In the standard cosmological model, it is dark matter which, with the help of the cosmic web, would be at the origin of the formation of the large structures of the Universe such as galaxies. However, even if the galactic models have been considerably improved over the last decades, the formation and evolution of galaxies still have gray areas. Nevertheless, the detailed observations of astrophysicists as well as the associated modeling have made it possible to put forward relatively solid hypotheses on the way in which galaxies acquire their morphology.
In the simplest form of classification, there are two categories for galaxy shapes:disk and elliptical. A disk galaxy, also called a spiral galaxy, is shaped like a fried egg, says Cameron Hummels, theoretical astrophysicist at Caltech. These galaxies have a more spherical center, like an egg yolk, surrounded by a disk of gas and stars — the egg white. The Milky Way and our closest galactic neighbor, Andromeda, fall into this category. Specifically, the Milky Way is a barred spiral galaxy, as it contains a bar of dense matter at its center.
In theory, disk galaxies initially form from hydrogen clouds. Gravity pulls gas particles together. As the hydrogen atoms get closer, the cloud begins to spin and their collective mass increases, which also increases the gravitational pull of the whole thing. Eventually, gravity causes the gas to collapse into a swirling disc. Most of the gas is in the outer rim, fueling star formation.
Edwin Hubble, who confirmed the existence of galaxies beyond our own only a century ago, called disk galaxies "late-type galaxies" because he suspected that their shape meant that they were formed later in the history of the Universe. When in reality, elliptical galaxies — what Hubble called "early-type galaxies" — appear to be older. Instead of spinning, like disk galaxies, stars in elliptical galaxies move more randomly, says Robert Bassett, an astrophysicist at Swinburne University in Melbourne.
Finally, a less common form, lenticular galaxies seem to be a mixture between an elliptical galaxy and a disk galaxy. It may be that when a disk galaxy uses up all of its gas and can no longer form new stars, the existing stars begin to interact. The gravitational tug of each other creates a shape that looks like a lens.
Elliptical galaxies are thought to be the product of galaxy mergers. When two galaxies of equal mass merge, their stars have gravitational effects on each other, disrupting star rotation and creating a more random orbit. Not all mergers result in an elliptical galaxy. The Milky Way is actually quite old and large, but retains its disc shape. Its mass has increased by attracting dwarf galaxies, which are much smaller than our home galaxy, and by absorbing surrounding free gas.
However, Andromeda, our neighboring disc-shaped galaxy, is heading directly towards the Milky Way. Thus, in billions of years, the two spiral galaxies could merge and each of the starry discs of the duo will compensate for the rotation of the other, creating a more random elliptical galaxy.
These mergers are far from instantaneous. They take hundreds of millions, even billions of years. In fact, there are mergers underway that are progressing so slowly — from our perspective — that they seem static. Hubble has given these galaxies their own classification:irregular galaxies. According to astrophysicists, these galaxies look like disordered multi-component structures.
What astrophysicists have discovered so far about galaxies and their 3D shapes has been inferred using thousands of 2D images and relying on other properties, such as galaxy color and motion. , to fill in observational gaps. For example, the young age of disk galaxies is supported by their blue color. Blue stars are generally larger and burn rapidly at higher temperatures (blue light has a higher frequency and is therefore more energetic than red light). While elliptical galaxies are filled with older stars — called red dwarfs — that don't burn out as fast.
However, the formation and evolution of galaxies remains one of the greatest open questions in the field of astronomy and astrophysics, and current galactic models are just waiting to be completed and clarified.
