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The Milky Way system
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.
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 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
Xray 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'.
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.
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