We can see several types of galaxies, differring for example for
their form, dimensions, brightness, mass, stellar contents and,
in the end, for the energy emission distribution in the
different bands of the electromagnetic spectrum. The principal
classification, called Hubble Sequency, is based on the form and
it divides the galaxies between elliptical, spiral and irregular
ones. They show regular systems, approximately with a spherical
form, with just few dust and interstellar gas, fitted with a
really dense nucleus, whose superficial brightness decreases
from the centre towards the periphery. Their structure may
change from the circular form, called E0, to the extremely
crushed one, described like E7. The elliptical galaxies are
made, above all, by red stars (or Population II) that, according
to the theory of the stellar evolution, are very ancient. Stars
are in fact used to change their color becoming old. In the
first part of their life they show a blue color, becoming then
more yellow-red. They appear l