What is a galaxy exactly? In simple words, a galaxy is a system of stars, stellar remnants and other celestial objects, bound together by gravitational force. Galaxies can differ in shapes and size and each of them orbit around their own center of mass.
Our knowledge about the galaxies, including our Milky Way, has evolved from the philosophies of Aristotle and others in 5th century B.C, to Edwin Hubble’s early ground breaking discoveries in early 1920s to major scientific findings in the late 20th and 21st century.
It is estimated that there are about 2 trillion galaxies in the observable universe and most of the discovered galaxy to date have distinct features and varies largely in shapes and sizes. To categorize different galaxies, astronomers and researchers use a morphological classification developed by Edwin Hubble, which helps them to study individual galaxies precisely.
His sequence was, however modified by French astronomer Gérard de Vaucouleurs in 1959. Along with the morphological classification of galaxies, we have also discussed types of galaxies based on other characteristics. So space nerds, lets begin.
Hubble Classification scheme for galaxies
In 1926, Edwin Hubble put forward the first ever morphological classification scheme for galaxies. Hubble’s classification, recognizes three major types of galaxies; Elliptical, Spiral and Lenticular. These broad categories of galaxies are subdivided to form a system which we know as tuning fork diagram.
Elliptical galaxies are generally smooth and featureless. Hubble’s classification scheme, separate these galaxies based on their rate of ellipticity, E0, being almost spherical to E7, a highly elongated galaxy. One of the most noteworthy feature of this type of galaxy is they have very low number of open clusters and low rate of star formation. They generally consist of more evolved or older stars.
Some of the biggest elliptical galaxies in the observable universe are more than 700,000 light years across, and have a mass near 100000000000000 solar masses, that’s 10^13 solar masses. Messier 87 and IC 1101 are two most popular elliptical galaxies detected so far.
NGC 7793, a spiral galaxy Image Courtesy: NASA/JPL-Caltech/R. Kennicutt
At the very center of Hubble’s system, where two branches of spiral galaxies bifurcates, you can see an intermediate galaxy(s) attributed by the symbol S0. These types of galaxies are known as lenticular galaxies. They have a bright bulge at their core and are elliptical in appearance, however, unlike spiral galaxies they lack spiral arms and are not producing new stars at significant rates.
De Vaucouleurs Classification
Diagram of Hubble -de Vancouleurs Galaxy Morphology
Based on Hubble’s sequence, de Vaucouleurs established an extension of galaxies, which he thought was left out by Hubble. He argued that Hubble’s classification of spiral galaxies is incomplete and it doesn’t describe them in their full extent. In his system, he introduced a more detailed classification of spiral galaxies, which focuses on their ring pattern and lenses.
Barred spiral galaxy
Most of the spiral galaxies observed thus far are actually barred spiral galaxies. A barred spiral galaxy is simply a spiral galaxy with a linear bar in the central core, which extends outward from its either sides. Hubble designates them as SB followed by small English letters a, b and c, similar to the one in normal spiral galaxies.
These bars are believed to be temporary and are either caused by a release of huge energy wave outwards from the core or because of powerful tidal interaction with a neighbor galaxy.
Our Milky Way is now classified as a barred spiral galaxy, but do you know before the 1990s, it was considered as a simple spiral galaxy? Our galaxy, which contains nearly about 2 billion stars, one of which is our sun and has 600 billion times more mass than that of the Sun, consists of a bar-shaped region in the core surrounded by stars and gas.
By the late 1990s, researchers all over the world raised doubts about the true shape of the Milky way and suspect that we might be a barred galaxy. Their doubts turned out to be true after the successful observation by the Spitzer Space Telescope in 2005.
Some Other Types of Galaxies Based on their Morphology
Peculiar Galaxy: A Peculiar galaxy, as its name suggests, is a galaxy of strange shape, size and have an unknown composition. Only a small percentage of all discovered galaxies are categorized as Peculiar galaxies. AGNs (Active Galactic nuclei) and interacting galaxies are currently the two types of peculiar galaxies identified by astronomers.
These types of galaxies are believed to be a result of gravitational tug-of-war between two galaxies, when they come extremely close to each other. The two affected parties develop peculiar visual properties due to massive tidal interaction.
Hoag’s object; a ring Galaxy Image Courtesy: NASA and The Hubble Heritage Team
Ring Galaxy: A ring galaxy contains many massive, young and bright stars, encircling around a relatively less bright nucleus. The Hoag’s object is the perfect example of the ring galaxies located in the Serpens Caput about 612 mega light years away. One leading theory regarding their formation is the gravitational disruption caused by a near pass-by of a smaller galaxy near the core of a larger one.
Irregular Galaxies: Those galaxies which can neither be classified as elliptical nor as spiral are known as Irregular galaxies. They have a chaotic appearance and are without any spiral arm or a central bulge. Irregular galaxies can be divided into three subcategories; Irr-I galaxy, lrr-II galaxy and dI-galaxy, none of which cleanly aligns with Hubble’s scheme.
In the above section we have discussed galaxies based on their morphology or their appearance. But if a galaxy, irrespective of their shape, contains an active galactic nucleus, then it can also be classified as an active galaxy. What is an active galactic nucleus you ask, well it’s a compact region near the center of a galaxy, that have more than usual luminosity over almost the entire electromagnetic spectrum.
Active galaxies are divided into two categories; Radio-quiet AGNs and Radio-loud AGNs. Radio-quiet AGNs like Seyfert galaxies shows narrow and sometimes broad emission-lines, infrequent strong X-ray emission and weak radio jet. Other types of radio-quiet AGNs are LINERs, Quasar 2s and Radio-quiet quasars.
The Image captured by Hubble telescope shows the ejected jet of matter from Messier 87, an active galaxy travelling nearly at the speed of light.
On the other hand, “Blazars,” a type of radio-loud AGNs, are distinguished by high X-ray and radio emissions, jets and are highly variable. Other types of radio-loud AGNs are optically violent variable quasar and radio galaxies.
Starburst galaxies are known to generate new stars at an exceptionally high rate. This rate is so high that these galaxies are bound to use their entire star forming gas reservoir much faster than any other type of galaxy. Most of the observable starburst galaxies are either going through a galactic merger or are about to encounter one.
Over the years, astronomers marginally classified starburst galaxies based on their distinct apparent characteristics. They are Blue compact galaxies, Luminous infrared galaxies and Wolf-Rayet galaxies. One of them is described below.
Luminous infrared galaxy: Infrared galaxies are most probably single, gaseous spirals, which gets their infrared luminosity either from large numbers of stars packed in a compact region or from an active galactic nucleus. LIRGs are believed to have brightness more than 100 billion times that of the Sun.
It is generally considered that some luminous infrared galaxies create nearly 100 new stars compared to only 7 stars by the Milky Way each year, in this way they maintain their extremely high luminosity levels.
On the left is an actual Lyman-alpha emitter and on the right is an artist impression of the galaxy Image Courtesy: Chandra Observatory
Lyman-alpha emitter or LAE are galaxy types that emits a continuous spectrum of hydrogen known as Lyman-alpha radiation. These galaxies, located in the far reaches of the universe, are young and have typically low mass (10^8 solar masses).
Other characteristics include highest specific star formation rate that strongly suggests these galaxies are incredibly valuable to study the evolution of much older and evolved galaxies like our Milky Way itself.
Low Activity Galaxies
Ultra diffuse galaxies (UDG): UDGs are extremely-low density galaxies found in different galaxy clusters. Most of the Ultra diffuse galaxies are about the same size as the Milky Way but have as low as 1% of its visible star count. UDGs are largely inhabited by older stars due to the lack of star forming gas.
Low-surface-brightness galaxy: These type galaxies are mostly dwarf and most of their matter is in gaseous hydrogen form rather than stars. They are extremely faint due to the lack of star formation.