Milky Way

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Milky Way
Artist's impression of the Milky Way (updated - annotated) .jpg
Schematic representation of the Milky Way system. The areas in the spiral arms that glow red in the light of the H-α line of hydrogen are star formation areas.
Physical Properties
diameter 170,000-200,000 light years
thickness up to 15,000 light years (with bulge ), 1,000 light years (without bulge)
Mass (including dark matter ) about 1.5 billion solar masses
Stars 250 billion ± 150 billion (estimated)
Type Barred spiral galaxy
Speed ​​relative to the CMB 552 ± 6 km / s
Super pile Virgo supercluster / Laniakea
Galaxy clusters Local group
Subgroup Milky Way subgroup

The Milky Way , also called galaxy , is the galaxy in which the solar system is located with the earth . Corresponding to its shape as a flat disk made up of billions of stars , the Milky Way can be seen from Earth as a ribbon-shaped brightening in the night sky that extends over 360 °. According to its structure, the Milky Way is one of the barred spiral galaxies .

History and origin of the name

The shape of the Milky Way, as Wilhelm Herschel derived it from star counts in 1785. The solar system was - due to the interstellar extinction - assumed by him still in the center.

The name of the Milky Way System is derived from the star system after the Milky Way, ancient Greek γαλαξίας (κύκλος) galaxías (kýklos) , from γάλα gála “milk”, which appears as a clear-eyed interior view of the system from the earth like a milky brush stroke across the firmament. That this whitish band is actually composed of innumerable individual stars was asserted by Democritus and recognized in modern times in 1609 by Galileo Galilei , who was the first to observe the phenomenon through a telescope. According to current estimates, the Milky Way consists of around 100 to 400 billion stars.

An ancient Greek legend tries to explain this term mythologically: According to this, Zeus let his son Heracles , whom the mortal wife Alkmene had given him, drink from the breast of his divine wife Hera while she was sleeping. Heracles was to receive divine powers in this way. But he suckled so vehemently that Hera woke up and pushed back the strange baby; a stream of her milk was spattered all over the sky.

In late antiquity the Milky Way was referred to by the Manicheans as the “pillar of glory” because it enabled the light captured in the world to return to the home of light from which it once came. In Persia at that time, where the former world religion originated, it could be seen as a column of light emanating from the horizon and pointing upwards.

According to a Germanic legend, the Milky Way was named Iringsstrasse after the god of light, Heimdall , also called Iring . The African San gave the Milky Way the name "backbone of the night".

Wilhelm Herschel came up with the first idea of ​​the disk shape of the Milky Way system in 1785 on the basis of systematic star counts ( stellar statistics ). However, this method could not lead to a realistic picture, since the light of more distant stars is strongly weakened by interstellar clouds of dust , an effect whose true meaning was only fully understood in the first half of the 20th century. By investigating the distribution of globular clusters in space, Harlow Shapley came to realistic estimates of the size of the Milky Way system in 1919 and came to the conclusion that the sun was not - as it was before, e.g. B. by Jacobus Kapteyn , assumed - sitting in the center of the galaxy, but rather on its edge. Edwin Hubble's measurements of the distances of spiral nebulae showed that these lie outside the Milky Way system and are actually, like it, independent galaxies.


A 360 ° panoramic image of the starry sky shows the band of the Milky Way as an arc, taken in Death Valley National Park

The band of the Milky Way extends across the firmament as an irregularly wide, slightly milky-light strip . Its appearance is due to the fact that no single stars are perceived in it with the naked eye, but a multitude of faint stars of the galactic disk and the bulge (in the direction of the galactic center). From the southern hemisphere , the bright center of the Milky Way is high in the sky, while from the northern hemisphere one looks towards the edge. Therefore, the band of the Milky Way can best be observed from the southern hemisphere.

Part of the Milky Way on a 19th century astronomical drawing ( Trouvelot , 1881)

In December and January, the brightest area of ​​the Milky Way cannot be observed, or only very poorly, because the sun is located between the center of the galaxy and the earth. Good observation conditions are given with clear air and as little light pollution as possible. All around 6000 stars that can be seen with the naked eye in the entire sky belong to the Milky Way system. At a greater distance and outside the Milky Way, only the Andromeda Galaxy can be seen.

The Milky Way ribbon runs through the constellations Sagittarius (the galactic center is also in this direction), Eagle , Swan , Cassiopeia , Perseus , Carter , Gemini , Orion , the keel of the ship , Centaur , Southern Cross and Scorpio . The middle plane of the Milky Way system is tilted by an angle of about 63 ° with respect to the celestial equator .

Astronomers occasionally use a special galactic coordinate system , adapted to the geometry of the Milky Way , consisting of longitude l and latitude b . The galactic latitude is 0 ° in the plane of the Milky Way system, + 90 ° at the galactic north pole and −90 ° at the galactic south pole. The galactic longitude, which is also given in degrees , has its origin (l = 0 °) in the direction of the galactic center and increases to the east.

Infrared image of the Milky Way system by the COBE satellite . Disc and central bulge can be seen.
Representation of the Milky Way as a regular spiral galaxy, as it was widespread until the 1990s


General structure

Exploring the structure of the Milky Way system is more difficult than studying the structure of other galaxies because observations can only be made from one point within the disk. Because of the aforementioned absorption of visible light by interstellar dust , it is not possible to obtain a complete picture of the Milky Way system by visual observation. Great advances were only made when observations in other wavelength ranges, especially in the radio frequency range and in the infrared , became possible. However, many details of the structure of the galaxy are not yet known.

The number of stars and the total mass of the Milky Way can only be estimated on the basis of calculations and observations, which results in large tolerances in the numbers. The Milky Way System consists of around 100 to 300 billion stars and large amounts of interstellar and dark matter . The extent of the Milky Way in the galactic plane is about 100,000 light years (30  CCP ), the thickness of the disc about 3,000 light-years (920 pc) and of the central bulge (engl. Bulge ) about 16,000 light-years (5 CCP). For comparison: The Andromeda Galaxy (M31) has an extent of about 150,000 ly and the third largest member of the local group, the Triangle Nebula (M33), approx. 50,000 ly. However, the information on the thickness may have to be corrected up to twice that amount As the Australian scientist Bryan Gaensler and his team said in January 2008.

Until the 1990s, a relatively regular spiral galaxy was assumed, similar to the Andromeda galaxy. But the galaxy is probably a barred spiral galaxy of the Hubble type SBbc. The movement of interstellar gas and the distribution of stars in the bulge give it an elongated shape. This bar forms an angle of 45 ° with the line connecting the solar system to the center of the Milky Way system. As determined using the infrared - Spitzer Space Telescope is the beam structure with an area of 27,000 light years surprisingly long.

Likewise, the Milky Way also shows signs of weak central, ring-shaped structures of gas and stars around the bulge. In the De Vaucouleur system, the Milky Way is therefore classified accordingly as type SB (rs) bc.

Based on the known period of revolution of the sun and its distance from the galactic center, Kepler's third law can be used to calculate the total mass that is within the solar orbit. The total mass of the Milky Way system was previously estimated to be around 400 billion to 700 billion solar masses. According to more recent findings, the total mass in a radius of 129,000 light years around the Galactic Center is around 1,500 billion solar masses . This makes the Milky Way in front of the Andromeda Galaxy (800 billion solar masses) the most massive galaxy in the Local Group .

Galactic halo

The galaxy is surrounded by the spherical galactic halo with a diameter of about 165,000 light years (50 kpc), a kind of galactic " atmosphere ". In addition to the approximately 150 known globular clusters, there are other ancient stars, including RR Lyrae variables , and very low density gas. The hot Blue Straggler stars are an exception . In addition, there are large amounts of dark matter with around 1 billion solar masses, including so-called MACHOs . Unlike the galactic disk, the halo is largely dust-free and almost exclusively contains stars from the older, metal-poor Population II , whose orbit is very strongly inclined towards the galactic plane. The age of the inner part of the halo was given in a new method for determining the age presented in May 2012 by the Space Telescope Science Institute in Baltimore as 11.4 billion years (with an uncertainty of 0.7 billion years). The astronomer Jason Kalirai from the Space Telescope Science Institute succeeded in determining the age by comparing the halo dwarfs of the Milky Way with the well-studied dwarfs in the globular cluster Messier 4, which are located in the constellation Scorpio.

Galactic disk

The majority of the stars in the galaxy are almost evenly distributed across the galactic disk. In contrast to the halo, it mainly contains stars from Population I with a high proportion of heavy elements .


Most spiral galaxies are domed. There is no connection between the frequency of a bulge and the age of the galaxy. In 1957, measurements with radio telescopes based on the 21 cm radiation of neutral hydrogen showed that the Milky Way disk is also slightly curved in the direction of the Magellanic Clouds - like a very flat plate. Since both young and very old stars move in the same way in terms of curvature, the curvature is a consequence of the gravitational field.

Spiral arms

The spiral arms characteristic of the Milky Way system are also part of the disk . These contain enormous accumulations of hydrogen and also the largest HII regions , the star formation regions of the galaxy with many protostars , young stars of the T-Tauri type and Herbig-Haro objects . During their lifetime, stars move away from their birthplaces and spread across the disk. Very massive and luminous stars do not move so far away from the spiral arms due to their shorter lifespan, which is why they emerge. Therefore, the stellar objects located there mainly include stars of the spectral classes O and B, supergiants and Cepheids , all younger than 100 million years. However, they only make up about one percent of the stars in the Milky Way system. Most of the galaxy's mass is made up of old, low-mass stars. The "space" between the spiral arms is not empty, but just less bright .

Scheme of the observed spiral arms of the Milky Way system (see text)

The spiral structure of the galaxy was confirmed by observing the distribution of neutral hydrogen . The spiral arms discovered were named after the constellations in their direction .

The drawing on the right shows the structure of the Milky Way system schematically. The center is not directly observable in visible light, as is the area behind it. The sun (yellow circle) lies between the spiral arms Sagittarius (according to the constellation Sagittarius ) and Perseus in the Orion arm . This arm is probably not complete, see the brown line in the figure. In relation to this immediate environment, the sun moves at about 30 km / s in the direction of the constellation Hercules . The innermost arm is the Norma arm (according to the constellation angle measure , also 3 kpc arm ), the outermost arm (not in the figure) is the Cygnus arm (according to the constellation Swan ), which is probably the continuation of the Scutum-Crux arm (according to the constellations Shield and Cross of the South ) is.

Evaluations of infrared images from the Spitzer telescope published by the University of Wisconsin in June 2008 showed the Milky Way system only as a two-armed galaxy. Sagittarius and Norma were only recognizable as thin side arms with an excess gas distribution, while in the other two arms a high one Dense old reddish stars were noticed. A more recent study of the distribution of star formation regions and young stars, however, confirmed the familiar four-armed structure. The Milky Way therefore apparently consists of four spiral arms, which are primarily defined by gas clouds and young stars, with many older stars also concentrating in two arms. A clearly defined logarithmic spiral pattern is seldom found in spiral galaxies across the entire disk. Arms often have extreme branches and ramifications. The likely nature of the local arm as such an irregularity suggests that such structures could be common in the Milky Way.

Names of the spiral arms
Default name alternative name Astronomic
Norma arm 3 kpc arm (ring) -
Scutum crux arm Centaurus arm −II
Sagittarius arm Sagittarius Carina arm −I
Orion arm Local arm 0
Perseus arm - + I
Cygnus arm Outer arm + II
The Milky Way towards the Sagittarius Arm, on the right the Eta Carinae Nebula NGC 3372, an H-II region

How the spiral structure came about has not yet been clearly clarified. Stars belonging to the spiral arms are not a rigid structure that rotates in formation around the galactic center. If this were the case, the spiral structure of the Milky Way System and other spiral galaxies would wind up due to the different orbital speeds and become unrecognizable. The density wave theory offers an explanation . This sees spiral arms as zones of increased matter density and star formation that move through the disk independently of the stars. Disturbances in the orbits of stars caused by spiral arms can lead to Lindblad resonances .

Stars of the galactic disk

The stars of the galactic disk that belong to population I can be divided into three sub-populations with increasing scatter around the main plane and age. The so-called “thin disk” in a range of 700 to 800 light years above and below the galactic plane contains, in addition to the above-mentioned luminous stars of the spiral arms, which are only a maximum of 500 light years away from the plane, stars of the spectral classes A and F, some Giants of classes A, F, G and K, as well as dwarf stars of classes G, K and M and also some white dwarfs . The metallicity of these stars is comparable to that of the sun , but mostly twice as high. Their age is around a billion years.

Another group is that of the middle-aged stars (up to five billion years old). These include the sun and other dwarf stars of the spectral types G, K and M, as well as some sub and red giants . The metallicity is significantly lower here with only about 50 to 100 percent of that of the sun. The eccentricity of the orbit of these stars around the galactic center is also higher. They are no more than 1500 light years above or below the galactic plane.

The “thick disk” extends between a maximum of 2500 light years above and below the main level . It contains red K- and M-dwarfs, white dwarfs, as well as some sub-giants and red giants , but also long-period variables . The age of these stars reaches up to ten billion years and they are comparatively low in metal (about a quarter of the solar metallicity). This population also resembles many stars in the bulge .


A 900 light-year wide section of the central region of the Milky Way System

The center of the Milky Way system lies in the constellation Sagittarius and is hidden behind dark clouds of dust and gas so that it cannot be directly observed in visible light. Beginning in the 1950s, it was possible to obtain increasingly detailed images of the vicinity of the galactic center in the radio wave range as well as with infrared radiation and X-rays . A strong radio source has been discovered there, called Sagittarius A * (Sgr A *), which emits from a very small area. This mass concentration is orbited by a group of stars in a radius of less than half a light year with an orbital period of about 100 years and a black hole with 1,300 solar masses three light years away. The star S2 , which is closest to the central black hole, orbits the galactic center in a strongly elliptical orbit with a minimum distance of about 17 light hours in a period of only 15.2 years. Its path could now be observed over a full circuit. The observations of the movements of the stars in the central star cluster show that within the orbit described by S2 there must be a mass of an estimated 4.31 million solar masses. The most plausible explanation for this large mass concentration within the framework of the theory of relativity and the only one consistent with all observations is the presence of a black hole .

The galactic center is currently in a comparatively quiet phase. But still around 3.5 million years ago, i.e. only a blink of an eye back in cosmological times, the central black hole was very active. There is some evidence that it will release 100,000 to 1 million times more energy over a period of several 100,000 years than the sun will ever radiate during its entire lifespan. This discovery was made while studying the Magellanic Current , a gas bridge that connects the Milky Way with the two Magellanic Clouds. An unusually large number of ionized carbon and silicon atoms were found in the gas of the Magellanic Current, which indicates an extremely high dose of UV radiation resulting from this energy output. During this active phase, the Milky Way behaved like a Seyfert galaxy .

Gamma-ray emitting bubbles

On November 9, 2010, Doug Finkbeiner of the Harvard-Smithsonian Center for Astrophysics announced that he had discovered two giant spherical bubbles extending north and south from the center of the Milky Way. The discovery was made with the help of data from the Fermi Gamma-ray Space Telescope . The diameter of the bubbles is each about 25,000 light years; they extend in the southern night sky from the maiden to the crane . Its origin has not yet been clarified.

Size comparison

You get a clear idea of ​​the size of the Milky Way with its 100 to 300 billion stars if you reduce it to a scale of 1:10 17 and imagine it to be drifting snow over an area 10 km in diameter and around 1 km in height on average. Each snowflake corresponds to a star and there are about three per cubic meter. On this scale, the sun would have a diameter of about 10  nm , i.e. it would be smaller than a virus . Even Pluto's orbit, which is on average about 40 times as far from the sun as the earth's orbit, would be at the limit of visual visibility with a diameter of 0.1 mm. Pluto itself, like the earth, would only have atomic dimensions. This model thus also demonstrates the low average mass density of the Milky Way. Furthermore, in this model our radio signals , which have been emitted for about 120 years, would have been about 11 m away from us (120 ly ), and the Arecibo message sent about 4 m away  .

Photo mosaic of the entire Milky Way band

The sun in the Milky Way system

The brightest stars in the vicinity of the sun (long-range image with right ascension; declination neglected)
The direct environment of the sun (approx. 2200 ly. × 1800 ly.)

The sun orbits the center of the Milky Way system at a distance of 25,000 to 28,000 light years (≈ 250  Em or 7.94 ± 0.42  kpc ) and is located north of the median plane of the galactic disk within the Orion arm in a largely dust-free area, the known as the " local bubble ". For one orbit around the center of the galaxy, a so-called galactic year , it takes 220 to 240 million years, which corresponds to an orbit speed of about 220 km / s. The investigation of this rotation is possible by means of the proper motion and the radial speed of many stars; around 1930, the Oort rotation formulas were derived from them . Nowadays, the apparent movement of the center of the Milky Way towards background sources caused by the orbital movement of the solar system can also be observed directly, so that the orbital speed of the solar system can be measured directly. More recent measurements have shown a speed of around 267 km / s (961,200 km / h).

The solar system does not orbit the galactic center on an undisturbed flat Kepler orbit . The mass distributed in the disk of the Milky Way system exerts a strong disturbance, so that in addition to its orbit around the center, the sun also oscillates regularly up and down through the disk. The disk crosses it once every 30 to 45 million years. About 1.5 million years ago it passed the disk in a northerly direction and is now about 65 light years (approx. 20 pc) above it. The greatest distance will be about 250 light years (80 pc), then the oscillating motion will reverse again.

Larger datable craters on earth as well as geological mass extinctions seem to have a periodicity of 34 to 37 million years, which conspicuously agrees with the periodicity of the disc passages. It is possible that the gravitational fields, which become stronger in the vicinity of the disk, disturb the Oort cloud of the solar system during a disk crossing , so that a larger number of comets enter the inner solar system and the number of severe impacts on earth increases. However, the periods in question are not yet known precisely enough to definitively establish a connection; more recent results (disk passage every 42 ± 2 million years) speak against it. A new study by the Max Planck Institute for Astronomy has shown that the apparent periodicity of the impacts is statistical artifacts and that there is no such connection.


Immediate neighborhood

Companion galaxies of the Milky Way System

A few dwarf galaxies are gathered around the Milky Way . The best known of these are the Large and Small Magellanic Clouds , to which the Milky Way system is connected by a hydrogen gas bridge , the Magellanic Current , which is about 300,000 light years long .

The closest galaxy to the Milky Way is the Canis Major Dwarf , at 42,000 light years from the center of the Milky Way and 25,000 light years from the solar system. The dwarf galaxy is currently being torn apart by the tidal forces of the Milky Way system, leaving behind a filament of stars that winds around the galaxy, known as the Monoceros Ring . However, it is not yet certain whether this is actually the remains of a dwarf galaxy or a random, projection-related cluster. Otherwise, the Sagittarius dwarf galaxy, 50,000 light-years from the galactic center, would be the closest galaxy to be incorporated into the Milky Way.

The Milky Way system is constantly incorporating dwarf galaxies and thereby increasing in mass. During the merger, the dwarf systems leave behind currents of stars and interstellar matter, which are torn out of the small galaxies by the tidal forces of the Milky Way system ( see also: interacting galaxies ). This creates structures such as the Magellanic Current, the Monoceros Ring and the Virgo Current , as well as the other high-speed clouds in the vicinity of the Milky Way.

Local group

With the Andromeda Galaxy, the Triangle Nebula and a few other smaller galaxies, the Milky Way System forms the Local Group , the most massive galaxy of which is the Milky Way. The local group is part of the Virgo supercluster , which is named after the Virgo cluster at its center. This belongs to the even larger Laniakea structure , as new methods of measuring galaxy positions and their relative movements have shown. The core of Laniakea is the Great Attractor . The local group within Laniakea is moving towards this. The Laniakea supercluster moves towards the Shapley supercluster , which suggests that together they could belong to an even larger structure.

The Andromeda Galaxy is one of the few galaxies in the universe whose spectrum shows a blue shift : The Andromeda Galaxy and the Milky Way system move towards each other at a speed of 120 km / s. However, the blue shift only provides information about the speed component parallel to the line connecting the two systems. It is believed that in about three billion years, the two galaxies will collide and merge into one larger galaxy. Due to the lack of precise knowledge of the space velocities and the complexity of the processes occurring during the collision, only probability statements can be made for the course of the collision. After the two galaxies merge, the end product is expected to be a massive elliptical galaxy . As the name for this galaxy, Cox and Loeb used the working name “Milkomeda” in their 2008 article, a fusion of the English Milky Way and Andromeda.


According to measurements from 2004, the Milky Way is around 13.6 billion years old. The accuracy of this estimate, which determines the age on the basis of the beryllium content of some globular clusters , is given as about ± 800 million years. Since the age of the universe of around 13.8 billion years is considered to be quite reliably determined, this would mean that the formation of the Milky Way can be dated to the early days of the universe.

In 2007, the star HE 1523-0901 in the galactic halo of the ESO observatory of the La Silla Observatory was initially determined to be 13.2 billion years old. In 2014, the Australian National University documented an age of 13.6 billion years for the star SM0313 , 6000 ly from Earth . As the oldest known objects in the Milky Way, these dates set a lower limit, which is in the range of the measurement accuracy of the 2004 estimate.

Using the same method, the age of the thin galactic disk can be estimated from the oldest objects measured there, resulting in an age of around 8.8 billion years with an estimate of around 1.7 billion years. On this basis, there would be a time gap of around three to seven billion years between the formation of the galactic center and the outer disk.

See also

Observation of the center of the Milky Way using a device for creating an artificial guide star at the Paranal Observatory


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Web links

Wiktionary: Milky Way  - Explanations of meanings, word origins, synonyms, translations
Commons : Milky Way  - album with pictures, videos and audio files

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This version was added to the list of articles worth reading on May 8, 2006 .