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 like systems full of interstellar gas and dusts, based on a central bulge surrounded by a disc, from where run bright spiral filaments, called arms, site of an intense stellar formation.
We can also divide these spiral galaxies into two classes: normal ones (S), with a central and at least perfectly spherical central nucleus and spiral arms, and barred spirals (SB), different from the normal ones because of a central structure placed through the nucleus, a so-called bar-shaped structure.
They appear like systems full of interstellar gas and dusts, usually being inferior, for their mass, to the spiral and elliptical galaxies. They are usually called "irregular" because their aspect has no simmetry. They typically host young stars, or stars from the population I.
In 1965, while studying the ground noise of a radio antenna...
When two galaxies start to approach, the tidal attraction forces deform their structures...