What is Spiral Galaxy?

The galaxy is a vast system of stars, gases, and dust that makes up the universe. It contains somewhere between 100 and 400 billion stars, depending on the type and size of the galaxy. The gas and dust in the interstellar medium are the raw materials from which stars form. The spiral galaxy is the most commonly observed galaxy shape in the universe. They can contain multiple spiral arms and are known for their clusters of new star formations. Spiral galaxies are just one of many different types of galaxies. 

Some spiral galaxy examples are Pinwheel Galaxy, Andromeda Galaxy, Sunflower Galaxy, Triangulum Galaxy, and Whirlpool Galaxy. 


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Spiral galaxies are a  part of the Hubble sequence, and they consist of a flat disk, rotating with stars, gas, and dust. Most spiral galaxies have a bulge in their center, which is composed of stars. A halo of stars sometimes surrounds these. Spirals are branched out arms with clusters of young, hot stars in them. It is these stars that make the spiral brighter than the surrounding galactic disk. Roughly two-thirds of all spirals have a bar structure that begins at the end of the central bulge, with spiral arms branching off of it. 


Characteristics of Spiral Galaxy

  • They have a circular, flat disc surrounded by spiral arms with dark matter concentrations and fewer stars. A spiral nebula encircles the flat disc in a spiral galaxy. 

  • In the spiral galaxies, the dark matter movement is guided by a structured rotation path- orbital movement, unlike other galaxies that do not possess such rotations.

  • The disc of stars and space orbiting around the bulge creates separate spiral arms in the galaxy.

  • A spiral galaxy consists of millions of globular clusters apart from a disc of stars that revolve around the center of the entire galaxy.

  • The most common type of galaxy in the universe, and often the largest in terms of volume, is the spiral galaxy. The stars in a spiral galaxy are densest around the center and thin out as they reach the outer edge of the spiral arms.

  • One of the largest spiral galaxies known to man is NGC 6872. Spanning 522,000 light-years across, this galaxy is five times the size of the Milky Way.


Types of Spiral Galaxy 

Broadly, there are two types of spiral galaxies that space scientists have observed to date. 

  1. Normal Spiral Galaxy- Normal Spiral Galaxy is a type of galaxy with arms protruding from the nucleus in all directions. In the center of the galactic disc, a spiral nebula surrounds the nucleus from all sides. The arms themselves have no specific points of origin when it comes to a normal spiral galaxy. This is why it's also known as just a spiral galaxy.

  2. Barred Spiral Galaxy- A barred spiral galaxy is a spiral galaxy with a central bar-shaped structure made up of celestial bodies. Most of the spiral galaxies are barred, where the solar system is located; the spiral galaxy milky way galaxy is named a barred spiral galaxy. 


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Spiral Galaxies Classification

Spiral galaxies can be classified based on a number of factors, including how tightly they are spiralled, how lumpy their arms are, and the size of their central bulge. These galaxies can contain different amounts of gas and dust, which help portray these differences. There are currently three different kinds of classical spiral galaxies and three different kinds of barred spiral galaxies. They are as follows.

  • SA Spiral Galaxies-  They have a big, central bulge and smooth, broad spiral arms. About 2% of their mass comes from gas and dust. This means that a relatively small number of Sa galaxies are forming stars.

  • SB Spiral Galaxies- SB galaxies have a medium-sized nucleus with spiralling arms around the nebula. This galaxy looks less smooth and has fewer stars than the SA spiral galaxy.

  • Sc Spiral Galaxies- They have a little bulge in the centre as well as narrow, well-defined arms. These galaxies contain about 15% gas and dust.

  • Barred Spiral Galaxy Type A(Sba)- They feature tightly bound arms.

  • Barred Spiral Galaxy Type B(Sbb)- They feature both tightly and loosely bound arms.

  • Barred Spiral Galaxy Type C(Sbc)- They have loosely bound arms.


Elliptical Galaxies

  • Elliptical galaxies are typically oval in shape and do not have any formation, such as spiral arms. They lack the internal structure common among spiral galaxies but still have a denser nucleus that accounts for about 20% of the galaxies in the universe. 

  • An elliptical galaxy may contain 10⁵ to 10¹³ solar masses and has the ability to create luminosity between 3×10⁵ and 10¹¹ solar luminosities. It contains a mixture of Population I and II stars, with the exception of globular clusters.

  • The diameters of elliptical galaxies vary from 1 kiloparsec to 200 kiloparsecs, with most galaxies hovering around 100 kiloparsecs. 


Elliptical Galaxy vs Spiral Galaxy

Spiral Galaxy

Elliptical Galaxy

Spiral galaxies have a flat and rounded shape. They consist of a bulging center and arms, which are made up of the galaxy's disk.

Elliptical galaxies are large and beautiful and have no visible internal structure.

Spiral galaxies have a dense nucleus and bulging regions of stars called the central bulge.

Elliptical galaxies also have dense centers, but they are not as protrusive as spiral galaxies.

The majority of galaxies are spiral galaxies, because they're the most common type, and they account for three-quarters of the total population.

Elliptical galaxies are one of the rarest types of galaxies. They contain only one-fifth of the galaxy population.

Spiral galaxies contain star-forming regions in their spiral arms, which makes them rich in Population I stars. Population I and II stars are found in the halo and the central bulge of a spiral galaxy.

Elliptical galaxies are made up of a mix of Population I and Population II stars.

FAQs on Spiral Galaxy

1. What are the causes of Spiral Galaxy rotation?

Spiral arms in galaxies can form by a combination of processes. Differential rotation is one of them. Basically, the speed at which everything orbits the center of the galaxy is different. For example, an inner star might orbit faster than an outer star. As a result, the inner star completes an orbit in less time than the outer star. Differential rotation is the process that naturally generates spirals in galaxies like the Milky Way. Typically, the galaxy rotates about 20 times in a single revolution. If this was the only process involved in generating spirals, we would see many tightly wound spirals, like a coil. But most spiral galaxies have only two to four major arms. This is evidence of a persistent pattern, not a specific star causing the arms.

2. Which is the largest and the closest Spiral Galaxy?

The Andromeda galaxy is a large spiral galaxy that is the nearest to our own. It is the brightest of all the other galaxies that can be seen from Earth. While there are other celestial objects closer to our Milky Way, none are as large or as bright as Andromeda. 


While Andromeda is the nearest spiral galaxy to our own Milky way, it is not the largest galaxy in our known universe. According to NASA’s GALEX, NGC 6872 is the largest known spiral to date. It is at a distance of 212 million light-years from the Milky Way, in the Pavo constellation.

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