Download 151 - ESO

The Andromeda Galaxy
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
Page 1 of 6
Group 151: Marie Kjærgaard, Karen Olsen and Line Andresen
Amtsgymnasiet in Sønderborg
The Andromeda Galaxy
The Andromeda Galaxy
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.
Page 2 of 6
Group 151: Marie Kjærgaard, Karen Olsen and Line Andresen
Amtsgymnasiet in Sønderborg
The Andromeda Galaxy
The Andromeda Galaxy
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.
Cepheids
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)
http://antwrp.gsfc.nasa.gov/apod/ap960406.html
Page 3 of 6
Group 151: Marie Kjærgaard, Karen Olsen and Line Andresen
Amtsgymnasiet in Sønderborg
The Andromeda Galaxy
The Andromeda Galaxy
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 ( 2.9  10 LY away from us) and NGC 205 (at the same distance away) are
also visible.
6
 Taken by us: Marie Kjærgaard, Karen Olsen, Line Andresen and guests.
Page 4 of 6
Group 151: Marie Kjærgaard, Karen Olsen and Line Andresen
Amtsgymnasiet in Sønderborg
The Andromeda Galaxy
The Andromeda Galaxy
With this photograph it is possible to determine the distance to the Andromeda-galaxy by
using the formula concerning magnitudes:
r 
m A  mB  5  log  A 
 rB 
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:
r 
m A  mB  5  log  A 
 rB 
 r 
 18,605  6,5  5  log  A 
 1km 
18,605  6,5
 r 

 log  A 
5
 1km 
r
 10 2, 421  A
1km
2, 421
 rA  10
km  264km
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.
Page 5 of 6
Group 151: Marie Kjærgaard, Karen Olsen and Line Andresen
Amtsgymnasiet in Sønderborg
The Andromeda Galaxy
The Andromeda Galaxy
 r 

m  M  5  log 
 10 pc 
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:
 r 

18.605  (6)  5  log 
 10 pc 
 r 

24.605  5  log 
 10 pc 
 r 
24.605

 log 
5
 10 pc 
10
24.605
5
r  10

r
10 pc
24.605
5
 10 pc  833681 pc  2.7178  10 6 LY
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:
 r 

18.605  (7.5)  5  log 
 10 pc 
 r 

26.105  5  log 
 10 pc 
 r 
26.105

 log 
5
 10 pc 
10
26.105
5
r  10

r
10 pc
26.105
5
 10 pc  1.66341  10 6 pc  5.42273  10 6 LY
6
6
The distances lie within the interval from 2.7  10 LY to 5.4  10 LY, which is correct
6
acording to the current distance used, 2.9  10 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.
Page 6 of 6
Group 151: Marie Kjærgaard, Karen Olsen and Line Andresen
Amtsgymnasiet in Sønderborg
The Andromeda Galaxy