The Milky Way system
On a dark night we can often see a band of light stretching across the sky. If we look at this band with binoculars or a small telescope we see that it is partially resolved into stars. This band we call the Milky Way and indeed it is composed of a band of stars most of which are too faint to be resolved so that we see their combined light as a faint glow.
This band is the plane of the disk of our galaxy. The Sun is one, rather faint, example of approximately 200,000,000,000 stars that make up our galaxy. These stars are mostly grouped into a flattened disk which has a bulge at its centre. The Sun is in this disk about two thirds of the way from its centre to its edge. When we look at the night sky we see the Milky Way when we look along the plane of this disk whereas when we look in other directions, out of the plane, we see far fewer stars.
There is a spherical component to our galaxy which contains very old stars and spherical clusters of old stars. These are often referred to as Population 2 objects. Population 1 being the objects found in the disk.
The size of our galaxy is huge; light would take about 100,000 years to cross the Galaxy.
Our galaxy has arms of younger stars and gas that appear to spiral out from the centre. In fact the objects in these spiral arms are in almost circular orbits about the centre of the Galaxy. The Sun takes about 200 million years to complete one orbit around the centre. About 30 percent of all galaxies have spiral arms. Some have arms that spiral directly from the nucleus while others have a linear feature, called a bar, from whose ends the arms originate.
Spiral galaxies are rich in gas and dust. Some are viewed face-on so that the spiral arms are easily seen whereas others are viewed edge-on. These show the presence of dust lanes which obscure the starlight coming from near the midline of the disk. We see this in our galaxy where the Milky Way is divided into two portions for much of its length. Indeed the centre of the Milky Way galaxy is invisible in ordinary light because the interstellar dust in that direction is so thick. Infrared light, however, penetrates the dust and recent measures have allowed astronomers to `see' the Galactic centre.
The majority of galaxies show no spiral features, nor are they flattened disks; they take the form of ellipsoids. They show only small evidence for young stars, dust or gas. They are very different in size ranging from giant ellipticals with masses of about 1 million million times that of the Sun to dwarf ellipticals with masses closer to those of the globular clusters.
Some galaxies are neither ellipsoidal nor are they spirals. Some of these are obviously objects which have been tidally distorted by the presence of another near-by galaxy but there are some, such as the Magellanic Clouds (see below), which have little symmetry to their structure.
Some galaxies show evidence for the generation of enormous amounts of energy from the vicinity of their nucleus. These are often strong radio emitters and often show complex lobe structure extending for millions of light years. Other galaxies have such energetic nuclei that we only see the bright nucleus and not the underlying galaxy; we call these objects quasars (quasi-stellar objects).
The presence of black holes at the centres of these objects is thought necessary by many astronomers to explain their nature. Because they are the brightest objects known in the universe it is not surprising that quasars are the objects that have been traced out furthest from us. The furthest known are so far away that the light we see coming from them must have originated when the Universe was only one tenth of its present age.
Clusters of galaxies
There are many clusters of galaxies. Members of some of the closest can be seen with a small telescope in the constellations Virgo and Coma Berenices. We can trace clusters of galaxies out to the furthest distances that we can reach. Some of these clusters contain thousands of galaxies. Near their centres giant ellipticals are often found and it is thought that these arise from the collision of several galaxies which have combined.
X-ray studies have shown that there is very hot gas between the galaxies in a cluster but this gas does not solve one of the great puzzles in astronomy which is that these clusters require a certain total mass to explain how they are held together but we only can account for one tenth of this mass. This is known as the `missing mass problem'.
Unfortunately those of us who live in the northern hemisphere cannot see the two closest galaxies, called the Magellanic Clouds, which are rather like two satellite galaxies to the Milky Way.
They can easily be seen by the naked-eye and their brightest stars can be seen with binoculars. These two galaxies are much smaller than the Milky Way and are about 200,000 light years away.
In the northern sky we can see two galaxies with the naked-eye. The Andromeda galaxy, M31, is a faint fuzzy patch that appears, with binoculars, as a lens shaped object. It is a galaxy rather like ours at a distance of about 2 million light years. It has two dwarf elliptical satellites which can be seen with a small telescope.
The other galaxy (M33 in Triangulum) is much harder to see although it is at a similar distance to the Andromeda galaxy. This is because it is smaller and less bright intrinsically. It too is a spiral galaxy.