Spiral Galaxy vs Irregular Galaxy (How Are They Different?

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The main differences between spiral and irregular galaxies would include their shapes. The former has a spiral shape and the latter has no discernable shape, spiral galaxies are larger than the Milky Way roughly 5 – 10 times that of your average irregular galaxy and that spiral galaxies make up 60% of all galaxies in the universe with the latter between 20 – 25%.

For a more detailed breakdown of both galaxy types as well as any other differences between the 2, continue reading.

What Is A Spiral Galaxy?

Barred Spiral Galaxy

Spiral galaxies are named for their shape which is characterized by arms of brighter stars spiraling out from a flat spinning disk of duller stars.

The overall appearance is an effect of the fact that these galaxies are spinning at speeds of hundreds of kilometers per second. Our galaxy, the Milky Way, is a spiral galaxy that rotates at 210 km per second.

A dense bulging cluster of older stars forms the center of the spiral galaxy’s disk, with a supermassive black hole believed to lie at the center of this bulge. In the Milky Way, our own Sun is just one of 100 – 400 billion stars spinning around a supermassive black hole called Sagittarius A*, whose mass is as great as four million suns.

A bar-shaped distribution of stars runs across the bulge in around two thirds of spiral galaxies, including the Milky Way. The mass, brightness, and size of a spiral galaxy and its components vary.

Spiral galaxies are the most common type of galaxy observed by astronomers to date, making up more than two thirds of the galaxies observed in a 2010 survey of Hubble Space Telescope data, and significantly outnumbering observed elliptical or irregular galaxies.

NB This means that they are brighter and easier to spot, rather than being more numerous. Elliptical galaxies are believed to outnumber spiral galaxies but are less visible.

What Is An Irregular Galaxy?

Irregular Galaxy

Irregular galaxies as their name would suggest, are galaxies distinguished by their unique shape and structure and makeup roughly 25% of all galaxies in the universe. In essence, these galaxies don’t have a discernable shape like elliptical or spiral galaxies.

Although it isn’t quite clear how irregular galaxies were formed, there are a few hypotheses stating how they were made with the most common being that the collisions between other galaxies are what forms the unusual shape of an irregular galaxy

Furthermore, these galaxies are considered young, as they are measured to be older than spiral galaxies but younger than elliptical galaxies.

What this means is that irregular galaxies might have initially been separate, distinct spiral galaxies that ended up colliding with one another. Once this happened, the spiral shape was lost and the irregular shape emerged.

Being a galaxy also means it should have a supermassive black hole at its center, and that does seem to be case for the few irregular galaxies we observed like NGC 6240 for example.

As for the different types of irregular galaxies, the Hubble classification has established 2 different ones, Irr I and Irr II.

Irr I have some semblance of a shape, but not enough to be able to be placed in either the elliptical or spiral categories. Irr II are lacking any semblance of a shape.

Similarities Between Spiral And Irregular Galaxies

Both spiral and irregular galaxies have a supermassive black hole at the center of their ecosystem, they are both made up of several star clusters, dust clouds, nebulae, and beyond.

Irregular galaxies tend to form when two galaxies collide, therefore taking into account that spiral galaxies are the most abundant in the universe the formation of these undiscernible galaxies are likely to materialize when 2 spiral galaxies collide.

Differences Between Sprial And Irregular Galaxies

As for the differences between these 2 galaxy types, they include the following:

  • Spiral galaxies like our Milky Way, have a discernable shape whereas irregular galaxies don’t have a common feature amongst them.
  • Spiral galaxies make up around 60% of the observable galaxies whereas irregular galaxies make up around 20 – 25%.
  • Irregular galaxies are roughly 20,000 light years across whilst spiral galaxies will range between 16,300 – 163,000 light years across. For reference, the Milky Way is on the larger end spanning 100,000 light years across.
  • Spiral galaxies form when a disc shape galaxy like an elliptical galaxy has rotational disturbance causing the galaxy to spiral outwards whilst Irregular galaxies form when two galaxies collide.
  • As a result of the larger size on average, spiral galaxies will have more stars and interstellar objects in their vicinity.
  • Spiral galaxies have a halo effect around them whilst irregular galaxies don’t.
  • Spiral galaxies are far brighter than your average irregular galaxy


At the end of the day, both are galaxies that draw millions of stars towards their vicinity with a supermassive black hole in the general center of each entity. The only major differences are how they look and how they were formed.

Other than this, both allow a variety of unique celestial objects to reside within their domain and number in the billions themselves too.

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