It is still used today for many purposes, such as bulk classification of galaxies with the citizen science website Galaxy Zoo, where you can categorize scientific images of faint galaxies, helping scientists, and improving automated image-analysis tools. Hubble’s system is a bit incomplete and also somewhat subjective, which can lead to different people assigning different classifications to the same galaxy. Hubble lumped anything that did not fit into the other three categories into the irregular (Irr) category, with Irr I denoting galaxies with resolved star clusters and Irr II for those that did not have any resolved star clusters. Our own Milky Way is an SBc galaxy, while nearby Andromeda is an SAb galaxy, lacking a bar and with tighter spiral arms than our own. Hubble also adds a letter to denote how tightly wound the spiral arms are, with a denoting the tightest and c the least tight, with later astronomers adding the d class for extremely loose spirals. Spiral galaxies can be differentiated between barred spirals, which have a bar going through the center of the galaxy, and unbarred spirals, or SA and SB, respectively some astronomers drop the A and refer to unbarred spirals as just S-type. Spiral galaxies were thought in Hubble’s day to have evolved from elliptical and lenticular galaxies today we know that they probably were some of the first galaxies with regular structure and that all giant elliptical and most lenticular galaxies are probably the results of mergers between smaller spiral galaxies. Lenticular galaxies are either aging spirals or the products of galaxy mergers our own Milky Way and Andromeda galaxies could become lenticular galaxies when they merge a few billion years from now. Lenticular galaxies have little ongoing star formation, which you can see in images as the dust lanes lack any bright bluish or pinkish spots corresponding to young blue stars or hydrogen-rich star-forming regions. They have the spherical shape of elliptical galaxies at the center, surrounded by a disk of concentric rings of dust that can look like a spiral when viewed edge-on, or an elliptical galaxy if the disk is not very bright. Lenticular galaxies are sort of an intermediate class between elliptical and spiral galaxies and are classified as S0 or ES in the Hubble system. Hubble was also unaware of the differences between dwarf spheroidal/elliptical galaxies and their larger counterparts, which we’ll get into later. Hubble classified some galaxies that were thought to be elliptical all the way to E7, but we now know that those were just lenticular galaxies viewed at an odd angle. Completely spherical elliptical galaxies are referred to as E0 types, flattening out to more oval or American football-shaped galaxies as E1, E2, and so on all the way to E4. Hubble’s system of classifying elliptical galaxies is not based on size, spectrum, or origin but rather on shape and structure (more commonly referred to as morphology). By contrast, dwarf elliptical galaxies are frequently found circling larger spiral or elliptical galaxies. Giant elliptical galaxies make up some of the largest galaxies in the Universe. This is probably the only classification you’ve ever heard of, but it’s a little incomplete.Įlliptical galaxies are galaxies with little dust and gas left. He came up with a system that is still used today. In 1926, around the same time that it was proven that the odd “spiral nebulae” in the sky and some of the other “faint fuzzies” seen through telescopes were actually separate galaxies or “island universes” from our own, astronomer Edwin Hubble sought to classify these objects. Origins of Galaxy Types & Classification Hubble’s Sequence of Galaxies The galaxies we’ve listed in each category are those that we’ve picked because they show the defining feature of their given morphological type many other well-known galaxies fit into each type we’ve listed. This article will give an overall layout of the classification and known types of galaxies, as well as some you can observe yourself with a fairly modest telescope under suburban or dark skies. However, there are quite a few exotic types, some of which can be observed with backyard telescopes, and numerous intermediate classifications and variations between the most commonly described galaxy types. You may have heard of the classes of spiral, elliptical, and irregular galaxies, along with perhaps lenticular galaxies, too. There are many more types of galaxies than common thinking might suggest.
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