What's a galaxy ?

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A bit of history

It was not before the XVIIth century that Galileo, observing the whitish band which crosses the sky and that we call the Milky Way, realizes that it is in fact a large number of slightly luminous stars.
One century later, Thomas Wright makes the assumption that this band could be a huge disk of stars surrounding the Earth. Immanuel Kant takes up this idea again, and applies it the laws of gravity of Newton : this disk could be rotating, as the solar system is rotating. He is even thinking of the possibility that others "island universes" could exist, and that some of the nebulosities that can be noticed could be of the same nature as our Milky Way.

The past astronomy had no idea about the distances of celestial bodies, so this idea will not be used again until the beginning of the XXth century. At this moment, everybody thought that all the fuzzy spots observed in the sky were nebulae, in a single galaxy : our Milky Way.

In 1923, with the 100 inch telescope of Mount Wilson, the most powerful one of the time, Edwin Hubble was able to measure the distance to a star in the Andromeda galaxy, at 2 million light years. He reveals two fundamental facts :

Shape of galaxies

His first study was to classify the galaxies according to their shape.
In the same way that we can give stellar types, we can classify galaxies according to their shape.

Hubble's classification

Hubble's classification scheme, beginning with the elliptical galaxies and separating into spiral galaxies and barred spirals. This scheme is often called the tuning fork diagram.
The mass of the galaxies increases from the right to the left.

Contrary to Hubble's first ideas, this is not really an evolutionary sequence.
If there is an evolution, it is from the right to the left : spiral galaxies can become elliptical ones through collision mechanisms. The opposite direction is impossible.

 The S0 galaxies, between the elliptical galaxies and the spiral ones, are called lenticular galaxies, or sometimes hybrid galaxies.

Composition of galaxies

Of course, in a galaxy, one can find stars. But there is also gas and dust, which are the basis of other stars building.
Gas can be ionized when it is heated by massive stars with powerful radiations, or if it is situated near cataclysmic phenomenons like supernovae. On the opposite, in quiet areas, this gas will be able to generate stable molecules.
In the planetary nebulae, or in the atmosphere of giant stars, heavy elements will be able to condensate in dusts.

To obtain informations about the age of the stars of a galaxy, hence about its activity, we can use the radiated light :

Interpretation of data is of course not so easy.
Especially, we must take into account the presence of dust. And the dust catches more light on the blue side of the spectrum than on the red one, hence the light of stars which crosses dust areas is reddened when we see it.