The Milky Way:
Our home galaxy consists of about 200 billion stars, with the Sun being a fairly typical specimen. The Milky Way 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! Our galaxy is made up predominantly of Population I stars, which tend to be blue and are reasonably young, spanning a million and ten billion years in age.
A light-year is how astronomers measure astronomical distances and it is equivalent to the distance that light travels in one year, nearly 9.46 trillion kilometres.
The bulge at the centre of the galaxy is a flattened ellipse of dimensions 1000 parsecs by 6000 parsecs. This is a high-density region where Population II stars predominate---stars that tend toward red and are very old: about 10 billion years. There is growing evidence for a very massive black hole at its centre.
Black holes are the monsters of space! Anything that gets too close to a Black Hole is pulled to it with such a strong force that it has no chance of escape. Even light – the fastest thing in the Universe – is doomed if it goes near one of these monsters. This is why black holes are black. However, they are not really holes and they are not empty. Black Holes are actually filled with a lot of material that is crammed into an extremely small region.
The halo, a diffused spherical region, surrounds the disk. It has a low density of old stars mainly in globular clusters (consisting of between 10,000 and 1,000,000 stars). The halo is believed to be composed mostly of dark matter, which may extend well beyond the edge of the disk.
According to scientists roughly 80 per cent of the mass of the universe is made up of material that they cannot observe directly. This material, known as dark matter, does not emit light or energy.
Classification of Galaxies:
There are two main categories of galaxies, the spiral and elliptical, and two others lenticular and irregular.
Our galaxy belongs to the spiral category. Spiral galaxies 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 that have developed a bar in the interior region of the spiral arms are labelled SBa, b, c, d, or m. Spiral galaxies are characterised by the presence of gas in the disk, which implies that star formation is active at the present time, hence the younger population of stars. Spirals are usually found in a low-density galactic field where their delicate shape can avoid disruption by tidal forces from neighbouring galaxies.
These forces are a secondary effect of the gravitational forces between two objects orbiting each other, such as the Earth and the Moon. Tidal forces are responsible for the fluctuation of the tides as well as for the synchronous rotation of certain moons as they orbit their planets.
Elliptical galaxies are placed in 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 a 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.