ESA Science & Technology03-Aug-2005 11:04:50

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 1: Shape & Structure of our Galaxy

The Milky Way has a distinct shape, formed by the three major components that make up the galaxy:

  1. A thin disk 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.
  2. A central bulge made up of older stars.
  3. 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.

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