What exactly is a galaxy? 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 shape, size and each revolve around their center of mass.
Our Knowledge about the galaxies (including that of the Milky War), has evolved from the philosophical thinking of Aristotle in the 5th century B.C., to Edwin Hubble’s groundbreaking discoveries in the early 1920s to major scientific findings in the late 20th and 21st century.
There are an estimated two trillion galaxies or more in the observable universe. Most of the discovered galaxies to date have distinct features and vary largely in shapes and sizes.
To categorize different galaxies, astronomers and researchers use a morphological classification known as Hubble Sequence (developed by Edwin Hubble), which helps them to study individual galaxies precisely.
Hubble’s method was later modified by French astronomer Gérard de Vaucouleurs in 1959. Based on these classifications and a few other characteristics, we have discussed different types of galaxies below. So, space nerds, let’s begin.
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Hubble Classification scheme for galaxies
In 1926, Edwin Hubble put forward the first-ever morphological classification scheme for galaxies; the Hubble’s classification. It recognizes three major types of galaxies; Elliptical, Spiral, and Lenticular. These broad categories of galaxies are subdivided to form a system called 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 features of elliptical galaxies is they have a very low number of open clusters (a group of few thousand stars) and a low rate of star formation. These galaxies generally consist of older, more evolved stars.
The largest galaxies in the observable universe are ellipticals. Many of them are more than 700,000 light-years across and have a mass near 100000000000000 solar masses, that’s 10^13 solar masses.
Examples of elliptical galaxies: Messier 87, IC 1101, and Maffei 1 (closest elliptical galaxy).
NGC 7793, a spiral galaxy Image Courtesy: NASA/JPL-Caltech/R. Kennicutt
The spiral galaxies are recognized by their bright spiral arms (mostly two) and a central bulge, inhabited chiefly by older stars. In Hubble’s classification, spiral galaxies are denoted by the English letter ‘S’ followed by a, b, or c, which indicates the stretch of spiral arms (‘a’ being close armed).
The arms of a spiral galaxy are distinctively visible due to the presence of young, still-forming stars in abundance.
Barred Spiral Galaxies
A barred spiral galaxy is, basically, a spiral galaxy with a bar-like structure at the center, which extends outward from either side. More than half of all spiral galaxies observed to date are, in fact, barred spiral galaxies. Hubble designates them as S.B., followed by small English letters a, b, and c, similar to the one in normal spiral galaxies.
These galactic bars are assumed to be temporary (they decay over time) and are caused either by an outward release of energy from the core, or due to powerful tidal interaction with a neighbor galaxy.
The Milky Way, which contains two billion stars (one of which is the Sun), was once classified as a spiral galaxy but is now confirmed as a barred spiral galaxy.
Examples of (Barred) Spiral Galaxies: Milky Way, Andromeda Galaxy, and Whirlpool Galaxy.
At the very center of Hubble’s system, where two branches of spiral galaxies bifurcate, 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.
Examples of Lenticular Galaxies: Cartwheel Galaxy, NGC 2787
De Vaucouleurs Classification
Diagram of Hubble-de Vancouleurs Galaxy Morphology
Based on the Hubble’s sequence, French astronomer de Vaucouleurs developed an extension of galaxy morphological classification. He argued that Hubble’s classification is incomplete and doesn’t describe them to their full extent.
While the De Vaucouleurs system maintains the primary classification of galaxies, ellipticals, spirals, lenticulars, and irregulars, it introduces a more detailed categorization of galaxies, which focuses on their rings, bars, and spiral arms.
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 has 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 a 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 its 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 has 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 show 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 traveling nearly at the speed of light.
On the other hand, “Blazars,” a type of radio-loud A.G. Ns, 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 get 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 emit 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 the 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 types of 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.