The main classes

Below you can see some elliptical galaxies.  

The lenticular galaxy M84 (Bill Keel, Univ. Alabama)

 

The lenticular galaxy (SEDS)

The elliptical galaxy M32 (Bill Keel, Univ. Alabama)

As you can see, the appearance of the elliptical galaxies is quite uniform. The luminosity is higher towards the center and decreases more and more going outwards. The last image shows that they have a yellow-orange color.  

Now you must remember what you discovered when you studied the life of stars. That is, a star system changes its color as it grows old. In particular, when it is just born it has a blue color and becomes more and more yellow as time goes by. This happens because the color of the total light is dominated by the most luminous stars. In a young system, the luminous stars are those blue ones of the main sequence, while in an aged system, the luminous stars are the rarefied red giants.  

The color of an elliptical galaxy then tells us that it does not contain young stars.  

The following pictures show two examples of spiral galaxies.  

M33 (David Malin, AAT)

M104 (Sombrero) (David Malin, AAT)

As you can see, the appearance of these galaxies is very different from that of the elliptical galaxies. M33 is seen almost "face on", and you can see that it has spiral arms superimposed to a less luminous disk. At its center there is a bulge more luminous than the disk. The color of the arms and of the disk is mostly bluish, while the bulge is yellow-white. There are also some clouds of hot gas which appear as luminous stains (for example in the upper left arm).  

A spiral galaxy seen edge on is just like M104. As you can see the disk is immersed into a halo about spherical in shape, and yellow, whose luminosity decreases going outwards. Moreover, it is evident that inside the disk a large amount of dust is stored.  

These considerations tell us that a spiral galaxy is much more vital than an elliptical galaxy. It has young stars and hot gas where stars are still forming, and it contains dust. But it also has a fraction of older stars, distributed in its halo and bulge.  

Besides these two classes of galaxies, there are irregular types like the following one.

The irregular galaxy M82 (Bill Keel, Univ. Alabama) 

 Generally, even irregular galaxies present the variety of stellar populations, gas and dust of the spiral galaxies.  

By counting the galaxies in a fixed volume of the Universe, one finds that the most numerous galaxies are the irregular ones, followed by the spiral galaxies and finally by the elliptical ones.  

Special cases 

 Galaxies tend to group in clusters of galaxies. At the center of these clusters giant elliptical galaxies can be found, like M87, which can be seen in the following photo.  

The elliptical galaxy M87 (David Malin, AAT) 

 Finally, cases of cannibalism between two or more galaxies are not uncommon. The perhaps most famous case is that of the M51 system. On the left you can see a bridge of matter connecting the two galaxies, which demonstrates that the two galaxies are interacting.

M51 (SEDS)

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