05-05-2007 Galaxies - cities of stars (Article 84)

Moderator: SAC Committee

Post Reply
Simon Kenny
Posts: 527
Joined: Tue Feb 12, 2008 9:17 pm
Location: Shannon, Co. Clare, Ireland

05-05-2007 Galaxies - cities of stars (Article 84)

Post by Simon Kenny » Wed Feb 04, 2009 10:56 am

Galaxies - cities of stars (Article 84)

All the stars that you can see in the night sky are members of an enormous grouping of thousands of millions of stars called the Galaxy. It is usually given a capital “G” to distinguish it from other galaxies and is frequently referred to as the Milky Way.

Only a century ago most astronomers thought that the entire Universe was just a few thousand light years across. During the 20th century one of the most important discoveries was that this was completely wrong! We know now that our galaxy, the Milky Way, is merely one of billions of galaxies strewn across billions of light years.

Astronomers classify galaxies according to their shape i.e. elliptical, spiral, barred spiral and irregular. Our Milky Way is a spiral galaxy and if we could travel outside the galaxy and look down we would see something like the image below (courtesy of NASA/JPL-Caltech.Hurt SSC).

Our Galaxy is not just made up of thousands of millions of stars but also vast quantities of gas and dust, all of which is gravitationally bound by the galaxy and is not free to wander off into intergalactic space. All the stars, gas and dust clouds travel around the galactic centre mostly in circular orbits travelling at 800,000kph! Stars and material nearer to the centre orbit more quickly than those further out. Our Sun for example takes about 225 million years to make just one orbit! Interestingly, only recently have astronomers discovered that most of the Milky Way’s mass is not in the stars but in a halo of mysterious dark matter that emits no measurable radiation.

A spiral galaxy, like ours, has three main regions called the central bulge, disk and halo.
The scale of our galaxy structure is so huge that astronomers often use the following analogy to help visualise it. Imagine two fried eggs stuck back-to-back representing a diameter of 100,000 light years (if light travels at 186,000 miles in one second — work out the mileage in one calendar year, then 100,000 years and appreciate the enormity of the distances involved).

Our Sun lies in the disk region in a spiral arm about 30,000 light years from the centre i.e. in the white part of the eggs, which would average about 2,000 light years thick.
The central bulge of our Galaxy, which is the densest part of the bulge, would be represented by the two egg yolks with a thickness of about 10,000 light years. Above and below the central bulge (the two egg yolks) lies a halo of very old stars. Most of the stars of the halo are in clusters of stars called globular clusters and these travel around the galaxy centre in elliptical paths.

Post Reply