Catch a star:

The Andromeda Galaxy

Made by: Christian Tversted, Louise Vind Nielsen, Cecil Marie Schou Pallesen and teacher Jens Peter Diget, Morsø Gymnasium, Limfjordsvej 95, DK-7900 Nykøbing Mors, Denmark. 01.11.02.

  The Andromeda galaxy is our nearest neighbour galaxy. It is one of the few that is visible to the naked eye, therefore it has been known for a long time. It has been known as the “little cloud” as early as 905 AD by the Persians. It was described by the Persian astronomer Abd-al-Rahman Al-Sufi in 964 AD. The galaxy has also appeared on a Dutch Starmap in 1500. Simon Marius was the first to give a telescopic description of the galaxy in 1612, by the order of Charles Messier.

Without very good telescopes M31 can only be seen as a Nebula. Therefore it was believed for a long time that Andromeda was a nebula. The first person, who discovered a difference between gaseous nebulae and “nebulae” with line spectra like Andromeda, was William Huggins. The error was not discovered until 1953, when the 200-inch Palomar telescope was completed and had started observing.  

The myth of Andromeda

– The chained princess.

The ancient Greek astronomers named the constellation, Andromeda, which means “Chained Princess”. According to the Greek Mythology, she was the daughter of King Cepheus and Queen Cassiopeia. The constellations Cepheus and Cassiopeia can be seen near Andromeda. In the myth Andromeda believed she was very beautiful, and she made no attempt to keep quiet about it. She said that she was even more beautiful than the Nereids, sea nymphs. This made Poseidon, the god of the sea, angry. Poseidon sent the sea monster, Cetus, to terrorise the kingdom. Cepheus and Cassiopeia got the advice by an oracle that the only way to soothe the beast was to sacrifice Andromeda. They chained her to a rock near the sea and left her to be taken by Cetus. However, just as Cetus was going to devour the princess Perseus, came riding on his winged horse, Pegasus. He had just killed the ugly sisters, the Gorgons. If you looked into the eyes of the most ugly sister of them, medusa, you would turn to stone. Therefor Perseus pulled the head of Medusa, which he had decapitated out of his bag. Cetus saw it and turned to stone. Cetus, Perseus and Pegasus are also constellations, which can be found the same vicinity of the sky as Andromeda. [1]

Galaxy types

In the 1920’s the American, Edwin Hubble, categorized the galaxies, which he had observed. Hubble formulised a so-called “fork diagram. Hubble places the galaxies in categories after form and structure, and not like the former classifications that places them after size. The four types of galaxies are:

-          Spiralled galaxies

-          Barred galaxies

-          Elliptical galaxies

-          Irregular galaxies

Spiralled galaxies are flat and have a centre from which the spiral arms rotate. Many people at first think that there are only stars in the arms, because only the arms are visible on most pictures, but there are stars in the whole galaxy. It’s just that in the arms new, blue and hot stars, which shine more than the old, are produced. There are variations between size of the central core and density of the spiral arms. These variations can again place the spiral galaxies in new categories:     

-          Sa: Big central core and very close arms

-          Sb: Not as big core and not as close arms

-          Sc: Small central core and not close arms.

Here is a picture of the Andromeda galaxy:

[2] The Andromeda Galaxy M31

We here see that the Andromeda galaxy is a spiral galaxy of the type Sb or Sa, like our own galaxy, the Milky Way.

Barred galaxies look a lot like spiralled galaxies. The difference is that instead of a core they have through the centre a bar rotates like a spiral.

These galaxies are also categorized almost like the spiral galaxies:

-          SBa: Big central core and a big, clear and broad bar.

-          SBb: Not as big central core and bar.

-          SBc: Small core and small slender bar.

Elliptical galaxies don’t have a spiral structure and are classified after how round or oval they are.

-          E0: Almost round as a ball.

-          E7: Very flat.

As the name indicates, irregular galaxies don’t have a specific structure or form. They often contain big amounts of dust and gas. Irregular galaxies often produce many stars.

The Magellan clouds are an example of an irregular galaxy.

[3] Large Cloud of Magellan (LMC)

There also exists a type of galaxy, which is a combination of a spiralled and an elliptical galaxy. This type of galaxy is often seen as a spiral galaxy, because it has the same form, but it doesn’t have the spiral arms. These are called S0. [4]

Here we see Edwin Hubble’s diagram, with real galaxies to illustrate the categorization:



A guide how to find the Andromeda galaxy in the night sky.

It is not difficult to find the Andromeda galaxy in the sky. Most people know how to find the constellation Cassiopeia. If you see Cassiopeia as a “W” you take the last to stars and draws a line trough them. The line then goes through a clear star under the “W”. This is a part of the constellation Andromeda. If you then move your eye to the right, you see another two clear stars. These three stars you now see on a line bent a little upwards is one of the “legs” in the Andromeda constellation.

When you see the three stars it is easy to find the Andromeda galaxy. You look at the star in the middle and move your eye up where you can see two not as clear stars. A little to the right of the last of these stars you can see the Andromeda galaxy as a small cloud. It can be difficult to see it at first, but after a little while your eye can focus on it.

[6] Cassiopeia is seen in the central of the map. And the constellation Andromeda and the galaxy are seen under Cassiopeia.

Theory about the Andromeda Galaxy

- Visual brightness: 3,4 mag.

- Distance: 2,9 million light years = 2,9 Mpc x 1/3,26

- Apparent dimension: 178 x 63 arc min.


Find the diameter of the Andromeda Galaxy

M31 is a large spiral galaxy, slightly larger than our own galaxy, the Milky Way. It is our closest normal-galaxy companion in the local galaxy cluster (which also includes M32 and M110, two bright dwarf elliptical galaxies). Actually, from a very distant point, the Andromeda Galaxy and the Milky Way would appear as a pair or a double galaxy system, if the smaller spiral galaxy, M33 was not to be seen. The Persian astronomer Al-Sufi called M31 a ”little cloud”, and it is visible to the naked eye. At the centre of the galaxy, there is a brilliant point of light, which is a very tightly packed star cluster. The entire galaxy is rotating in space with the lower portions approaching while the upper parts recede. The rotation is not completely smooth, showing bumps where the spiral arms occur, which are probably due to the spiral density wave that maintains the arms. The disturbance in the spiral structure is apparently also because of the interaction with M32.

Under normal viewing conditions, the apparent size of the visible Andromeda Galaxy is about 3 x 1 degrees. Its distance is 2,9 million light years, and its mass is estimated at 1,23 trillion times that of the sun. The mass of the Milky Way is 1,9 trillion times that of the sun.   

The Hubble Space Telescope has revealed that the Andromeda Galaxy has a double nucleus. This suggests that either it has actually two bright nuclei, probably because is has “eaten” a smaller galaxy which once intruded its core, or parts of its only one core are obscured by dark material, probably dust. In the first case, this second nucleus may be a reminder of a possibly violent dynamical encountering event in the earlier history of the local group. In the second case, the duplicity of Andromeda´s nucleus would be an illusion caused by a dark dust cloud obstructing parts of a single nucleus in the center of M31 [7]

How many stars in the Andromeda Galaxy?

If we know the distance of the galaxy, we can decide how much light is send out and then, how many stars the galaxy contains. We just have to compare with a lamp in a known distance and with a known light intensity. Because of the very faint light intensity from the galaxies, we have to use the Sun instead of a lamp, and then decide, how many times stronger the Andromeda Galaxy´s light is compared to the light from the sun.

m = visual brightness

r = distance

m sun = -26,74 mag

m andromeda = 3,4 mag

r sun = 150 million km = (150 million km/9,468 x 10 12 km)ly =

1,58 x 10 -5 ly = (1,58 x 10 -5 ly/3,26 ly) pc = 4,85 x 10 -6 pc

r andromeda = 2,9 million ly = (2,9 million ly/3,26 ly)pc = 889570,5521 pc

M = magnitude

L = light intensity

M sun = m – 5log r + 5 = -26,74 mag – 5log (4,85 x 10 -6 ) + 5 = 4,83 m

M andromeda = 3,4 mag – 5log(889570,5521) + 5 = -21,346 m

M andromeda – M sun = - 2,512log(F andromeda (10)/F sun (10)) = -2,512log(L andromeda /L sun ) «

(M andromeda - M sun )/-2,512 = log(L andromeda /L sun ) «

10 (Mandromeda - Msun)/-2,512 = (L andromeda /L sun ) «

10 (-21,346-4,83)/-2,512 = 2,63 x 10 10

L andromeda /L sun =2,63 x 10 10 →

The light intensity of the Andromeda Galaxy is 2,63 x 10 10 times stronger than the light intensity of the Sun.

That means, that the light intensity of the Andromeda Galaxy is:

(2,63 x 10 10 ) x 3,90 x 10 26 = 1,0257 x 10 37 W

If we decide that every star in the Andromeda Galaxy on average sends out as much light as our Sun, there are about 2,63 x 10 10 stars in the galaxy. In the Milky Way there are about 200 billion stars.

The future of the Andromeda Galaxy ?

The Milky Way and the Andromeda galaxy are approaching each other with a speed of 480.000 km per hour or 133,33 km/s

It's not certain yet whether the collision will be a head-on collision or a simple sideswiping by the massive galaxy, which is a near twin to the Milky Way. Astronomers will first need to use powerful new telescopes to precisely measure Andromeda's tangential motion across the sky.

A direct collision would lead to a grand merger between the two galaxies, and the Milky Way would no longer be the spiral galaxy, as we know today, but would change into a huge elliptical galaxy.

It would happen no sooner than five billion years in the future! By then the Sun may have burned out, and the Earth reduced to a cold, lifeless planet.

The collision will take several billion years to fully run its course, so it will be hard for any one civilization, like ours, to fully understand the consequences both in time and space of the collision.

However, by studying pairs of other colliding galaxies and using computer simulations, astronomers can get  a series of snapshots of the collision process and get an idea of what might eventually happen to our galaxy.

Scenario of how the Milky Way might change if were are going to have a head-on collision with Andromeda.

The Andromeda galaxy appears simply as a  smudge of  light in the northern autumn sky. As the two galaxies approach each other, Andromeda will steadily increase its size in the sky, and it will look like a glowing sword.

When the Andromeda galaxy and our Milky Way galaxy are close enough, huge clumps of cold, giant molecular clouds, each measuring approximately  tens to hundreds of light-years across, will be compressed. These dark knots will light up as millions of stars burst into life. Most of these stars will be in brilliant blue clusters, many of them 100 times brighter than the original globular star clusters already present in the two galaxies.

The disk of dust and stars that for billions of years showed  the lane of our galaxy and the Andromeda galaxy, will begin to move away under the gravitational pull of the two galaxies. As Andromeda swings past our galaxy, the sky will grow increasingly mixed with  lanes of dust, gas, and brilliant young stars and star clusters.

So many new stars will be born, that the fraction of massive stars that are present will increase dramatically. These stars will begin popping off like a string of firecrackers as they self-destruct as supernovae.

After swinging by our galaxy, Andromeda will take perhaps 100 million years to make a slow and elegant  U-turn, before  crashing nearly directly into the Milky Way's core. Another, even more spectacular burst of star formation will then occur, with the winds from the supernovae driving most of the remaining gas and dust out of the galaxy. Soon both the old and new stars of  the two galaxies will be mixed into each other and form a single elliptical-shaped galaxy.

As the stars gravitationally fit  into their new “home”, through a dynamic process called "violent relaxation", any hint of the Milky Way and Andromeda as majestic spiral galaxies will be gone. The band known as the Milky Way will be gone, but far in the future some astronomers might look  out onto a starry sky and look all the way into the core of the new elliptical galaxy. They would have no idea that there were once two majestic spiral galaxies, called the Milky Way and Andromeda by a long forgotten civilization.

Galactic collision: (future of the Milky Way and Andromeda)


Figure 1. Galactic Collision - Interacting galaxies NGC 2207 (left) and the smaller IC 2163, 114 million light-years away in the constellation Canis Major. The pair is similar to the Andromeda Galaxy and the Milky Way.

Observations indicate that, billions of years from now, the IC 2163 is destined to swing past the larger galaxy again and eventually merge into it. (image: NASA and The Hubble Heritage Team)

Determining distances

This has always been one of the biggest problems in astronomy. At the very beginning the astronomers found the distances to the nearest objects such as the Sun, the Moon and the planets by using many different methods of measurement. The discovery of the Cepheid variable star gave  the astronomers a new tool to distance-determination which was needed very much because the conventional method such as parallax-measuring only was useful when the measured objects were not more than 10 kpc from earth.

The discovery of the first galaxies.

           In 1845 William Parson constructed a telescope with a massive mirror which measured 6 feet in diameter. For many years this was the largest telescope in the world. With this new telescope W. Parson examined many of the nebulae [8]

            that had been discovered and catalogued by William Herschel. He observed that some of these nebulae have a special spiral structure. At that time he had no camera so he made some drawings of what he saw and actually these drawings compared very good with modern photographs. After having examined the nebulae and seeing the spiral structure Parson began to think about what the German philosopher Immanuel Kant had suggested less than a century before. It was the idea that vast collections of stars lie outside the Milky Way – a kind of “island universes”

Much later in 1923 the astronomer Edwin Hubble took a historic photograph of the Andromeda “Nebulae”. First he thought it was a nova but later he realized that the object was actually a Cepheid variable star. He  used the Cepheid periode-luminosity relation, which constants were determined by observing Cepheids in the Globular Clusters of the Milky-Way.

( 30 years later Walter Baade made a more precise calibration based on the Population I Cepheids)

But what is the Cepheid variable technique actually and how precise is it ?

In 1912 , the American astronomer Henrietta Leavitt reported her important discovery of the period-luminosity relation for Cepheid variables. She discovered the relation by studying numerous Cepheids in the Small Magellanic Cloud.

A Cepheid variable is recognized by the characteristic way in which its light output varies from rapid brightening followed by gradual dimming. They are very important and useful because they have two properties that allow astronomers to determine the distance to very remote objects.

First because of their high luminosity (ranging from a few hundred times solar luminosity to more than 10.000 (L,sun)) the Cepheids they can be seen at distances of millions of parsecs. Second, because there is a direct relationship between a Cepheid´s period and its average luminosity.

(See fig.) This fig shows: the more luminous the Cepheid, the longer its period and the slower its pulsations. As shown there are two types of Cepheids, Type I and II. The difference is that Type I  Cepheids are metal-rich, Population I stars, and Type II Cepheids are metal poor Population II stars.

How do astronomers use this relationship between luminosity and period ?

The results of the work by Levitts was the famous Period-luminosity relation given in  this equation:

M = a log P + b

M: absolute mag a and b are constants. These constants were found by making calibrations.

Baade found a = -2,5 and b =1,7

When M is found you can determine the distance to the Cepheid  by using the relation between a star’s apparent magnitude and absolute magnitude given by this equation:

  m-M = 5 log d –5

m-M is called the distance modulus and log d is the logarithm of the distance in parsecs.

By isolating d we can determine the distance to be:

              d = 10 ( m-M +5)/5   parsecs

Using this method astronomers are able to calculate the distances to other galaxies with great accuracy and all the following steps on “the distance ladder” are based on this technique.

[5] The illustrations are: E0: M87 / NGC4486, E7: M59 / NGC4621, S0: M86 / NGC4406, Sa: M104 / NGC4594,

Sb: M63 / NGC5055; Sc: M51 / NGC5194, Sba: M83 / NGC 5236; SBb: NGC1300, SBc: NGC 1365.



[8] A cloud of gas and interstellar dust.