A spiral galaxy , also obsolete spiral nebulae , is a disk-shaped galaxy whose appearance shows a spiral pattern . The central area, called the bulge , is spheroidal and consists mainly of older stars. The disk shows a spiral structure with mostly several spiral arms . Spiral galaxies contain a relatively large amount of gas in the disk. This means that new stars can be formed permanently. The spiral arms appear bluish due to the newly formed stars. The galaxy is embedded in a halo of invisible dark matter . Together with the lenticular galaxies , spiral galaxies are also grouped together as disk galaxies . Galaxies in which a bar is visible starting from the bulge, to which the spiral arms attach, are called bar-spiral galaxies . The Milky Way itself is a barred spiral galaxy. Within a radius of around 30 million light years around the Milky Way, around 34 percent of the galaxies are spirals, 13 percent ellipses and 53 percent are irregular and dwarf galaxies .
The first telescope observers like Charles Messier saw no further structures in the foggy spots in the sky and could therefore not make any distinction between galaxies and nebulae . It was not until 1845 that William Parsons, 3rd Earl of Rosse , first recognized the spiral structure of some of these foggy spots in the Whirlpool Galaxy with what was then the largest telescope in the world . However, it was still unclear whether these nebulae are part of the Milky Way or independent and distant objects. This ambiguity was central to the great debate that took place in 1920 between the astronomers Harlow Shapley and Heber Curtis .
It was not until 1926 that Edwin Hubble discovered a certain type of periodic variable stars in several 'spiral nebulae' Cepheids , the luminosity of which correlates closely with the period, so that their distance can be determined. This made it clear that spiral galaxies are very distant objects. In 1936 he described the spiral galaxies in more detail in his book The Realm of the Nebulae .
The following structures can be seen in a spiral galaxy:
- A flat and rotating disk of stars, gas and dust. The disk can be divided into a thin component, which contains a lot of gas and newly formed stars, and a thick disk, which mainly contains older stars. The thin disk contains 65% of the visible mass of the galaxy, the thick disk only 5%.
- A central component, called a central body or bulge . This consists mainly of older stars. The central body contains 33% of the visible matter.
- It is now believed to be certain that there is a supermassive black hole in the center of every galaxy .
- The galactic halo consists of widely dispersed, older stars and a multitude of globular clusters that slowly orbit the galaxy. The halo contributes only 1% to the visible matter. However, it contains 95% of all matter in the galaxy in the form of dark matter .
Classification according to the Hubble scheme
The most common ordering scheme for galaxies is the Hubble scheme . The galaxies are classified according to their visual impression. Although the Hubble scheme does not allow a history of evolution of the galaxies to be derived, many physical properties can be assigned to the individual classes. Spiral galaxies are classified according to the ratio of the brightness of the bulge and the disk as well as the opening angle of the spiral arms in the classes Sa to Sd (more precisely as SAa to SAd). Bar spirals are named SBa to SBd. These galaxies have a long bar extending from the center, at the end of which the spiral arms attach.
If one compares the different classes from Sa to Sd, one finds the following properties:
- From Sa to Sd the gas content in the galaxy increases. This also increases the number of young stars and the rate of star formation.
- From Sa to Sd, the ratio of disk to central body increases.
- The opening angle of the spiral arms increases from Sa to Sd, from about 6 ° for Sa galaxies to about 18 ° for Sc galaxies.
Expression of the spiral structure
Spiral galaxies can also be classified based on the shape of the spiral pattern.
- Grand Design spiral galaxies show two clearly defined and symmetrical spiral arms. These make up 10% to 20% of the known spiral galaxies.
- Flocculent spiral galaxies show a disrupted structure. Whole spiral arms cannot be followed, in some cases only the beginnings of arms are present. About 20 to 30% of spiral galaxies show this type.
- About 60% of spiral galaxies show more than two spiral arms.
- One-armed spiral galaxies, called Magellanic Spirals, are very rare . These are named after their model, the Large Magellanic Cloud.
The luminosity of a galaxy correlates with the development of the spiral structure. Therefore, a division into so-called luminosity classes (Roman IV) can be created. This division extends the Hubble classification.
- Luminosity class I: high surface brightness, well-defined spiral arms
- Luminosity class III: torn and short spiral arms
- Luminosity class V: only spiral arm extensions available
Examples / table
|designation||description||Dimensions||Spiral galaxy||Barred spiral galaxy|
|SAa / SBa||coherent and tight-fitting arms, large bulge||0.2 to 6 · 10 12 M ☉||
M104 Type: SA (s) a
NGC 1291 type: (R_1) SB (l) 0 / a
|SAb / SBb||arms slightly open, medium-sized bulge||0.2 to 5 · 10 12 M ☉||
M 81 type: SA (s) from
NGC 1365 type: (R ') SBb (s) b
|SAc / SBc||weak bulge, arms wide open and torn||0.2 to 4 · 10 12 M ☉||
M 74 type SA (s) c
NGC 1300 type: (R ') SB (s) bc
|SAd / SBd||Spiral structure dissolves, transition type to an irregular galaxy||1 · 10 10 M ☉||
NGC 300 type: SA (s) d
NGC 1313 Type: SB (s) d
Since spiral galaxies basically have the shape of a thin disk, the impression changes very much depending on the viewing angle we have on the galaxy. In the so-called “Face On” view, you can see the galaxy from the front and see the entire spiral structure. With "Edge On" you can see the edge of the pane. Here you can usually see a horizontal division into two parts by dark dust regions along the edge.
Direction of rotation
During the initial analyzes of the Sloan Digital Sky Survey , the theory emerged that spiral galaxies prefer to rotate in one direction. To confirm or disprove this, the online project Galaxy Zoo was launched, in which thousands of amateurs rated galaxy images according to their direction of rotation. However, a preferred direction of rotation was not found here.
In spiral galaxies that we see from the side, the Doppler effect can be used to measure how fast the disk rotates: one half of the disk comes towards us and shows a blue shift , and the other half shows a red shift . With the help of Kepler's laws one can predict how fast a star has to move around the galaxy at a certain distance from the center. It is also taken into account that the visible mass of a galaxy is not concentrated in one point as in the solar system, but is distributed in the disk. During the measurements it turned out, however, that the orbital speed of the stars increases strongly with the distance from the center, as expected. But instead of a decrease in speed with increasing distance from the center, this remains almost constant up to the edge of the disk. This is explained with a halo of dark matter in which the galaxies are embedded, which greatly influences the rotation of the disk.
Thin and thick slice
The disk of a spiral galaxy can be divided into a thin disk and a thick disk. This subdivision has been studied in the Milky Way and observed in other galaxies. The thin disk contains relatively young stars (<9 billion years) with a high metal content . The spiral arms and the interstellar material are embedded in it. It has a thickness between 100 and 400 pc. The thick disk is up to ten times the height of the thin disk and consists of low-metal, old stars (> 12 billion years). It could consist of remnants of smaller galaxies that merged with the spiral galaxy as it formed. These two components can be differentiated by the age and the speed of the stars.
In some spiral galaxies, an S-shaped deflection of the disk was found. The bending usually begins at the edge of the visible pane and continues through the more extensive gas pane. This bending is called a warp and could result from merging processes with smaller galaxies. Research has shown that at least 50% of all spiral galaxies contain a warp.
Most of the gas in the disk consists of neutral hydrogen. The gas disk expands far beyond the visible star disk, sometimes up to twice its diameter. Embedded in it are colder molecular clouds in which star formation begins. As soon as stars emerge from the collapsed molecular clouds, the most luminous of them ionize the surrounding gas. This creates HII regions that expand and thereby create cavities in the neutral gas disk.
The most striking characteristics of the spiral galaxies are their spiral arms . The stars themselves cannot form a solid spiral structure, as the spiral arms would then wrap themselves closer and closer around the center due to the differential rotation of the galaxy after a few galactic revolutions. In order to explain the formation of the spiral structure, several theories have been put forward that can explain the observed structures well.
As early as 1925, Bertil Lindblad proposed the theory that the orbits of stars in galaxies are in resonance with one another. As a result, the orbits are synchronized with each other and density waves are created. This theory of density waves was further developed by Chia-Chiao Lin and Frank Shu in the 1960s. The stars and gas clouds move in and out of such a density wave several times during their orbit around the galaxy. The gas is compressed and new stars are created. The most massive and therefore very short-lived among them shine bright and blue and thus mark the spiral arms. Due to their short lifespan, they never leave the spiral arm, but rather explode beforehand and the shock waves that occur promote further star formation.
A density wave can be compared well with a traffic jam behind a mobile construction site on the motorway. Cars drive into the traffic jam (traffic density increases) and out again after the construction site. The mobile construction site moves slowly at a constant speed. Even if the stars appear to only exist in the spiral arms, there are also relatively many stars between the arms. In the area of a spiral arm, the density is about 10 to 20 percent more than outside the arm. Stars and gas masses in the vicinity are thereby attracted a little more.
The "Stochastic self-propagating star formation" theory tries to explain the spiral structure through shock waves in the interstellar medium. This creates shock waves through supernova explosions, which in turn promote star formation in gas. The differential rotation of the galaxy creates a spiral pattern. However, this theory cannot explain the large-scale and symmetrical spiral structures that can be seen in grand design spirals.
Orbits of stars
The stars in the disk all move in the same direction in elliptical orbits around the center of the galaxy, but not like planets in the solar system. The mass of the galaxy is not concentrated enough for that. After one orbit the star does not return to its starting point, so the path forms the shape of a rosette . In addition, a star moves up and down in the plane of the disk due to the attraction of the disk. This gives the pane its thickness. So that stars remain trapped in the gravitational field of a bar, they follow complicated orbits. Most of the tracks are elongated ellipses along the beam, but there are also loop tracks and reversals in the direction of movement.
The stars in the bulge and in the halo, on the other hand, move in all possible directions and at different angles around the galaxy.
About 50% of the spiral galaxies show a bar structure. A bar forms when the orbits of the stars become unstable and deviate from a more round orbit. The orbits become elongated and the stars begin to move along the beam. Further stars follow these in a resonance behavior. This creates an axially symmetrical and cigar-shaped disturbance that is visible as a bar. The beam itself rotates as a rigid structure. Bars are an important factor in the evolution of the galaxy as they allow gas to flow to the center of the galaxy on a large scale and fuel star formation there.
A bulge in the center of the spiral galaxy consists mainly of older, metal-poor stars. Some bulges have properties similar to those of an elliptical galaxy , others are just condensed centers of the disk. It is believed that there is a massive black hole in the center of the bulge . The mass of the black hole seems to be directly related to the mass of the bulge: the greater the mass of the bulge, the more massive the black hole.
Halo and Corona
The visible area of the halo around a spiral galaxy is marked by a large number of globular clusters and some old stars of Population II. These objects are left over when the original gas collected in the disk during the formation of the galaxies. The globular clusters consist of very old, metal-poor stars and were all formed at the same time. It is sometimes assumed that the halo consists of the remains of small satellite galaxies that were collected during the formation of the galaxies. However, the main component of the halo is invisible in the form of dark matter. Due to its gravitational influence, this matter determines the entire evolution of the galaxy. The exact extent of the halo can usually not be determined exactly.
Another component of the halo is the corona. It consists of gas at a temperature of millions of degrees. This gas could be detected with the Chandra X-ray telescope near the galaxy NGC 4631. Such a gas corona was expected from the development of supernova remnants that extend beyond the disk and carry hot gas into the halo.
Cosmic matter cycle
Spiral galaxies are very dynamic systems. Star formation is still in progress due to its high gas content. This creates complex interactions between the individual components of the galaxy. Atomic gas (HI) is compressed by the density waves described above and molecular gas clouds are formed. Some of the molecular gas clouds begin to collapse, and new stars are formed inside them, many of them of low mass, a few of them very massive. These massive stars explode as a supernova very early after only a few million years. The explosions enrich the interstellar medium with heavy elements. Gas is strongly accelerated by the supernovae and stellar winds, and shock fronts are generated in the surrounding gas. These in turn condense further gas clouds with which the star formation cycle begins again.
The supernova explosions also create so-called hot bubbles in the gas disk, spaces that are swept empty by the accelerated and ionized gas. Multiple explosions can connect multiple bubbles. If there is such an empty space at the edge of the disk, the hot and ionized gas can leave the disk plane due to the lack of resistance and rise up into the halo as a galactic fountain . This could be a source of something called high speed clouds. These fall back onto the disc at a later point in time at a speed of around 200 km / s. Here, too, an impulse is given for further star formation.
Magnetic fields are an important component in the interstellar medium of spiral galaxies. In spiral galaxies, magnetic fields have been observed that are aligned along the spiral arms and have a strength of one hundred thousandth of a Gauss (Earth 0.5 Gauss). Since the interstellar gas is not electrically neutral, the magnetic fields influence the gas flow in the spiral arms. The origin of the fields has not yet been precisely clarified. When the galaxy is formed, the star formation means that magnetic fields must already be present. However, these fields cannot hold up to the present day. So there has to be a mechanism to maintain the magnetic field. According to the dynamo model, the galactic magnetic field is fed by turbulence that forms during star formation, through supernova explosions and through the impact of cold gas into the galaxy's disk. Another source of energy for the field is the differential rotation of the disk.
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