Monday, 27 April 2020

Astrognome A-Z of Constellations # 1 Andromeda


Andromeda – autumn sky
We start our tour of the constellation with a fairly easy one to find and is involved in probably the most famous mythological story.
Andromeda is the princess linked with the story of Perseus and the Medusa plus Cassiopeia, Cepheus and Cetus (Kraken). Andromeda is depicted chained to a rock, however needless to say the pattern is nothing like a princess.
In mythology Andromeda is the daughter of queen Cassiopeia and King Cepheus, Cassiopeia boasted that her daughter was more beautiful than the Nereids these were 50 charming daughters of Nereus the wise old man of the sea. This was a decidedly tactless thing to say, offended by Cassiopeia’s remarks they complained to their protector the sea god, Neptune.
In anger Poseidon struck the water with his trident flooding the lands of the Palestine coast and calling up from the deep the sea monster the Kraken or Cetus. (There could be a basis in truth here because the great flood could have been caused by a meteor strike 3,700 year ago in the Eastern Mediterranean. The meteor crashed into where Austria is found but chunks could have fallen off into the sea causing the flood).
Cepheus consulted the oracle as to how to save his kingdom and was told that his land could only be saved if his daughter Andromeda was sacrificed to the monster. Accordingly Andromeda was chained to rocks near Joppa. Jaffa is the modern name for (Joppa) The name Joppa appears for the first time in the list of cities that Thutmose III captured (15th century BC). The legend of Andromeda being bound to the rock was first associated with Joppa by Strabo (1st century A.D.).
With Andromeda chained to the rocks and the monster appearing everything seemed lost, however at the very last minute Perseus riding the winged horse Pegasus appeared on the scene. He had just killed the Medusa. Anyone looking at Medusa would turn to stone. By chance Perseus still had the head of the medusa with him, he showed it to the monster who turned to stone. Perseus then landed and rescued Andromeda. They were married and lived happily ever after. All these characters can be found in the night sky.
For some reason in 1930 the International Astronomical Union the controlling body of world astronomers decided to move one of the stars from the square of Pegasus to the neighbouring constellation of Andromeda. Therefore Delta Pegasi whose name is Alpheratz and was the top left hand star of the square became alpha Andromedae. The other main stars in Andromeda are marked by a rather irregular line from the square of the Pegasus pointing towards the left to the constellation of Perseus. In order they are alpha or Alpheratz, delta, Mirach or beta, and Alamac or gamma.

Starting with alpha or Alpheratz or sometimes Sirrah which means the Horses Navel and clearly has nothing to do with a maiden chained to a rock is a star of magnitude 2.0 which means it is as bright as the North Star. The star is a B9 class star which means that with a surface temperature of around 13,500 degrees it is hotter than the Sun which has a surface temperature of only around 5,800 degrees, Alpheratz it lies about 97 light years away.
The next star on this line moving left from the square of Pegasus is delta Andromedae, it’s a K3 giant star which means it is not as hot as Alpheratz , it lies about 105 light years away. The star is fainter than Alpheratz being of mag 3.3 and its temperature is 4,000 degrees. It looks orange in colour.
Beta Andromedae or Mirach which means girdle, is next and is at mag 2.0 the same brightness as Alpheratz but at 200 light years away is actually brighter. It is an M0 giant star with a surface temperature of around 3,500 degrees which means that it is the coolest stars in this line. It looks more red than orange.
The final star in this line is gamma, whose name is Almach which means a small animal like a badger. Almach is about 350 light years away. It is at mag 2.1, the main star is K1 giant star with a surface temperature of 4,200 degrees.
Beta or Mirach can be used as a guide to find the Andromeda galaxy (M31). A line drawn upwards and slightly to the right from the star Mirach leads to the faint star Mu magnitude 3.9 then continue the line slightly further along to Nu magnitude 4.5. The Andromeda Galaxy is close to Nu slightly to the right. You will need a very dark site but it can be seen with the naked eye, if you know where to look. It is not spectacular with the naked eye but there is always the satisfaction of seeing the most distant object to be seen without optical aid. It is about 2.2 million light years away. It is larger than our galaxy and in about 3.5 billion years’ time the two galaxies will pass through each other!!.
A pair of binoculars will show it easily as a fuzzy patch in the sky.
Other stars you might try to find in Andromeda
Epsilon Andromedae is just below delta and is magnitude 4.4 and is 155 light years away. It is a G class giant star our Sun is also a G class star, however while the Sun is a dwarf star it will be slightly hotter than epsilon
Continue the line from delta through epsilon and further downwards and you will reach zeta Andromedae which is classified as an orange K-type bright giant with a magnitude of 4.1. It is cooler than our Sun and zeta is about 180 light years away.
Just to the left of zeta is eta Andromeda at magnitude 4.4 it is slightly fainter than zeta. It lies 240 light years away and is another G class giant star.
Located just above delta is pi Andromedae at magnitude of 4.4 and is located approximately 600 light-years from Earth. It is a B class dwarf star meaning it is much hotter than the Sun.
Groombridge 34 - One of the closest double stars to Earth at 11.7 light-years distant. Although not bright or spectacular Groombridge 34 is an interesting double since it consists of two red dwarf stars in near circular orbit. The two stars shine at magnitudes +8.1 and +11.1 far too faint to be seen with the naked eye. They are easily within the range of medium size telescopes. In August 2014, a planet orbiting Groombridge 34 was discovered.



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