Category:List of Systems

The Milky Way

The galaxy consists of about 200 billion stars, with the Terran sun, Sol, being a fairly typical specimen. It is a fairly large spiral galaxy and it has three main components – a disk, in which the Sol system resides, a central bulge at the core, and an all-encompassing halo. These components are of varying ages, but the determining factor for any galaxy’s age is the composite age of its halo stars. In the Milky Way, halo stars have a rough age estimate of 12 billion years. This, compared with other galaxies, renders the Milky Way a mature adult galaxy, nearly twice the age of nearby Andromeda (four to six billion years) but not nearly as old as Maelstrom (a galaxy with an amazing age of more than 90 billion years).

• Disk

The disk of the Milky Way has four spiral arms and it is approximately 30 kiloparsecs wide and one kiloparsec thick. It is made up predominantly of Population I stars which tend to be blue and are reasonably young, spanning an age range between a million and ten billion years old.

• Bulge

The bulge, at the centre of the galaxy, is a flattened spheroid ten kiloparsecs wide and two and a half kiloparsecs thick. This is a high-density region where Population II stars predominate – stars which tend toward red and are very old, about ten billion years or more in age.

• Halo

The halo, which is a diffuse spherical region, surrounds the disk. It has a low density of old stars mainly in globular clusters (these consist of between 10,000 and 1,000,000 stars). The halo is believed to be composed mainly of dark matter, which may extend well beyond the edge of the Milky Way’s disk.

A star system can contain one star or multiple stars. Humans are more likely to find habitable planets in systems with single stars. In reality, more than half of all star systems have two or more stars, and these systems typically contain planets that are inhospitable to human life.

-> Classification of star types