SOLAR SYSTEM

Milky Way Galaxy - our celestial backyard

On dark, clear nights we can sometimes see a faint, hazy band of light studded with many stars and stretching across the sky. This is the body of the Milky Way, the galaxy in which we live. Here in the southe rn part of Australia, the centre of the Milky Way passes almost directly overhead so we can obtain an excellent view of our galaxy.

Our understanding of the Universe has changed vastly over the last 400 years. The Earth is no longer regarded as the centre of the Universe. Now we know that we are like "suburban residents" of the Milky Way, situated well away from the centre of the galaxy. Our solar system is about 30,000 light years out from the galactic centre and orbits around it at a speed of 250 kilometres per second. The astronomer Harlow Shapley, in the 1920s, was the first to realise that we are not at the centre of the Milky Way.

Containing over 100 billion stars (some of which may have planets!), and different types of interstellar gas and dust, the "body" of our galaxy is shaped like a great disc 300 light years thick and 100,000 light years across. It is a spiral galaxy, and our Sun is about two thirds of the way out from the centre along one of the spiral arms. Humans like to think they are important; but in the vastness of the Milky Way, our Earth is like a grain of sand on the beach - and the Milky Way is only one of millions of galaxies wheeling through space!

The galaxy has four main parts:

Nuclear Bulge:	The galaxy is shaped like a pancake with a bulge at the centre. This "nuclear bulge" is about 16,000 light years in radius, and contains mainly old stars and interstellar gas and dust.
Galactic Disc:	The part of the pancake outside the bulge, extending about 50,000 light years out from the centre. The disc contains all the young stars, and more interstellar gas and dust.
Halo	The spherical region surrounding the disc to a radius of about 65,000 light years. The halo contains old stars, globular star clusters, and thinly spread interstellar gas and dust.
Galactic Corona	The galactic corona is an enormous sort of outer halo that may extend as far as 300,000 light years in radius. The corona is now believed to contain most of the mass of the galaxy.

The galaxy is slowly rotating: our Sun takes about 250 million years to do one orbit of the galactic centre. Hence our solar system must have made only 20 or so orbits.

source:http://www.perthobservatory.wa.gov.au//information/milky_way.html#