What kind of galaxy is Andromeda

The Andromeda Galaxy

If the Andromeda galaxy was called the "sister of the Milky Way" for a long time, we now know that it is much larger and also differs in structure.

The central area


Naeh the star gamma of the constellation Andromeda one can see the object, which is the furthest away from us at 2.5 million light years, which can still be seen with the naked eye Andromeda Galaxy.

The location of the Andromeda galaxy can be seen from the small star map opposite. It bears the name M 31 of the Messier catalog or NGC 224 (New General Catalog). Its two largest companion galaxies, M 32 and NGC 205, are also indicated. The Andromeda galaxy is a spiral galaxy of the typeSb and thus differs significantly from the Milky Way. Unfortunately we can only see it distorted, because it is inclined by 13 ° to our line of sight. This galaxy is also known as Andromeda nebulawhat has historical origin. Because of their blurred appearance and their unknown nature, the spiral galaxies were called Spiral nebula and it was believed for a long time that they were part of the Milky Way. In general, it was previously believed that the entire universe consisted only of the galaxy!


D.och in 1923 could Edwin HubbleAmerican astronomer, discovered the Hubble constant, recognized that M 31 lies outside the Milky Way and developed the Hubble sequence, a classification of galaxies. (1898-1953) resolve single stars of the Cepheid type in the outer spiral arms and thus indicate the distance to 900,000 light years. Cepheids are variable stars that change their brightness with a highly precise period and thus as so-called Standard candle serve to determine the distance. It was now clear for the first time that the Andromeda Galaxy could not belong to the Milky Way and that the universe must be much larger than previously thought. 1944 could Walter Baade (1893-1960) also dissolve single stars in the inner areas. He was able to detect Cepheids, Novae, giant stars, globular clusters and interstellar matter in the galaxy. In 1952, he finally realized that Hubble had fallen into the wrong class of Cepheids and corrected the distance to 2 million light years. According to recent measurements, the distance to us is 2.52 million light years.

If you look at the galaxy, you can clearly see the large proportion of interstellar matter in the spiral arms. Here are the nurseries of many young stars. The zone of interstellar matter is only a few 100 [pc] thick and is arranged around the main plane of the system. No clouds of dust can be seen in the outer edge zones. While it was previously believed that the Andromeda galaxy was the "little sister" of the Milky Way, we now know that it is 150,000 light years across, and according to recent measurements (Raja Guhathakurta, University of California) is even larger by 200,000 light years (galaxy: 100,000 [ly]). The different numbers of stars also make the difference in size clear: our Milky Way can boast around 300 billion copies, but the Andromeda Nebula consists of up to 1 billion stars.

Clicking on the picture opens a fantastic large view.

My special thanks go to Robert Gendler for the permission to use this (copyrighted!) Recording

Because the Andromeda Galaxy is easy to see with the naked eye (visual brightness +3.5m), it must have been observed since early human history. However, it was first mentioned in 905 by a Persian astronomer (Al-Sufi). What can be seen with the naked eye is only the brightly shining central area of ​​the galaxy. In reality it extends over an area of ​​180 minutes of arc, that is six times the extent of the full moon!

The central area

D.he central region of the Andromeda Galaxy consists of a small, star-like core. This area cannot be broken down into individual stars, but we know today that, like in our Milky Way, a black hole is up to the center of the galaxy, but here it is a "heavyweight" of 140 million solar masses (!) (Source: John Kormendy and teams of Alan Dressler and Douglas Richstone; Hubble Site), more than 3 times as difficult as previously believed.

The core (nucleus) of M 31 is even strangely a double. This shows us this picture of the Hubble telescope from 1991. The area shown covers just 30 light years and it was initially believed that two black holes orbited each other there. However, that is no longer tenable today. There are two scenarios that can be used to explain the dual core. One possibility is that the Andromeda galaxy long ago merged with another and incorporated its matter, trapping the central area as well. However, gas and dust clouds could also obscure the view somewhat and in this way simulate a double core. From the earth only the brighter part could be seen and it was believed to be the galactic center. However, the Hubble Space Telescope's studies show that the center is in the fainter zone.

With kind permission of T. R. Lauer (KPNO / NOAO) et al., HST

But more was discovered with the Hubble Telescope. A strange blue light was found that surrounds the massive black hole in the center.

After more than a decade of intensive research and observation with the spectrograph on board the telescope, it was found that more than 400 young, hot and bluish stars are circling the black hole. They must have formed about 200 million years ago and surround the black hole in a disk just one light-year in diameter. According to the Doppler effect, light becomes more energetic when it comes towards us and less energetic when the source moves away. From the resulting shifts of the spectral lines in the blue or red area of ​​the spectrum, it was possible to determine the speeds of these stars. You circle the hole with 1000 [km / s], that is a proud 3.6 million [km / h]! At this speed, one of these stars would orbit the earth in 40 seconds and it would take just 6 minutes to reach the moon. They can make one cycle in 100 years. It is completely unknown by which mechanism the star disk was formed. Stars are formed from interstellar clouds of gas and dust, but when the gas swirls around the black hole at such speeds, star formation is hardly imaginable. But the stars are here now! Because they are massive and therefore short-lived, it cannot be ruled out that a star ring was created several times in Andromeda's more than 12 billion year old history and that this will also be repeated in the future. This disk of blue stars is embedded in a ring of older and therefore reddish stars, which was already known from earlier Hubble observations.

For a more detailed large view of this artist's impression, simply click on the picture.

Courtesy NASA, STScI

Another view of the center of the Andromeda Galaxy shows us this picture taken by the Chandra X-ray telescope. It shows many individual X-ray sources, including the absolute center, roughly in the middle with the small bruise. This zone is where the super heavy black hole is located. Strangely enough, this X-ray source is relatively cool with "only" 1 million [K]; one should expect ten times the temperature here due to the size of the black hole. But there is still no explanation for this. Our neighboring galaxy is constantly confronting us with new riddles and questions, the solution of which is far from in sight.
Courtesy S. Murray, M. Garcia, et al., (CfA) SAO, CXO, NASA


W.he so-called differential rotation also prevails in the Andromeda galaxy in our Milky Way. This means that the galaxy does not rotate like a rigid body, but that the different areas have different orbital speeds around the core.

From the center up to a distance of about 5 [pc] it increases quite evenly to about 60 [km / s]. If you go further outwards, however, it drops again, namely at a distance of 20 [pc] from the center to almost 0. Now the orbital speed of the stars increases again up to a center distance of 400 [pc], namely up to to 225 [km / s]. After this the speed decreases again steadily with increasing distance, a minimum is then again at about 2 [kpc]. A maximum speed of 270 [km / s] is found at about 10 [kpc] from the center. At a distance of 9 [kpc], which roughly corresponds to the distance of the sun from the center of the Milky Way, the orbital speed is 225 [km / s]. This corresponds to the speed of the sun, so a star at this distance also needs 220 million years to orbit the center of the Andromeda galaxy.
The small sketch shows the rotation relationships.
The rotation speed slowly decreases towards the edge of the galaxy, but remains constant at 220 [km / s] from a distance of around 20 [kpc] to the outer edge.

The mass distribution can now be deduced from the rotation ratios of the galaxy. Today one comes to the result that in the core a mass of about 107 Solar mass is concentrated. In the center, up to a distance from the center of 2 [pc], one calculates with 6 · 109 Solar masses, at 25 [pc] it becomes 2 · 1011 be. As in our Milky Way, the rotation speed does not decrease up to about 30 [kpc], which is why one can assume that high mass concentrations must also be present in the edge zones. The total mass of the Andromeda galaxy is around 1.2 to 3.7 · 1011 Solar masses estimated, 1% of which is due to interstellar matter.


D.he Hubble Space Telescope has provided us with a lot of new knowledge during its long period of operation, including about the Andromeda galaxy. As you can see in this picture, it has discovered a beautiful globular cluster (designation: G1), which orbits the center of the galaxy. It is the brightest globally known globular cluster in the Local Group and contains more than 300,000 stars. These stars are as old as those in the globular clusters of our Milky Way and are therefore among the oldest objects in the universe. As in the Milky Way, the globular clusters belong to the halo objects of the Andromeda Galaxy; their number is estimated at 400 to 500. A halo is a spherical space surrounding a galaxy in which, in addition to globular clusters, there are also individual stars. In the case of the Andromeda Galaxy, this structure has a diameter of 1 million light years, making it the largest object in the Local Group, our "home" galaxy cluster.

Courtesy of Michael Rich, Kenneth Mighell, and James D. Neill (Columbia University), and Wendy Freedman (Carnegie Observatories), Hubble Space Telescope, and NASA

Not only are globular clusters in the halo of the Andromeda Galaxy, but also individual stars in the halo of the Milky Way. Again with the HST (Hubble Space Telescope) amazing things came to light: Based on HDF- (Hubble Deep Field) Recordings determined the ages of the stars in the halo by astronomers.

About a third of the stars are only 6 to 8 billion years old, much less than the 11 to 12 billion year old stars of the Milky Way halo. How can this difference be explained? Well, the Andromeda Galaxy might have merged with another, larger galaxy billions of years ago, or it might have incorporated several smaller galaxies. In any case, these relatively young stars have a higher content of heavy elements (their Metallicity is bigger) than the stars in our halo. The distribution of the chemical elements is characteristic of stars from large, massive galaxies. There are three ways in which the young stars got into the halo:
  • Collisions destroyed the young disk of the Andromeda Galaxy and propelled disk stars into the halo
  • A single collision with an invading galaxy destroyed her, and stars from her and M31 were hurled into the halo
  • The stars were formed directly on the spot during or by the collision
Unfortunately, not all stars in Andromeda's halo can be examined, only the massive giant stars can still be photographed. After all, their number is estimated at around 300,000. "Normal gravity" stars like our sun are far too faint at this distance. But what can still be seen in this Hubble image are thousands of background galaxies, down to a brightness of 31m. Many of these galaxies are involved in collisions, which clearly shows us that we live in a dynamic, ever-changing universe. A large view of further details can be obtained by clicking on the picture.

Courtesy of NASA, ESA, and T.M. Brown (STScI)

With the help of the Hubble telescope, completely new insights into the halo of the Andromeda Galaxy could now be obtained (May 2015). Accordingly, its expansion is 6 times larger and its mass is a thousand times higher than previously thought. The halo consists mainly of very thin gas and is actually invisible. If it were to shine, it would take up a hundred times the diameter of the moon in the night sky. The science team was able to "see" it by analyzing the light from distant quasars in the UV range. If the light passes through the halo, it is attenuated in defined frequency ranges.

Simulations show that a halo is formed at the same time as the galaxy and initially only contains hydrogen and helium. Over the billions of years, it is enriched with heavier elements ejected by supernovae. Halos have also been observed in other galaxies, but the fact that Andromeda stretches over 1 million light years is astonishing - it almost meets the Milky Way halfway. We don't know whether our galaxy also has such an immense halo, we don't see the forest for the trees, so to speak. But maybe both halos are already in contact. If not yet, it is inevitable ...

Courtesy of the Hubble Space Telescope, NASA, Space Telescope Science Institute

Uur Milky Way and the Andromeda Galaxy are the largest galaxies in the Local Group, i.e. the associated galaxy cluster. But not only for this reason they represent something special, because the Andromeda galaxy is approaching our system with about 300 [km / s], and probably with increasing speed. It is therefore unavoidable that our two galaxies will begin to merge in around 2 billion years. However, one shouldn't think of a galaxy collision as a hard collision. The individual stars are so far apart that there will hardly be any contact. Only in the centers with their high star densities will this happen more often. Whether there are still people who can watch this surely grandiose spectacle is still questionable, considering the current development of mankind with all its self-made problems. From a still intact, habitable planet, however, one can observe how the band of the Milky Way initially dissolves to give way to a sky strewn with billions of stars. From both galaxies, an elliptical giant galaxy will have formed in 5 billion years, the earth will be displaced into the outer area and will be 100,000 light years away from the center (based on simulations by Thomas Cox and Avi Loeb, Harvard-Smithsonian Center for Astrophysics, Cambridge). But then no one will be able to observe this on earth, because the sun, which will then mutate into a red giant, will have extinguished all life on our planet by now.


E.Just like our Milky Way, the Andromeda galaxy is also accompanied by other galaxies, we know today of a total of 14. The most famous are M 32 and M 110 (=NGC 205).

This figure shows the elliptical companion galaxy M 32. It is a dwarf elliptical galaxy, but it was probably once much larger. It can be assumed that in the past it still had an intensive "relationship" with the Andromeda galaxy as a spiral galaxy, which led to the loss of the star disk at M 32, but at the same time caused star formation in the central area to increase by leaps and bounds.

Courtesy of the 1.1 Meter Hall Telescope, Lowell Observatory, Bill Keel, U. Alabama

In a series of new images taken by the Spitzer telescope in IR light, astronomers were able to demonstrate the rather turbulent past of the Andromeda galaxy. They found that the companion galaxy M 32 migrated through one of the two spiral arms a few million years ago. This created a gap in the arm and the star formation areas were pushed away in an arc. You can see this arch at the bottom right of the picture.

Courtesy NASA / JPL-Caltech / K. Gordon (University of Arizona)

As a further example of the companions, we take a look at the galaxy M 110, also an elliptical dwarf galaxy. Eight globular clusters were discovered in their halo; their total mass is estimated at only 3.6 to 15 billion solar masses. Alongside M 32, it is the second lighter companion of M 31.


Here is a tabular overview of all known companions (taken from http://en.wikipedia.org):

(Million [ly])
NGC 185dE52.01+11m1787
NGC 147dE52.2+12m1829
Andromeda IdSph2.43+13m21970
Andromeda IIdSph2.13+13m1970
Andromeda IIIdSph2.44+10m31970
Andromeda IV *dIm?  1972
Andromeda VdSph2.52+15m41998
Pegasus Dwarf
(Andromeda VI)
Cassiopeia Dwarf
(Andromeda VII)
dSph2.49 1998
Andromeda VIIIdSph2.7+9m12003
Andromeda IXdSph2.5+16m22004
Andromeda XdSph2.9+16m22005
Triangle galaxy
SA (s) cd2.59+6m271654?

*Whether a companion galaxy is still uncertain; d, dwarf = dwarf

Courtesy of 2MASS / UMass / IPAC-Caltech / NASA / NSF

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