Galaxies and the Expanding Universe
Structure of Milky Way
The Milky Way Galaxy, the home of our Solar System and approximately 400 billion other stars and their planets, has a spiral structure. With a diameter of around 120 000 light years, the Milky Way's mass could amount to one trillion solar masses. Everything in the Milky Way galaxy orbits around a centre of mass called the Galactic Centre. At the distance of the Sun a full orbit takes 225 million years - even though the Solar System is travelling at 250 km per second.
Figure 6.1: Structure of Milky Way
The Milky Way has a distinct shape, formed by the three major components that make up the galaxy:
- A thin disc made up of spiral arms. Within these arms are plentiful quantities of the gas and dust out of which stars are produced, and these are constantly being formed.
- A central bulge made up of older stars.
- A more mysterious element - a dark halo of unknown composition. There are no luminous stars in this halo, but we know it is there because it exerts a gravitational pull on visible matter.
The Sun is located around 28 000 light-years from the Milky Way’s centre, equivalent to roughly two-thirds of the way from the centre, a distance confirmed by data acquired by ESA’s astrometric satellite Hipparcos. The Solar System is found in a smaller spiral arm called the Local or Orion Arm. This arm connects two much bigger arms, the Sagittarius Arm and the Perseus Arm.
||Types of Galaxy
Last Update: 14 May 2013