The Milky Way

of these articles treats the Milky Way in the astronomical sense; for the French film of the same name see the Milky Way
in such a way could the Milky Way to an outside viewer present itself. One considers the central bar. The star formation areas are additionally red marked in the spiral arms

The Milky Way is the name for the bandförmige clarification in the night sky, which almost re-clamps the sky ball as stepping out symmetry plane of the milchstrassensystems along a great circle. The term stands occasionally also as shortening name for the milchstrassensystem , also galaxy mentioned. That Star system of the type of a bar spiral galaxy is the homeland of our solar system. Others, extragalactic star islands are called galaxy.

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history and origin of the name

Die Milchstraße am Nachthimmel, abgebildet ist die Region zwischen Deneb (unterhalb Bildmitte) und Cassiopeia (oben rechts)
thoseThe Milky Way in the night sky, shown is the region between Deneb (below picture center) and Cassiopeia (right above)

in the antiquity was well-known already the Milky Way as bright, narrow strip in the night sky. An antique Greek legend reports the their of the goddess Hera,Stepson Herakles satisfied. As this zubiss, the whole milk was sprayed too impetuously over the sky. From the Greek word „Gala “(milk) developed the Greek term γαλαξίας, (galaxías male - the milchige star fog), therefore the word galaxy comes.

Also the African shrub men found a very poetic name for the Milky Way: „The backbone of the night “.

Only 1609 could it Galileo Galilei the genuine explanation for the Milky Way discover and could at least partly into its individual stars divide.

To firstConception of the disk form arrived already to William Herschel in the year 1785 due to systematic star countings (stellar statistics).


the volume of the Milky Way extends as irregularly broader, weakly milk-industrial union-bright strip over the Firmament. Its feature agitates therefore,that in it with the naked eye no single stars are noticed, but a multiplicity of faint stars. In order to see it, very good observation conditions are necessary, like clear air and the absence of artificial sources of light within some kilometers around the observation place. Over itoutside all belong to that approximately 6000 with the naked eye stars visible in the night sky to the milchstrassensystem.

structure of the galaxy

general structure

infrared photograph of the Milky Way by the satellite COBE. Disk and central bulge are recognizable.

ThatMilchstrassensystem is four or fünfarmige bar spiral galaxy. It consists of approximately 300 billion stars and large quantities of interstellar subject, which constitutes again 600 million to some billion solar masses. The mass of this internal range of the galaxy becomes with approximate3,6×10 41 kg estimates. Their expansion in the galactic level amounts to about 100,000 light-years (30 kpc), the thickness of the disk about 3000 light-years (920 PC) and those the central bulge (English. Bulge) about 16,000 light-years (5 kpc). Outthe movement of interstellar gas and the star distribution in the Bulge an oblong form results for this. This bar forms an angle of 45° with the connecting line of the solar system for the center of the Milky Way. The Milky Way is thus probably a bar spiral galaxy ofHubble type SBc. In accordance with a regulation assistance of the infrared - space telescope pencil sharpener is surprisingly with an expansion of 27,000 light-years long the straight bar pattern.

The total mass of the Milky Way becomes estimated on 1.9 trillion solar masses, thus is it the galaxy most massive with distancethe local group (see. Andromedanebel: 1,2 trillion).

the galactic halo

surrounding is more galactic the galaxy of the spherical galactic halo with a diameter of approximately 165,000 light-years (50 kpc), a kind of „atmosphere “. In itare beside that about 150 globular star clusters only further old stars, under it RR Lyrae variable one, and gas of very small density. Exception are the hot Blue Straggler - stars. In addition come large quantities of dark subject with approximately 1 trillion solar masses, under italso so-called MACHOs. Differently than the galactic disk the halo is to a large extent dust free and contains almost exclusively stars of the older, metal-poor population II, whose orbit is very strongly bent against the galactic level.

the galacticDisk

the majority of the stars within the galaxy is approximately evenly distributed on the galactic disk. It contains contrary to the halo above all stars of the population I, which elements heavy by a high portion are characterised.

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Spiral arms

part of the disk are also the spiral arms characteristic of our Milky Way. In the spiral arms are enormous accumulations of hydrogen and also the largest HII regions, the star formation areas of the galaxy. Therefore are there also many Protosterne,young stars of the T-Tauri-type and Herbig Haro objects. During their lifetime stars of their birth places move away and distribute themselves on the disk. Particularly massive and luminous stars depart however due to their shorter life span not so far the spiral arms,why these step out. Therefore above all stars of the spectral classes O and B, over giant and Cepheiden , everything belong younger than 100 million years to the stellar objects there present. They place however only about a per cent of the stars in the Milky Way.The largest part of the mass of the galaxy consists of old, mass-poor stars. The “gap” between the spiral arms is thus not empty, but is simply only less shine strong.

Schema der beobachteten Spiralarme unseres Milchstraßensystems (siehe Text).

The spiral structure of the galaxy could by the observation of the distribution of neutral hydrogen to be confirmed. The discovered spiral arms were designated after the constellations of stars lying in their direction.

The design right represents the structure of the Milky Way schematically. The center is not visible, it shades also the range behind it.Our sun (yellow circle) is appropriate for Orionarm between the spiral arms Sagittarius and Perseus, on the local arm, also mentioned. Probably this arm is not complete, sees brown line in the illustration. In relation to this direct environment movesthe sun with approximately 30 km/s toward the constellation Herkules. The internal arm is the Norma arm (also 3 k PC - arm), the extreme (not in the illustration) is the so-called Cygnus arm, which probably the continuation of the Scutum Crux arm is.

The distance to W3OH, a star formation area in the Perseus - arm (blue cross) could be measured by means of Triangulierung and VLBA directly. Them amount to 6357 ± 130 light-years [1]. The measurement is based on a VLBA with a basis length ofapprox. 8000 km with a radio frequency of 12 GHz.

The Milky Way toward the saying CCIT air US arm, on the right of the Eta Carinae nebulas NGC 3372, a HII region
of spiral arms
standard name alternative name astronomical
Norma arm 3kpc-Arm (- ring) no
Scutum Crux arm Centaurus arm - II
saying CCIT air US arm Sagittarius Carina arm - I
Orion arm of local ones arm 0
Perseus arm +I
Cygnus arm outside arm +II

which processes for the emergence of the spiral structure is not responsible is clearly clarified so far not yet. However it is clear that the stars belonging to to the spiral arms do not represent rigid structure, itself in formationaround the Milky Way center turns. This would be the case became the spiral structure of the Milky Way and other spiral galaxies, due to which different path speeds roll up, within relatively short time and become unrecognizable. So-called density waves offer an explanation. The spiral arms place zonesincreased subject density and star formation, which move independently of the stars by the disk.

stars of the galactic disk

the stars of the galactic disk ranking among the population I leave themselves with increasing dispersion around the main level andOlder in three Unterpopulationen divide. The so-called “Thin disk” within a range from 700-800 light-years over and below the galactic level contains of the spiral arms, which depart only maximally 500 light-years from the level beside the luminous stars specified above,Stars of the classes A and F, some giants of the classes A, F, G and K, as well as Zwergsterne of the classes G, K and M and also some white dwarves. The Metallizität of these stars is comparable to the sun with that,usually in addition, twice as highly, their age is with approximately a billion year.

A further group is those of the centralold stars (age up to five billion year). In addition the sun and further Zwergsterne of the spectral types G, K count andM, as well as some under and red giants. The metal content is here clearly smaller with only about half to once as many as with the sun. Also the Bahnexzentrität of the galactic orbits of these stars is higher and it continues to be notas 1500 light-years over or below the galactic level.

Between maximally 2500 light-years above and below the main level the “ Thick disk” extends. There are red K and M of dwarves, white dwarves, as well as some under and red giants, in addition, long-periodic variable ones. Their age reached up to ten billion year and it are comparatively metal-poor (about a quarter of the Sonnenmetallizität). This population resembles also many stars in the Bulge.

The galactic disk is not perfect straight,by gravitative reciprocal effect with the Magellanic Clouds it is easily in their direction downward curved.


a 900 light-years broad cutout of the central region of the Milky Way

major item galactic center

the center of the Milky Way lies in the constellation contactorand is hidden behind dark gas clouds, so that it cannot be observed in the visible light directly. Beginning into the 1950er years succeeded, in the radiowave range as well as with infrared radiation and x-ray increasingly more highly detailed pictures from the close environment of theto win galactic center. One discovered there a strong radio source, called Sagittarius A* (Sgr A*), which radiates from a very small area. Within this region is, concentrated on an area of 15.4 million km diameter, a massfrom estimated 2.6 to 4 million solar masses. One assumes generally it concerns thereby supermassif a black hole. This mass concentration becomes from a group of stars in a radius of less than a half light-yearwith a scan time of approximately 100 years, as well as a black hole with 1300 solar masses in three light-years distance circles. The central black hole, the galactic center at a distance of approximately 17 light hours rotates star S2 next lyingin one period of only 15.2 years at immense speed. In January 2005 by the Chandra Röntgenteleskop brightness outbreaks were observed in close proximity to Sgr A*, which suggest that itself in the periphery of approx. 70 light-years around Sgr A*10.000 to 20,000 black holes finds, which circle the super+substantial central black hole in Sgr A*.

size size of

one gets a descriptive conception of the size of our galaxy with their 300 billion stars, if one it itself inYardstick 1:10 of 17 reduced as Schneetreiben in an area of 10 km diameters and a height of approximately 1 km on the average presents. Each flake corresponds thereby to a star and it gives about 3 pieces per cubic meter. Our sun would havein this yardstick a diameter of approximately 10 Nm, would be thus smaller than a virus. Even the Plutobahn would be with a diameter of 0.1 mm because of the border of the visual recognizability.Pluto would have only atomic ones just like the earthDimension. Thus this model demonstrates also the tremendously small mass density in the cosmos, which seems in the contradiction to the impressing photos of galaxies as close fire wheels to stand.

situation of the sun in the milchstrassensystem

Die direkte Umgebung der Sonne (Maßstab 250 Lj.)
the direct environment of the sun(Yardstick 250 Lj.)

the sun circles the center of the milchstrassensystems in a distance of 25.000 to 28,000 light-years and is about 15 light-years north the centre plane of the galactic disk within the Orion arm, in a to a large extent dust free space, thatas “restaurants blister “admits is. For a circulation around the center of the galaxy, which needs galactic year so mentioned, it 220 to 240 million years, which corresponds to a rotation speed of approximately 220 km/s. The researchis possible for this rotation by means of the independent movement and the radial velocity of many stars; from them around 1930 the Oort rotation formulas were derived.


with the Andromeda Galaxy and unite other smaller galaxies forms the Milky Way the local group,whereby the Milky Way is the most massive galaxy under it although it does not possess the largest expansion. Around the milchstrassensystem some irregular dwarf galaxies are met. The most well-known of it are the large one and the small Magellanic Cloud, with those the Milky Wayover one about 300,000 light-years long hydrogen gas bridge, Magellanic Stream, is connected which.

Situation of the Canis major dwarf galaxy and the mono cerium OS ring in our Milky Way

those the milchstrassensystem next convenient galaxy is the Canis major dwarf, with a distance of only 42,000 light-yearsfrom the Milky Way center and 25,000 light-years of our solar system. The dwarf galaxy is torn apart at present by the tidal forces of the Milky Way similarly as the 50,000 light-years of the galactic center removed saying CCIT air US dwarf galaxy and leaves themselves thereby a filament from stars, around thoseThe Milky Way winds, the so-called mono cerium OS ring.

Loud generally recognized computer simulations move the Andromeda Galaxy and the Milky Way on collision course. They approach with a speed of 120 to 300 km/s and in approximately two to six billion years will each other penetrateand into an accordingly larger star system merge. At present the Milky Way still moves in a distance of approximately 2.5 million light-years around the Andromedanebel. The data in detail deviate still very from each other.


measurements from thatYears 2004 according to the milchstrassensystem about 13.6 billion years is old. The accuracy of this estimation, which determines the age on the basis the beryllium portion of some globular star clusters, is indicated as approximately 800 million years.

see also


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