Spiral Galaxy: Types and Features

  • Soumyaa Rawat
  • Sep 15, 2021
  • Science & Technology
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What is a Galaxy?


The universe is a vast space for humans to discover. Astronomy, defined as the study of the universe beyond Earth’s atmosphere, has helped humans discover and explore what exists beyond the atmosphere of our planet. 


Unlike Earth, all other planets are lifeless which implies that Earth is the sole planet with a supportive environment for the existence of humans. Anything beyond this planet is still getting explored. 


Even though the Milky Way, the galaxy we live in, has been explored, there remain about a million galaxies that are out there in the universe. Don't  you want to know how big the universe is, do you? Follow the link.


A galaxy is defined as a collection of a million-trillion dust and gas particles and dark matter that are bound together by the force of gravity. Scientists suggest that most of the galaxies in the universe have enormous black holes in the center.  


With millions of stars and numerous planets revolving around each other with the help of space orbits, galaxies in the universe have for long occupied the interest of millions of astronomers and space scientists since time immemorial. 


For instance, the Milky Way is a galaxy that we live in along with 7 other planets and millions of stars. 


As scientists suggest, there are around a billion more galaxies other than the Milky Way that humans live in. Broadly, there are 3 types of galaxies that are known to astronomists so far. 


The 3 types of galaxies are - Elliptical, Spiral, and Irregular. Elliptical galaxies are oval or spherical in shapes and have randomly moving stars with no standard rotation. While an elliptical galaxy is spherical in shape, a spiral galaxy consists of flat and rotating discs that have prominent spiral arms. 


The third of all types, irregular galaxies have no regular shape and have oddly spread stars and dark matter that are bound together by the force of gravity. An irregular galaxy has an amorphous appearance. In this blog, we will be looking at the concept of Spiral Galaxy in depth. Let us get started. 


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Spiral Galaxy


All galaxies consist of stars, dust, gas particles, and dark matter. Spiral galaxies do so too. A spiral setting of space particles and dark matter, held together by the force of gravity is what defines a spiral galaxy. 


Spiral galaxies consist of flat and rotating discs that have prominent spiral arms. Herein, stars appear in a pinwheel pattern, revolving the spiral nucleus from all edges. 


Originally mentioned by Edwin Hubble in 1936 as a part of the Hubble Sequence. Similar to the movement of a spiral beyblade that has protruding spiral arms, a spiral galaxy too has arms forming a spiral nebula that holds space matter together. 


With a flat, circular blue-white disc in the center, a spiral galaxy has a concentration of stars in the center. This central concentration of stars is known as the bulge. 


From here on, the concentration of stars becomes much fainter and lighter, as opposed to the center where the concentration is dense and much clearer to observe. 


The flow of stars takes place from inwards to outwards, forming a spiral shape with the center of the galaxy having the highest concentration of dark matter and outer space particles. 


“Spirals are characterized by circular symmetry, a bright nucleus surrounded by a thin outer disk, and a superimposed spiral structure.”Characteristics of Spiral Galaxies


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Features of Spiral Galaxies


As we have already learned about spiral galaxies, we will now enlist a few characteristics of the concept to understand the concept in a better way.


  1. They have a flat, circular disc surrounded by spiral arms with a lighter concentration of stars and dark matter. A spiral nebula surrounds the flat disc in a spiral galaxy. 


  1. The movement of dark matter in spiral galaxies is dominated by a structured rotation path - orbital movement, unlike other galaxies that do not have such structured movement rotations.


  1. The disc of stars and space matter revolving around the bulge form separate spiral arms that form the spiral galaxy.


  1. A spiral galaxy consists of millions of globular clusters (spherical arrangement of stars) apart from a disc of stars that revolve in the center of the entire galaxy.


  1. The most common types of galaxies found in the universe are spiral galaxies that have a thick concentration of stars in between and a thin concentration of stars in the region of spiral arms. 


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  1. Based on the bond of stars and dark matter in a spiral nebula, spiral galaxies can be classified as follows -


  • Sa Spiral Galaxies- The first class of spiral galaxies is the SA spiral galaxy. With a large spiral nebula, tightly wound spiral arms around the nucleus, SA spiral galaxies are defined as having the closest set-up among all classes of the category. 


In addition, the SA galaxies represent a smooth center with a dense concentration of stars. 


“NGC 1302 is an example of the normal type of Sa galaxy, while NGC 4866 is representative of one with a small nucleus and arms consisting of thin dust lanes on a smooth disk.”

Sa Spiral Galaxies 


  • Sb Spiral Galaxies- The next class of spiral galaxies is the SB Spiral Galaxy. This class is defined by its structure of a medium-sized nucleus with more widely spread arms around the nebula.


The appearance of this spiral galaxy is less smooth and also has a reduced concentration of stars, as compared to the SA spiral galaxy.


  • Sc Spiral Galaxies- The third class of all, the SC Spiral Galaxy is represented by its much smaller nucleus size and far more widely spread arms around the nucleus. 


These galaxies have lumpy arms that have an increased number of stars and dark matter as compared to that of the SA and SB spiral galaxies. 


“In terms of the nucleus, Sa has the largest while Sc has the smallest.  The galaxies that appear to have a spiral disc but no visible arms are called S0. “Classification of Spiral Galaxies 



Types of Spiral Galaxy 


Broadly, there are two types of spiral galaxies that have been observed by space scientists to date. Here is a brief introduction of the two types of spiral galaxies -


  1. Normal Spiral Galaxy


Normal Spiral Galaxy is a spiral galaxy that has its spiral arms protruding from the nucleus all over. With a flat and circular disc in the center, normal spiral galaxies have a spiral nebula that surrounds the nucleus from all ends. 


There are no specific points of origin of spiral arms when it comes to a normal spiral galaxy. While a normal spiral galaxy can also be referred to as simply a spiral galaxy, it still is considered to be a variant of this class of galaxy.  



  1. Barred Spiral Galaxy


Unlike normal spiral galaxies that have their spiral arms emanating from all over the nucleus, a barred spiral galaxy is slightly different. With spiral arms emanating at the ends and coiling the nucleus from there on, a barred spiral galaxy has a different structure than that of a normal spiral galaxy. 

This image is a visual representation of a barred spiral galaxy.

Barred spiral galaxy

As opposed to a normal spiral galaxy, the spiral arms of a barred spiral galaxy emanate from the 2 opposite ends of the nucleus. 


The Milky Way, our galaxy, is a typical example of a barred spiral galaxy that is one of the most widely known galaxies in the world. It was discovered in 1610 by Galileo Galilei.   


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Case Study - The Milky Way


A typical example of spiral galaxies is the Milky Way galaxy. Discovered by Galileo Galilei in 1610, the Milky Way galaxy is the world we live in. It is one of the most renowned spiral galaxy examples.  


Consisting of our solar system - the Sun and 8 planets- Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune, the Milky Way galaxy is named after its appearance. 


This is because the center of this galaxy appears to be milky (white) in texture and has a dense concentration of stars and spiral nebulae. 


A spiral galaxy, the Milky Way is a typical example of a barred spiral galaxy (a spiral galaxy with spiral arms emanating on the ends of the nucleus). Earlier, astronomers used to believe that the Milky Way comprised the whole universe. 


However, with advanced studies at leading organizations like NASA revolving around the galaxy, it was found that the Milky Way was only one such galaxy, apart from millions of such galaxies of different types - elliptical, spiral, and irregular. 


“Our Milky Way Galaxy is just one of billions of galaxies in the universe. Within it, there are at least 100 billion stars, and on average, each star has at least one planet orbiting it. This means there are potentially thousands of planetary systems like our solar system within the galaxy!” The Milky Way  





To sum up, a spiral galaxy is a class of galaxies as mentioned by Edwin Hubble. Consisting of a flat disc that is circular in shape, a spiral galaxy can be defined as a spiral nebula that has spiral arms emanating from the edges of the nucleus. 


With a densely concentrated bulge in the center, the concentration of the stars in the arms becomes fainter and thinner as we move to the end. A typical example of the spiral galaxy is the Milky Way galaxy that we live in. 


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As astronomers claim and as has been found, there are millions of galaxies like the Milky Way and a major chunk of the universe remains to be unexplored to date.