Glitter Your Milky Way

The Milky Way:

Our own galaxy consists of about 200 billion stars, with our own Sun being a fairly typical specimen. It is a fairly large spiral galaxy and it has three main components: a disk, in which the solar system resides, a central bulge at the core, and an all-encompassing halo.

• The disk of the Milky Way has four spiral arms and it is approximately 300 parsecs thick (1 parsec is approximately 3.26 light years) and 30,000 parsecs in diameter. Compare this to the four hours it takes light to travel from the Sun to Neptune, showing the scale of the Milky Way is immense. 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.

• The bulge, at the centre of the galaxy, is a flattened spheroid of dimension 1000 parsecs by 6000 parsecs. This is a high density region where Population II stars predominate—stars which tend toward red and are very old, about 10 billion years. There is growing evidence for a very massive black hole at its centre.

• 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 - 1,000,000 stars). The halo is believed to be composed mainly of dark matter which may extend well beyond the edge of the disk.

Classification of Galaxies:

There are two main categories, the spiral and elliptical galaxies, but also lenticulars and irregulars.

• The spiral category includes our own galaxy. They fall into several classes depending on their shape and the relative size of the bulge: ordinary spirals are labelled either Sa, b, c, d or m while those which have developed a bar in the interior region of the spiral arms are SBa, b, c, d or m. Spiral galaxies are characterized by the presence of gas in the disk which means star formation remains active at the present time, hence the younger population of stars. Spirals are usually found in the low density galactic field where their delicate shape can avoid disruption by tidal forces from neighbouring galaxies.

• The elliptical galaxies are placed in the categories E0-7 depending on their degree of ellipticity, with E0 being least elliptical. They have a uniform luminosity and are similar to the bulge in a spiral galaxy, but with no disk. The stars are old and there is no gas present. Ellipticals are usually found in the high density field, at the centre of clusters.

• Lenticulars are labelled S0 and, although they possess both a bulge and a disk, they have no spiral arms. There is little or no gas and so all the stars are old. They appear to be an intermediate.

• Irregulars are small galaxies, labelled Irr, with no bulge and an ill-defined shape.