Catch a star Marie Kjærgaard, Karen Olsen and Line Andresen The Andromeda Galaxy

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Catch a star

© Marie Kjærgaard, Karen Olsen and Line Andresen

The Andromeda Galaxy

By: Marie Kjærgaard, Karen Olsen and Line Andresen

Teacher: Mogens Winther

Group 151 Amtsgymnasiet in Sønderborg, Denmark

The mythology of Andromeda

Andromeda was the princess of Ethiopia, daughter of Cepheus and Cassiopeia.

Cassiopeia was a boastful woman, and foolishly bragged that she was more beautiful than Juno (the queen of the gods, and the Nereids). In order to avenge the insult to his nymphs, Neptune sent a sea monster to ravage the Ethiopian coast.

The horrified king Cepheus consulted Ammon (the oracle of Jupiter) who said that Neptune could be appeased only by sacrificing Cassiopeia's beautiful virgin daughter, Andromeda, to the monster. Andromeda was duly chained to a rock on the coast, fully exposed to the monster. Fortunately for her, the hero Perseus happened to be flying by on his way back from killing the Gorgon Medusa.

When Perseus saw the princess and her arms was chained to the hard rock, he would have taken her for a beautiful statue, if not the light had breeze stirred her hair, and warm tears streamed from her eyes. Without realizing it, he fell in love. Amazed at the sight of such rare beauty, he stood still in wonder, and almost forgot to keep his wings moving in the air. As he came to a halt, he called out: "You should not be wearing such chains as these, the proper bonds for you are those which bind the hearts of fond lovers! Tell me your name, I pray, and the name of your country, and why you are in chains."

At first she was silent; for, being a girl, she did not dare to speak to a man. She would have concealed her face modestly behind her hands, had they not been bound fast. What could she do? She started filling her eyes with tears. When Perseus persisted, questioning her again and again, she became afraid lest her unwillingness to talk might seem due to guilt. So she told him the name of her country, and her own name, and she also told him how her mother, a beautiful woman, had been too confident in her beauty.

Before she had finished, the waters roared and from the ocean wastes there came a horrible monster, its breast covering the waves far and wide. The girl screamed. Her sorrowing father was close at hand, and her mother too. They were both in deep distress, though the mother had more cause to be so.

Perseus says to Andromeda's parents that he'll kill the monster if they agree to give him their daughter's hand in marriage. They of course give him their consent, and Perseus kills the monster. He took the head of Medusa and showed it to the sea monster. In the same second Perseus lifted the head up the sea monster turned into stone. Andromeda is freed, and the two joyously marry.

Today Andromeda is represented in the sky as the figure of a woman with her arms outstretched and chained at the wrists.

Information on the Andromeda galaxy

The Andromeda galaxy is also known as the messier object M31 in the Messier-catalogue.

The Andromeda galaxy holds the equivalent of some 300 billion solar masses, which makes it half as wide as the Milky Way, but twice as massive. In other ways Andromeda is very much similar to the Milky Way. The two galaxies are the dominated ones in The Local Galaxy-group.

The Andromeda galaxy exists 2,2 million light years away, and the great spiral-galaxy is more than 125,000 light years across. The intensity of the galaxy is 31 billion times as strong as the Sun and the full extent of the galaxy covers over 3° of sky on its longest side.

The central bulge consists mostly of older stars, and is surrounded by starmaking clouds of dust and gas, which shines blue as the light of hot young stars.

The galaxy-type of the Andromeda galaxy

The Andromeda galaxy is of the type Sb, meaning that the central part of the galaxy is not very big, but with fairly tight wound arms.

Observations with the Hubble Space Telescope suggest that the Andromeda Galaxy may have two cores. This is thought to be the result of a past merger with another galaxy. Such processes were once common in the Universe and still occur today. Indeed, the galaxy is approaching us at a speed of 68 kilometres per second and a collision with the Milky Way is a possibility in the distant future.


A Cepheid is a regular, varying and pulsating star, meaning that the intensity of the light of the star changes. These pulsations can last several hundreds of days and nights, but the pulsation of a Cepheid endures 60 days and nights at the most. Another special characteristic of a Cepheid is its way of pulsating. The intensity or the magnitude increases much quicker than it decreases. This diagram indicates the regular pattern of changing brightness as a function of time:

During one of these pulsations the size of the star also changes. The radius of the star can alter with 10-20%.

Harvard astronomer Miss Henrietta Leavitt was the first person to determine distances using Cepheids. Therefor in 1912 she made it possible to prove that “spiral-nebulas” (?) are independent galaxies millions of light-years away. Later on in 1923 american astronomer Edwin Hubble found a Cepheid in the Andromeda-galaxy and showed that the object is not just a small cluster of stars and gas within our own galaxy, but a large galaxy. The distance from us has later been determined to be 2,2 million light-years.
The historic plate on which Edwin Hubble found a Cepheid in a spiral-galaxy and marked it by writing “var” (variable)

Our own picture

This was taken during two nights on October 26th and 28th. The exposure time used:

Blue - 8200 sec

Green - 3300 sec

Red - 4100 sec

The galaxies M32 (LY away from us) and NGC 205 (at the same distance away) are also visible.
Taken by us: Marie Kjærgaard, Karen Olsen, Line Andresen and guests.

With this photograph it is possible to determine the distance to the Andromeda-galaxy by using the formula concerning magnitudes:

The biggest magnitude we can find on our photograph is 18,605 as shown on the following picture:

The human eye can at its maximum see a candlelight 1 km away, which means a magnitude of 6,5. By comparing this magnitude to the absolute magnitude that we were able to measure we can estimate the distance that our telescope can see a candelight:

So we are able to see a candlelight in Copenhagen from our Gymnasium in Sønderborg.

Using the same method we can find an approximate distance to the Andromeda galaxy.

m is the meassured magnitude while M is the absolute magnitude in the distance of 10pc.

We assume that the marked star is a giant, very strong star. Using a Hertzsprung-Russel diagram we can see that giant stars have an absolute magnitude of –6 or –7.5.

So now we are able to estimate the distance to the galaxy for both possibilities:

By multiplying with 3,26 we find the distance in light years, LY. The same procedure is used on the absolute magnitude of –7.5:

The distances lie within the interval from LY to LY, which is correct acording to the current distance used, LY. If we had time to find a Cepheid in the galaxy it would have been possible to find a very exact brightness of a star and from that calculate a more correct distance.

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Group 151: Marie Kjærgaard, Karen Olsen and Line Andresen

Amtsgymnasiet in Sønderborg

The Andromeda Galaxy

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