The Milky Way
The Milky Way is one of the highlights of our late autumn and early winter skies. A glorious soft band of light curves across the evening sky from south west to north east after dark. In ancient times it was thought to be just that – a band of light. It wasn’t until 1610, with the invention of the telescope, that Galileo discovered the Milky Way was really composed of a huge number of distant stars.
Dust and Dark Nebulae
Once you have allowed your eyes adapt to the night sky, you will begin to see dark patches within the arc of the Milky Way. These are regions of dust and cold gas that may eventually turn into new stars. But these clouds also obscure our view of our galaxy beyond them making it difficult for early observers to understand the shape of the Milky Way.
By the mid 1700’s Immanuel Kant and others were suggesting that the Milky Way might be a huge, flattened swarm of stars. But it was only in the twentieth century that the true spiral nature of the Milky Way emerged, with the discovery of other galaxies.
The Milky Way is a vast spinning disc of over 100 billion stars with a diameter of 100,000 light years. (A light year is the distance that light travels in one year. Light takes about eight minutes to go from the Sun to the Earth).
Our nearest significant galactic neighbour is in the constellation of Andromeda. Although it is 2,500,000 light years away you can see it with your naked eye on clear nights. It is visible in the picture above, appearing like a nebulous bright star in the top left hand corner of the image.
Andromeda is known as “the Chained Lady”. That name refers to Andromeda’s role in the Greco-Roman myth of Perseus: Cassiopeia, the queen of Ethiopia, boasted that her daughter was better looking than the Nereids, sea nymphs favoured with exceptional beauty. Offended by her remark, the nymphs asked Poseidon to punish Cassiopeia for her impudence and Poseidon ordered the sea monster Cetus to attack Ethiopia. Concerned for his kingdom, Andromeda’s father Cepheus consulted the Oracle of Ammon, only to be told to sacrifice his daughter to the monster. So Andromeda was chained to a rock by the sea. However the hero, Perseus appeared in a nick of time and used the head of the gorgon, Medusa to turn the monster into stone.
To find the Andromeda galaxy in the night sky we will have to go star hopping. First of all find the Great Square of Pegasus. Andromeda is the constellation to the north east of the Square. Starting at the north eastern corner move three stars to the north east counting the corner of the Square as the first star. The star you have landed on is the first in a line of three pointing to the north west. Go to the third star in line. Just to the west of it is the faint glow of the core of the Andromeda galaxy.
The view is improved through binoculars if you have them to hand. The scale of this galaxy is vast. Although the disc and spiral arms are not visible with the naked eye, their diameter extends to six times the width of the full moon!
In the picture above the core and spiral arms of M31 can clearly be seen. In between the spiral arms are dark lanes of dust and gas. You can clearly see that stars of our galaxy are in the foreground and that M31 itself lies beyond the Milky Way.
As a bonus we get two other smaller galaxies in the picture. Both M32 (above) and M110 (below left) are dwarf elliptical galaxies, satellites of the larger M31.
Measurements indicate that we are on a collision course with M31, which will be due in around 3.75 billion years time. But I guess that there will be other things to worry about before that happens!