc Arm) was discovered in the 1950s by astronomer van Woerden and collaborators through radio measurements of HI.
It was found to be expanding away from the central bulge at more than 50. It is located in the fourth galactic quadrant at a distance of about 5.2 from the and 3.3 kpc from the. The Far 3 kpc Arm was discovered in 2008 by astronomer Tom Dame (Harvard-Smithsonian CfA). It is located in the first galactic quadrant at a distance of 3 (about 10,000 ) from the Galactic Center. A simulation published in 2011 suggested that the Milky Way may have obtained its spiral arm structure as a result of repeated collisions with the. It has been suggested that the Milky Way contains two different spiral patterns: an inner one, formed by the Sagittarius arm, that rotates fast and an outer one, formed by the Carina and Perseus arms, whose rotation velocity is slower and whose arms are tightly wound. In this scenario, suggested by numerical simulations of the dynamics of the different spiral arms, the outer pattern would form an outer, and the two patterns would be connected by the Cygnus arm.
The long filamentary molecular cloud dubbed 'Nessie' probably forms a dense 'spine' of the Scutum–Centarus Arm Outside of the major spiral arms is the (or Outer Ring), a ring of gas and stars torn from other galaxies billions of years ago. However, several members of the scientific community recently restated their position affirming the Monoceros structure is nothing more than an over-density produced by the flared and warped of the Milky Way.
Halo The Galactic disk is surrounded by a of old stars and globular clusters, of which 90% lie within 100,000 light-years (30 kpc) of the Galactic Center. However, a few globular clusters have been found farther, such as PAL 4 and AM1 at more than 200,000 light-years from the Galactic Center. About 40% of the Milky Way's clusters are on, which means they move in the opposite direction from the Milky Way rotation. The globular clusters can follow about the Milky Way, in contrast to the of a planet around a star. Although the disk contains dust that obscures the view in some wavelengths, the halo component does not. Active takes place in the disk (especially in the spiral arms, which represent areas of high density), but does not take place in the halo, as there is little gas cool enough to collapse into stars. Are also located 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, the possibility of the disk of the Milky Way extending further is apparent, and this is supported by evidence from the discovery of the Outer Arm extension of the and of a similar extension of the. With the discovery of the 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, it was found that a ring of galactic debris from its interaction with the Milky Way encircles the Galactic disk.
The of the northern sky shows 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 Milky Way. The proposed likely interpretation is that a is merging with the Milky Way.
This galaxy is tentatively named the and is found in the direction of Virgo about 30,000 light-years (9 kpc) away. Gaseous halo In addition to the stellar halo, the, and have provided evidence that there is a gaseous halo with a large amount of hot gas. The halo extends for hundreds of thousand of light years, much further than the stellar halo and close to the distance of the Large and Small. The mass of this hot halo is nearly equivalent to the mass of the Milky Way itself. The temperature of this halo gas is between 1 and 2.5 million K (1.8 and 4.5 million oF). Observations of distant galaxies indicate that the Universe had about one-sixth as much (ordinary) matter as dark matter when it was just a few billion years old. However, only about half of those baryons are accounted for in the modern Universe based on observations of nearby galaxies like the Milky Way.
If the finding that the mass of the halo is comparable to the mass of the Milky Way is confirmed, it could be the identity of the missing baryons around the Milky Way. Sun’s location and neighborhood.
Diagram of the stars in the Solar neighborhood The is near the inner rim of the, within the of the, and in the, at a distance of 26.4 ± 1.0 kly (8.09 ± 0.31 kpc) from the Galactic Center. The Sun is currently 5–30 parsecs (16–98 ly) from the central plane of the Galactic disk. The distance between the local arm and the next arm out, the, is about 2,000 parsecs (6,500 ly). The Sun, and thus the Solar System, is located in the Milky Way's.
There are about 208 stars brighter than 8.5 within a sphere with a radius of 15 parsecs (49 ly) from the Sun, giving a density of one star per 69 cubic parsecs, or one star per 2,360 cubic light-years (from ). On the other hand, there are 64 known stars (of any magnitude, not counting 4 ) within 5 parsecs (16 ly) of the Sun, giving a density of about one star per 8.2 cubic parsecs, or one per 284 cubic light-years (from ). This illustrates the fact that there are far more faint stars than bright stars: in the entire sky, there are about 500 stars brighter than 4 but 15.5 million stars brighter than apparent magnitude 14. The apex of the Sun's way, or the, 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 near the constellation of, at an angle of roughly 60 sky degrees to the direction of the Galactic Center. The Sun's orbit about the Milky Way 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 passes through the Galactic plane approximately 2.7 times per orbit.
This is very similar to how a works with no drag force (damping) term. These oscillations were until recently thought to coincide with periods on Earth.
However, a reanalysis of the effects of the Sun's transit through the spiral structure based on CO data has failed to find a correlation. It takes the Solar System about 240 million years to complete one orbit of the Milky Way (a ), so the Sun is thought to have completed 18–20 orbits during its lifetime and 1/1250 of a revolution since the. The of the Solar System about the center of the Milky Way is approximately 220 km/s (490,000 mph) or 0.073% of the. The Sun moves through the heliosphere at 84,000 km/h (52,000 mph). 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.
The Solar System is headed in the direction of the zodiacal constellation Scorpius, which follows the ecliptic. Galactic rotation. 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. The stars and gas in the Milky Way rotate about its center, meaning that the rotation period varies with location. As is typical for spiral galaxies, the orbital speed of most stars in the Milky Way does not depend strongly on their distance from the center.
Away from the central bulge or outer rim, the typical stellar orbital speed is between 210 and 240 km/s (470,000 and 540,000 mph). Hence the 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 have significantly different velocities associated with them. The rotation curve (shown in the figure) describes this rotation.
Toward the center of the Milky Way the orbit speeds are too low, whereas beyond 7 kpcs the speeds are too high to match what would be expected from the universal law of gravitation. If the Milky Way contained only the mass observed in stars, gas, and other baryonic (ordinary) matter, the rotation speed would decrease with distance from the center. However, the observed curve is relatively flat, indicating that there is additional mass that cannot be detected directly with electromagnetic radiation. This inconsistency is attributed to dark matter. The rotation curve of the Milky Way agrees with the of spiral galaxies, the best evidence for the existence of in galaxies. Alternatively, a minority of astronomers propose that a may explain the observed rotation curve.
Main article: The Milky Way began as one or several small overdensities in the mass distribution in the shortly after the. Some of these overdensities were the seeds of globular clusters in which the oldest remaining stars in what is now the Milky Way formed. Nearly half the matter in the Milky Way may have come from other distant galaxies. Nonetheless, these stars and clusters now comprise the stellar halo of the Milky Way.
Within a few billion years of the birth of the first stars, the mass of the Milky Way was large enough so that it was spinning relatively quickly. Due to, this led the gaseous interstellar medium to collapse from a roughly spheroidal shape to a disk. Therefore, later generations of stars formed in this spiral disk. Most younger stars, including the Sun, are observed to be in the disk. Since the first stars began to form, the Milky Way has grown through both (particularly early in the Milky Way's growth) and accretion of gas directly from the Galactic halo.
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The Milky Way is currently accreting material from two of its nearest satellite galaxies, the and Magellanic Clouds, through the. Direct accretion of gas is observed in like the. However, properties of the Milky Way such as stellar mass, and in its outermost regions suggest it has undergone no mergers with large galaxies in the last 10 billion years. This lack of recent major mergers is unusual among similar spiral galaxies; its neighbour the Andromeda Galaxy appears to have a more typical history shaped by more recent mergers with relatively large galaxies.
According to recent studies, the Milky Way as well as the Andromeda Galaxy lie in what in the is known as the 'green valley', a region populated by galaxies in transition from the 'blue cloud' (galaxies actively forming new stars) to the 'red sequence' (galaxies that lack star formation). Star-formation activity in green valley galaxies is slowing as they run out of star-forming gas in the interstellar medium. In simulated galaxies with similar properties, star formation will typically have been extinguished within about five billion years from now, even accounting for the expected, short-term increase in the rate of star formation due to the collision between both the Milky Way and the Andromeda Galaxy. In fact, measurements of other galaxies similar to the Milky Way suggest it is among the reddest and brightest spiral galaxies that are still forming new stars and it is just slightly bluer than the bluest red sequence galaxies.
Age and cosmological history. Night sky from a hypothetical planet within the Milky Way 10 billion years ago. Globular clusters are among the oldest objects in the Milky Way, which thus set a lower limit on the age of the Milky Way. The ages of individual stars in the Milky Way can be estimated by measuring the abundance of long-lived such as and, then comparing the results to estimates of their original abundance, a technique called. These yield values of about 12.5 ± 3 billion years for and 13.8 ± 4 billion years for.
Once a 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 billion years. Age estimates of the oldest of these clusters gives a best fit estimate of 12.6 billion years, and a 95% confidence upper limit of 16 billion years. Several individual stars have been found in the Milky Way's halo with measured ages very close to the 13.80-billion-year. In 2007, a star in the galactic halo, was estimated to be about 13.2 billion years old. As the oldest known object in the Milky Way at that time, this measurement placed a lower limit on the age of the Milky Way.
This estimate was made using the UV-Visual Echelle Spectrograph of the to the relative strengths of caused by the presence of and other created by the. The line strengths yield abundances of different elemental, from which an estimate of the age of the star can be derived using. Another star, is 14.5 ± 0.7 billion years old. The age of stars in the galactic has also been estimated using nucleocosmochronology. Measurements of thin disk stars yield an estimate that the thin disk formed 8.8 ± 1.7 billion years ago. These measurements suggest there was a hiatus of almost 5 billion years between the formation of the and the thin disk. Recent analysis of the chemical signatures of thousands of stars suggests that stellar formation might have dropped by an order of magnitude at the time of disk formation, 10 to 8 billion years ago, when interstellar gas was too hot to form new stars at the same rate as before.
The satellite galaxies surrounding the Milky way are not randomly distributed, but seemed to be the result of a break-up of some larger system producing a ring structure 500,000 light years in diameter and 50,000 light years wide. Close encounters between galaxies, like that expected in 4 billion years with the Andromeda Galaxy rips off huge tails of gas, which, over time can coalesce to form dwarf galaxies in a ring at right angles to the main disc. Main article: The Milky Way and the are a of giant spiral galaxies belonging to a group of 50 closely bound galaxies known as the, surrounded by a Local Void, itself being part of the.
Surrounding the Virgo Supercluster are a number of voids, devoid of many galaxies, the Microscopium Void to the 'north', the Sculptor Void to the 'left', the to the 'right' and the Canes-Major Void to the South. These voids change shape over time, creating filamentous structures of galaxies. The Virgo Supercluster, for instance, is being drawn towards the, which in turn forms part of a greater structure, called. Two smaller galaxies and a number of in the Local Group orbit the Milky Way.
The largest of these is the with a diameter of 14,000 light-years. It has a close companion, the. The is a stream of neutral gas extending from these two small galaxies across 100° of the sky. The stream is thought to have been dragged from the Magellanic Clouds in tidal interactions with the Milky Way.
Some of the are (the closest), and. The smallest dwarf galaxies of the Milky Way are only 500 light-years in diameter. These include, and.
There may still be undetected dwarf galaxies that are dynamically bound to the Milky Way, which is supported by the detection of nine new satellites of the Milky Way in a relatively small patch of the night sky in 2015. There are also some dwarf galaxies that have already been absorbed by the Milky Way, such as. In 2014 researchers reported that most satellite galaxies of the Milky Way actually lie in a very large disk and orbit in the same direction. This came as a surprise: according to standard cosmology, the satellite galaxies should form in dark matter halos, and they should be widely distributed and moving in random directions.
This discrepancy is still not fully explained. 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 orbit the Milky Way, causing vibrations when they pass through its edges.
Previously, these two galaxies, at around 2% of the mass of the Milky Way, were considered too small to influence the Milky Way. However, in a computer model, the movement of these two galaxies creates a dark matter wake that amplifies their influence on the larger Milky Way. Current measurements suggest the Andromeda Galaxy is approaching us at 100 to 140 km/s (220,000 to 310,000 mph).
In 3 to 4 billion years, there may be an, depending on the importance of unknown lateral components to the galaxies' relative motion. If they collide, the chance of individual with each other is extremely low, but instead the two galaxies will merge to form a single or perhaps a large over the course of about a billion years. Velocity Although states that there is no 'preferred' in space with which to compare the Milky Way, the Milky Way does have a velocity with respect to cosmological. One such frame of reference is the, the apparent motions of galaxy clusters due to the.
Individual galaxies, including the Milky Way, have relative to the average flow. Thus, to compare the Milky Way to the Hubble flow, one must consider a volume large enough so that the expansion of the Universe dominates over local, random motions. A large enough volume means that the mean motion of galaxies within this volume is equal to the Hubble flow. Astronomers believe the Milky Way is moving at approximately 630 km/s (1,400,000 mph) with respect to this local co-moving frame of reference. The Milky Way is moving in the general direction of the and other, including the, behind it. The Local Group (a cluster of gravitationally bound galaxies containing, among others, the Milky Way and the Andromeda Galaxy) is part of a called the, centered near the: although they are moving away from each other at 967 km/s (2,160,000 mph) as part of the Hubble flow, this 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. Another reference frame is provided by the (CMB).
The Milky Way is moving at 552 ± 6 km/s (1,235,000 ± 13,000 mph) with respect to the photons of the CMB, toward 10.5 right ascension, −24° declination ( epoch, near the center of ). This motion is observed by satellites such as the (COBE) and the (WMAP) as a dipole contribution to the CMB, as photons in equilibrium in the CMB frame get in the direction of the motion and in the opposite direction. Etymology and mythology. The shape of the Milky Way as deduced from star counts by in 1785; the Solar System was assumed near center In (DK 59 A80), (384–322 BC) wrote that the (c. 500–428 BC) and (460–370 BC) proposed that the Milky Way might consist of distant. 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, in the region of the world which is continuous with the heavenly motions.' The philosopher ( c.
495–570 A.D.) criticized this view, arguing that if the Milky Way were, it should appear different at different times and places on Earth, and that it should have, which it does not. In his view, the Milky Way is celestial. This idea would be influential later in the. The astronomer (973–1048) proposed that the Milky Way is 'a collection of countless fragments of the nature of stars'.
The astronomer ( 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 in, citing his observation of a of Jupiter and Mars in 1106 or 1107 as evidence. (1292–1350) proposed that the Milky Way is 'a myriad of tiny stars packed together in the sphere of the fixed stars' and that these stars are larger than. According to Jamil Ragep, the Persian astronomer (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.' Actual proof of the Milky Way consisting of many stars came in 1610 when used a to study the Milky Way and discovered that it is composed of a huge number of faint stars. In a treatise in 1755, drawing on earlier work by, speculated (correctly) that the Milky Way might be a rotating body of a huge number of stars, held together by 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 visible in the night sky might be separate 'galaxies' themselves, similar to our own. Kant referred to both the Milky Way and the 'extragalactic nebulae' as 'island universes', a term still current up to the 1930s. The first attempt to describe the shape of the Milky Way and the position of the Sun within it was carried out by 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 Milky Way with the Solar System close to the center. In 1845, 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.
Photograph of the 'Great Andromeda Nebula' from 1899, later identified as the In 1917, had observed the nova within the ( 31). Searching the photographic record, he found 11 more. Curtis noticed that these novae were, on average, 10 fainter than those that occurred within the Milky Way. 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. In 1920 the took place between 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 is an external galaxy, Curtis noted the appearance of dark lanes resembling the dust clouds in the Milky Way, as well as the significant. The controversy was conclusively settled by in the early 1920s using the Mount Wilson observatory. With the of this new telescope, he was able to produce that resolved the outer parts of some spiral nebulae as collections of individual stars.
He was also able to identify some that he could use as a to estimate the distance to the nebulae. He found that the Andromeda Nebula is 275,000 parsecs from the Sun, far too distant to be part of the Milky Way. Mapping The spacecraft provides distance estimates by determining the of a billion stars and is mapping the Milky Way with four planned releases of maps in 2022.
See also. Notes. Pasachoff in his textbook Astronomy: From the Earth to the Universe states the term Milky Way should refer exclusively to the band of light that the galaxy forms in the, while the galaxy should receive the full name Milky Way Galaxy. See:. (1994). Astronomy: From the Earth to the Universe. Harcourt School.
For a photo see:. Chandra X-ray Observatory. Harvard-Smithsonian Center for Astrophysics. January 6, 2003. From the original on March 17, 2008.
Retrieved May 20, 2012.