Types of galaxies
What is a galaxy?
A galaxy is an immense group of billions of stars, interstellar gas and dust, and dark matter, all gravitationally bound. The Universe counts hundreds of billions of them.
When has the first galaxy been observed/recognized?
The first galaxy that has ever been observed is our Home Galaxy: the Milky Way. But where do the names “galaxy” and “Milky Way” come from?
The word “galaxy” originates from the Greek word galaxias which means “milky”. Ancient civilizations associated legends with the bright white band they see in the night sky. The Greeks, for instance, thought that this white strip is Hera’s milk splattered across the sky, hence the name galaxías kýklos or “milky circle”, which led to the word “galaxy”
The first galaxy, other than the Milky Way, that has ever been observed is our close neighbor Andromeda. Persian astronomer Abd Al-Rahman Al-Sufi was probably the first to record the observation of that galaxy, back in 964.
Types of galaxies
The major components of a galaxy are the bulge (a central, spherical, and very dense group of stars) and the disk that usually contains spiral arms.
We can categorize galaxies according to their shape, that is the shape of the bulge and disc. This what Edwin Hubble did in 1926. His observations led to the Hubble diagram, also known as the Hubble sequence or the tuning fork diagram.
As a rule, the more we move to the right, the smaller the bulge is, the larger spiral arms become, and the more young stars the galaxies hold. From left to right, we find the following categories:
1. Elliptical galaxies
labeled from E0 (most spherical) to E9 (most elongated). They represent about 1/3 of all galaxies. They have a relatively small content of gas and dust. Stars in this type of galaxies are old and active star formation stopped. Giant elliptical galaxies are the rarest. Scientists think that they are the result of the merger of galaxies. Giant ellipticals can reach up to 300,000 light-years across, while the smallest ones (a few thousand light-years across), also known as dwarf elliptical galaxies, are the most common. At the intersection of the two “prongs”, we find lenticular galaxies, labeled S0. Both elliptical and lenticular galaxies are also called “early-type galaxies”, as Edwin Hubble thought at first that they were the first stages of galaxy formation. However, researchers have shown that this was wrong, although the terms early-type and late-type are still used.
2. Spiral galaxies
they are on the right side of Hubble’s sequence. They have spiral arms, which are regions of active star formation. In the upper branch, we find normal spirals, labeled Sa, Sb, and Sc. The galaxies in the lower branch are the barred spirals and are labeled SBa, SBb, and SBc. They are called barred because they have a “bar” of stars that runs through the bulge and from which the spirals usually start. A significant fraction of all the galaxies in the local universe includes spiral galaxies. All these galaxies are also known as late-type galaxies. They were thought to be at their latest stages of evolution (which, again, turned out to be false). They appear flat, with a blue-white disk of gas, dust, and stars. They have yellowish bulges in the center. The Pinwheel Galaxy (M101) and The Leo Triplet are examples of spiral galaxies.
3. Irregular galaxies
on the far right, we find irregular galaxies, which are galaxies that have no regular shape. They have no disk or bulge. Most of them are very distant, which means that they were predominant in the early Universe, unlike spiral galaxies for example. Irregular galaxies used to be elliptical or spiral galaxies but were distorted by interactions or collisions with other galaxies.
4. Dwarf galaxies
they are the most common type of galaxies in the Universe. Dim, small, with a small number of stars, they are not easily detected from Earth. Most of the dwarf galaxies are irregular, and some of them are dwarf elliptical or spiral galaxies, like the Coddington’s Nebula.
Scientists identified other types of galaxies, based on the physics rather than the morphology of the galaxy, aside from the ones above; for example AGNs (active galactic nuclei, such as quasars, blazars, and Seyfert galaxies), interacting galaxies, starburst galaxies (having very high rates of star formation).