A galaxy is a system of stars, stellar remnants, interstellar gas, dust, and dark matter, bound together by the force of gravity. Since its first discovery, scientists have estimated there are about 200 billion of galaxies out there. These galaxies are classified or according to their morphological structure or visual appearance.
Perhaps the most famous classification is the Hubble sequence, invented by Edwin Hubble in 1926. This system divides galaxies into three groups : elliptical galaxies, spiral galaxies, and lenticular galaxies. The galaxies who does not fall into this type of group is simply classified as an “irregular galaxy.”
Elliptical Galaxies
IC 1101
Elliptical galaxies have smooth, featureless light distributions and appear as ellipses in photographic images. Unlike flat spiral galaxies with organization and structure, they are more three-dimensional, without much structure, and their stars are in somewhat random orbits around the center. Most elliptical galaxies are composed of older, low-mass stars and minimal star formation activity. They also tend to be surrounded by large numbers of globular clusters (a cluster of stars forming a ball). They are not the dominant type of galaxy in the universe overall. Elliptical galaxies are called “early-type” galaxies (ETG), due to their location in the Hubble sequence.
Examples of elliptical galaxies: M49, M59, M60, M87, NGC 4125.
Spiral Galaxies
The Pinwheel Galaxy (NGC 5457)
A spiral galaxy consists of a flattened disk, with stars forming a (usually two-armed) spiral structure, and a central concentration of stars known as the bulge. Roughly half of all spirals are also observed to have a bar-like structure, extending from the central bulge, at the ends of which the spiral arms begin. The bulges are often surrounded by a much fainter halo of stars, many of which reside in globular clusters.
Spiral galaxies are named by their spiral structures that extend from the center into the galactic disc. The spiral arms are sites of ongoing star formation and are brighter than the surrounding disc because of the young, hot stars that inhabit them. Together with irregular galaxies, spiral galaxies make up approximately 60% of galaxies in today’s universe.
Examples of regular spiral galaxies: M31 (Andromeda Galaxy), M74, M81, M104 (Sombrero Galaxy), M51a (Whirlpool Galaxy), NGC 300, NGC 772.
Lenticulars
The Spindle Galaxy (NGC 5866)
A lenticular galaxy is a type of galaxy intermediate between an elliptical and a spiral galaxy in morphology. Lenticular galaxies are disc galaxies (like spiral galaxies) that have used up or lost most of their interstellar matter and therefore have very little ongoing star formation. They consist mainly of aging stars (like elliptical galaxies). Despite the morphological differences, lenticular and elliptical galaxies share common properties. Both can be considered early-type galaxies that are passively evolving, at least in the local part of the Universe. Lenticular and spiral galaxies, taken together, are often referred to as disk galaxies.
Examples of lenticular galaxies: M85, M86, NGC 1316, NGC 2787, NGC 5866, Centaurus A.
Irregulars
The Large Magellanic Cloud (LMC) – a dwarf irregular galaxy
An irregular galaxy is a galaxy that does not have a regular shape, unlike a spiral or an elliptical galaxy. Irregular galaxies do not fall into any of the regular classes of the Hubble sequence, and they are often chaotic in appearance, with neither a nuclear bulge nor any trace of spiral arm structure.
They are thought to make up about a quarter of all galaxies. Some irregular galaxies were once spiral or elliptical galaxies but were deformed by an uneven external gravitational force. Irregular galaxies may contain abundant amounts of gas and dust.
Irregular galaxies are commonly small, about one tenth the mass of the Milky Way galaxy. Due to their small sizes, they are prone to environmental effects like crashing with large galaxies and intergalactic clouds.
Examples of irregular galaxies: M82, NGC 1427A, Large Magellanic Cloud, Small Magellanic Cloud.
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