Spiral galaxy

A spiral galaxy outdated, even spiral nebula, a disk-shaped galaxy, whose appearance shows a spiral pattern. The central area, called Bulge is spheroidal and consists mainly of older stars. The disc shows a spiral structure, usually with several spiral arms. Spiral galaxies contain relatively much gas in the disk. This new stars can be formed permanently. The spiral arms appear bluish by the newly formed star here. Nestled in the galaxy is a halo of invisible dark matter. Together with the lenticular galaxies Spiral galaxies are summarized as disk galaxies. Galaxies, where starting a beam is visible from the Bulge, at the start the spiral arms, called barred spiral galaxies. Our Milky Way is a barred spiral galaxy itself. Within a radius about 30 million light-years of the Milky Way are about 34 percent of the galaxies spirals, ellipses, and 13 percent 53 percent irregular galaxies and dwarf galaxies.

  • 4.1 disc 4.1.1 rotation curve
  • 4.1.2 Thin and thick disk
  • 4.1.3 Warp
  • 4.1.4 gaseous disk


The first telescopic observers such as Charles Messier discovered in the foggy spots in the sky no further structures, and could therefore make no distinction between galaxies and nebulae. Only 1845 saw William Parsons, 3rd Earl of Rosse with its largest telescope in the world at this time, the spiral structure of some of these foggy spots, first at the Whirlpool Galaxy. However, was still unclear whether these nebulae are part of the Milky Way, or separate and distant objects. This uncertainty was a central topic at the Great Debate, which took place in 1920 between the astronomers Harlow Shapley and Heber Curtis.

Only in 1926, Edwin Hubble discovered Cepheids, ie, a particular type of variable stars with well-known properties, in several spiral nebulae. This became clear that spiral galaxies are very far away objects are. In 1936 he described the spiral galaxies in more detail in his book, The Realm of the Nebulae.


In a spiral galaxy, the following structures can be recognized:

  • A flat, rotating disk of stars, gas and dust. The disk may be divided into a thin component containing a lot of gas and newly formed star, and a thick disc, which contains mainly older stars. The thin slice contains 65 % of the visible mass of the galaxy, the thick disk only 5%.
  • A central component, the central body or bulge called. This consists mainly of older stars. The central body contains 33 % of the visible matter.
  • It is now taken for granted that every galaxy is a supermassive black hole at the center.
  • The galactic halo consists of widely scattered, older stars, and a large number of globular clusters that orbit the galaxy slowly. The halo contributes only 1 % to the visible matter. However, it contains 95% of the total matter of the galaxy in the form of dark matter.


Classification according to the Hubble diagram

The most widely used classification scheme for galaxies, the Hubble diagram. Herein, the galaxies are classified according to their visual impact. Although the Hubble diagram can be derived no evolution of galaxies, it can still be many physical properties assigned to the individual classes. Spiral Galaxies are classified according to the ratio of the brightness of the bulge and the disk, and the opening angle of the spiral arms in the classes Sa to Sd ( more accurate than SAa to SAd ). Barred spirals will be named SBa to SBd. These galaxies have an outgoing from the center long beam, begin at the end of the spiral arms.

Comparing the different classes of Sa to south, we can see the following properties:

  • From Sat by Sd the gas content in the galaxy grows. This also increases the number of young stars and the star formation rate.
  • From Sat by Sd the ratio of disk to the central body grows.
  • From Sat by Sd the opening angle of the spiral arms increases of about 6 ° at Sa galaxies to about 18 ° in Sc galaxies.

Expression of the spiral structure

Spiral galaxies may also be classified based on the severity of the spiral pattern.

  • Grand design spiral galaxies show two clearly defined and symmetrical spiral arms. These make up 10% of 20 % of the known spiral galaxies.
  • Flocculent spiral galaxies show a rugged relief. Whole spiral arms can not be traced, in part, only approaches of arms are available. About 20 to 30% of spiral galaxies show that type.
  • Approximately 60% of spiral galaxies show more than two spiral arms.
  • Very rarely are one-armed spiral galaxies called Magellanic spiral. These are referred to by their example, the Large Magellanic Cloud.

The luminosity of a galaxy correlate with the severity of the spiral structure. Therefore, can also create a division into so-called luminosity classes (Roman IV). This classification 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 a spiral arm sets available

Examples / Table


NGC 4565: Galaxy in Edge On view

Since spiral galaxies, in principle, have the form of a thin disk, the impression varies greatly depending on the viewing angle, which we have on the galaxy. The so-called " Face On " view you can see a frontal view of the galaxy, and you can see the entire spiral structure. In " Edge On " you can see on the edge of the disc. Here one usually sees a horizontal division by dark dust regions along the edge.

Direction of rotation

In the first analyzes of the SDSS Sloan Digital Sky Survey, the theory arose that preferably spiral galaxies rotate in one direction. To confirm this or disprove the online project Galaxy Zoo was launched into being, in which thousands of amateurs galaxy images valued according to its direction of rotation. A preferred direction of rotation did not stand out here, however.



Rotation curve

In spiral galaxies that we see from the side, can be calculated using the Doppler effect to measure how fast the disc rotates: One half of the disc comes up to us and shows a blue shift, and the other half shows a redshift. With the help of the Kepler laws can predict how fast a star must move at a certain distance from the center to the galaxy. It is also considered that the visible mass of a galaxy is not concentrated in one point is distributed as in our solar system, but in the disk. In the measurements, however, it turned out that the rotational speed of the stars first, as expected, increases strongly with distance from the center. But instead of a speed decrease with increasing distance to the center it will remain almost constant up to the edge of the disc. This is explained with a halo of dark matter, in which the galaxies are embedded, which strongly affects the rotation of the disc.

Thin and thick disk

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 also observed in other galaxies. The thin disc contains relatively young stars (< 9 billion years) with a high metal content. In it, the spiral arms and the interstellar material are embedded. It has a thickness between 100 and 400 pc. The thick disk has a up to ten times the height of the thin disk and consists of metal-poor, old stars (> 12 billion years). They could consist of remnants of smaller galaxies that are fused in the formation of the spiral galaxy. Distinguishing these two components can be just by age and by the velocities of the stars.


In some spirals to an S-shaped bending of the plate could be determined. The bending usually begins at the edge of the visible disk, and continues through the more extended gas disk. This bending is called Warp and could arise by fusion processes with smaller galaxies. Investigations revealed that at least 50 % of all spiral galaxies contain a warp.

Gas disk

The principal portion of the gas in the disc consists of a neutral hydrogen. Here, the gas disk extends far beyond the visible stellar disk, sometimes up to twice the diameter. Embedded within it are colder molecular clouds where star formation begins. When stars are formed from the collapsed molecular clouds, so ionize the most luminous of them the surrounding gas. This results in HII regions that grow and thereby create voids in the neutral gas disk.

Spiral structure

The most striking feature of spiral galaxies whose spiral arms. The stars themselves can not form a solid spiral structure, since then the spiral arms would wrap due to the differential rotation of the galaxy after a few galactic revolutions ever closer to the center. To explain the formation of the spiral structure, several theories have been proposed that can explain the observed structures well.

Bertil Lindblad introduced in 1925, the theory that the orbits of stars in galaxies are in resonance with each other. Thus the orbits are synchronized with each other and there are density waves. This theory of density waves has been further developed by Chia- Chiao Lin and Frank Shu in the 1960s. The stars and gas clouds are moving in its orbit around the galaxy several times in such a density wave pushed in and out. The gas is compressed, it creates new stars. The most massive and therefore short-lived among them shine bright and blue and mark as the spiral arms. Due to their short lifetime they never leave the spiral arm, but explode in advance and promote the shock waves occurring further star formation.

A density wave can be compared behind a roadworks on the motorway with a good jam. Cars drive in the traffic jam inside ( the traffic density increases ), and after the site out again. The roadworks moves slowly at a constant speed. Although it looks as if the stars existed only in the spiral arms, there are relatively many stars also between the arms. In the range of a spiral, the density is about 10 to 20 percent more than the outside arm. Stars and gas masses in the vicinity are attracted by something stronger.

The " Stochastic self- propagating star formation" theory attempts to explain the spiral structure by shock waves in the interstellar medium. Here are caused by supernova explosions, shock waves, which in turn promote the star formation in gas. Due to the differential rotation of the galaxy so creates a spiral pattern. However, this theory can not explain the large-scale and symmetrical spiral structures, such as detected in the Grand- design spirals.

Orbits of the stars

The stars in the disk all do not move in the same direction in elliptical orbits around the center of the galaxy, but like planets in the solar system. For the mass of the galaxy is not concentrated enough. After a round of the star does not return to its starting point, thus the path forms the shape of a rosette. Furthermore, a star moved by the attracting force of the disc up and down in the disc plane. Thus, the disc receives its thickness. (PDF, 3.3 MB) This star remain trapped in the gravitational field of a bar, carry out these intricate webs. Most of the paths are elongated ellipses along the beam, but there are also loop lifts and reversals in the direction of movement.

The stars in the bulge and the halo, however, move in all possible directions and different angles around the galaxy.


Approximately 50% of spiral galaxies show a beam structure. A bar is off when the orbits of the stars become unstable and deviate from a circular orbit rather. The sheets are elongated and the stars start to move along the beam. In a resonance behavior of these other stars to follow. This axisymmetric and cigar-shaped disturbance is formed, which is visible as a bar. The bar itself rotates as a rigid structure. Bars are an important factor in the development of the galaxy because they allow gas to a large extent to the center of the galaxy flow, and there fan the star formation.


A bulge in the center of the spiral galaxy consists mainly of older, metal-poor stars. Some bulges have properties similar to an elliptical galaxy, others are just compressed centers of the disc. It is assumed that there is a massive black hole at the center of the bulge. The mass of the black hole appears 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 around will be marked by a large number of globular clusters and some old stars of Population II These objects are left, as the initial gas collected in the galaxy formation in the disk. The globular clusters consist of very old, metal-poor stars and have all emerged at the same time. In some cases it is assumed that the halo of the remains of the collected small satellite galaxies during galaxy formation is. The main part of the halo is, however, in the form of invisible dark matter. Through their gravitational influence this matter determines the overall development of the galaxy. The exact extent of the halos can usually not be determined precisely.

Another component of the halo is the corona. It consists of millions of degrees hot gas. This gas could be detected with the Chandra X-ray telescope in the galaxy NGC 4631. Such a gas corona was expected from the evolution of supernova remnants, which extend over the windshield out and transport of hot gas in the halo.

Cosmic Matter Circuit

Spiral galaxies are very dynamic systems. Due to their high gas content, the star formation is still in progress. This gives rise to complex interactions between the different components of the galaxy. Due to the density waves described above atomic gas (HI) is compressed, it formed molecular gas clouds. Some of the molecular gas clouds begin to collapse, and there are new stars in its interior, very many with low mass, a few very massive. These massive stars explode very early after only a few million years ago as a supernova. Due to the explosions, the interstellar medium is enriched with heavy elements. By supernovae and stellar winds, gas is accelerated, it generated shock waves in the surrounding gas. This in turn compress more gas clouds with which the star formation cycle begins again anew.

By supernova explosions occur in the gas disk, so-called hot bubbles, empty swept through the accelerated and ionized gas chambers. Through several explosions also several bubbles can connect. If there is such an empty space on the edge of the disc, then the hot and ionized gas can leave here by the lack of resistance of the disk plane, and ascend as a galactic fountain in the halo. This could be a source of so-called high velocity clouds. This fall at a later date at a speed of about 200 km / s on the disc back. Here, too, an impulse for further star formation is given again.

Magnetic field

Magnetic fields are an important component in the interstellar medium of spiral galaxies. In spiral galaxies magnetic fields were observed, which are aligned along the spiral arms and a strength of one hundred thousandth of a gauss have ( Earth 0.5 gauss). Since the interstellar gas is not electrically neutral, the magnetic fields affect the flow of gas in the spiral arms. The origin of the fields is not yet clearly understood. In the formation of the galaxy star formation must already be present magnetic fields. However, these fields can not keep up to the present time. Therefore, there must be a mechanism that maintains the magnetic field. According to the dynamo model, the galactic magnetic field turbulence, which are formed during star formation, supernova explosions and by incidence of cold gas in the galactic disk feeds. A further energy source of the field, the differential rotation of the disc.