A 2016 study estimated that the observable Universe has about 2 trillion galaxies, spanning more than 90 billion light years. We see them from Earth at different stages of life, and their forms tell us not only their age but also give us clues about their evolution.
Types of galaxies
Also called a spiral, it looks, in the words of Cameron Hummels, an astrophysicist at the California Institute of Technology (Caltech), “a fried egg”: a circular center surrounded by a disk of gas and stars spinning hundreds of kilometers per second .
This type is what astronomer Edwin Hubble, who classified galaxies in 1936 and gives the iconic space telescope its name, called it “late” because it would have formed some time after the Big Bang from hydrogen clouds joined by gravity.
Called by Hubble “initial”, this type of galaxy would have formed soon after the explosion that started the Universe and concentrates very old stars and little gas. Because of its random movement (it does not rotate like disk galaxies), it is believed that it forms in the merger of colliding galaxies.
Ellipticals are round, but some stretch out in the shape of a cigar. The largest known galaxies in the Universe are giant ellipticals, with 1 trillion stars and 2 million light years in length, but there are small versions called dwarf ellipticals.
If every elliptical galaxy is the result of a merger, not every merger produces an elliptical galaxy, and a good example is the Milky Way, still in disk form, but not forever: it is on a collision course with Andromeda, and the two disks should come together, creating a new elliptical.