school energy&environment

What galaxies are

Have you been invited to an astronomical dinner, and you don’t want to come hempty-handed? Here we have the perfect recipe for you!
Let’s take at least 100 billions of stars and all the interstellar gas as much as you like. Then add enough interstellar dust and dark matter. Cook your mixture at least at 10³² K and do not forget the fundamental ingredient: the gravity! Let’s make all cold for a billion of years and… voilà! You will have a new galaxy, ready to be waited on!
It seems easy to create a new galaxy, but really we did not know what a galaxy was, until a very few time ago. In the past, we didn’t have powerful observation instruments as today, so galaxies seemed small regions, placed everywhere in the skies and featured by a bright vagueness, called, exactly, nebulas.
Until the Twenties of the last century, scientists supposed that these nebulas were parts of our galaxy, whose real dimensions were still misterious. In 1924, astronomer Edwin Hubble, thanks to one of the most powerful telescope of his time, had been able to see some regions of the nebula of Andromeda, confirming that it was a single galaxy, external to ours. Already around 1929, Hubble discovered 18 galaxies, each one containing billions of stars. But are all these galaxies equal?