A galaxy is something we all know about from some source or another from our childhoods. Whether it comes from comic books or rudimentary scientific literature, most people do have some inkling as to what a galaxy is. While we know that a galaxy is incredibly vast, even in comparison to our solar system, we can define a galaxy as a gravitationally bound system of stars, interstellar gas, stellar remnants and dark matter. Galaxies are incredibly expansive and diverse, yet they have still been classified by scientists based on various parameters and the properties they possess. Before we get into the types and morphologies of different galaxies, let’s briefly review the observational history of galaxies.
It is quite a feat when you think about how we have been able to observe and classify galaxies in the universe. The realization that we live in a galaxy that is only one among many others was paired with other discoveries revolving around the Milky Way Galaxy and Nebulae. The proposal for the existence of the Milky Way stretches as far back as the Greek Philosopher Democritus, who proposed that the Milky Way Galaxy might consist of distant stars. Actual proof of the Milky Way Galaxy containing many stars came in the year 1610, from the Italian Astronomer Galileo Galilei, who was able to study the Milky Way with the help of a telescope and concluded that many faint stars were present.
However, further investigation and research piqued the curiosity of scientists to describe the exact shape of the Milky Way Galaxy. The first project undertaken to define the shape of the Galaxy was in the year 1785, conducted by William Herschel. He was able to produce the first preliminary diagram by placing the sun at the centre as the reference point, which allowed him to continue mapping around it. By the early 1900s, specifically in the 1930s, significant strides were made by studying open clusters and a much more refined picture now exists today.
A few galaxies outside the Milky Way have also been observed by the unaided eye in the night sky. The Andromeda Galaxy was studied and noted in the 10th century by the Persian Astronomer Al-Sufi. By the time it came to William Herschel, he was able to catalogue close to 5000 different nebulae. In 1845, further improvement of the visual capabilities of telescopes helped astronomers distinguish between elliptical and spiral nebulae. Advancements continued until the year 1920, which is considered a crucial year in astronomical history. It was in this year that a great debate broke out concerning the nature of the Milky Way and its dimensions. The key aspect of the debate related to whether the Andromeda galaxy was a part of the Milky Way or a separate entity unto itself. It was proved to be an independent entity with the help of the Doppler Shift!
Types and Morphology
Galaxies are broadly classified into three main categories. They are organized in what is known as the Hubble Sequence. The Hubble sequence acts as a morphological scheme that aids in the classification of galaxies. Hubble’s sequence classifies galaxies into three primary sequences, the first of them being elliptical galaxies. Elliptical galaxies are relatively smooth, featureless light distributions and appear in the form of ellipses in photographic images. When they are included in a picture, they are usually demarcated with the capital letter E followed by an integer n (which represents the degree of ellipticity). The ellipticity increases from left to right on the Hubble Diagram.
At the center of the Hubble tuning fork, where the spiral galaxy and elliptical galaxy branches join, lies an intermediate class of galaxies known as lenticulars. These galaxies consist of a bright central bulge, similar in appearance to an elliptical galaxy. However, these galaxies are also surrounded by an extended disk-like structure. Unlike spiral galaxies, these discs do not form stars of any significant quantity. When merely glancing at the image head-on, it is hard to distinguish them from elliptical galaxies. When viewed from the perspective of the edge, however, the disk becomes more apparent and prominent, and is sometimes visible in the optical wavelength.
On the right extreme of the Hubble progression, there are two parallel branches encompassing the spiral galaxies. A spiral galaxy consists of a flattened disk with stars forming a (typically two-armed) spiral structure, and a central concentration of stars known as the “bulge”. Roughly half of all spiral galaxies are also observed to have a bar-like structure, extending from the central bulge, at the ends of which the spiral arms begin. In the tuning-fork diagram, the regular spirals occupy the upper branch and are denoted by the letter S, while the lower branch contains the barred spirals, which are given the symbol SB. Although these are the most common types of galaxies, there are other kinds of galaxies that have been seen, but are not as significant, in terms of volume, as the other three types. In conclusion, we can state that nearly all types of galaxies in the known universe can be classified within the main categories outlined above.