Types of Spiral Galaxies

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  • 0:01 Classifying Spiral Galaxies
  • 0:53 Spiral Type A, B & C
  • 4:34 Barred Spiral Galaxies
  • 5:45 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

Spiral galaxies are classified into several different types, depending on the characteristics of certain parts of their galaxy. Specifically, this lesson will go over Hubble's classification scheme of spiral galaxies with a small nod to lenticular galaxies as well.

Classifying Spiral Galaxies

Classification systems exist for just about everything you can think of. Moreover, things can be classified in more than one way. Take cars for example: they can be classified by year, make, model, and even custom features. Such classifications obviously help people instantly picture what a car looks like in their mind. When I say something like, 'Porsche,' you think of a small car with two doors that lies close to the ground. When I say, 'Hummer,' you can imagine a very big car, four doors, and one that may need a step for some people to get into.

This is why astronomers classify spiral galaxies into different types: to help visualize what they look like through an easy name. This lesson will tell you how they're classified so that you'll know what they look like if someone mentions their name and we'll also reveal why they look the way they do.

Spiral Type A, B, & C Galaxies

Sa galaxies, where 'Sa' is short for 'Spiral Type A,' are spiral galaxies that have a big central bulge and smooth, broad spiral arms. Take a look at the image on your screen to see what I mean. Note how the bulge is very prominent and how the spiral arms are very tightly wound.

An Sa galaxy
An Sa galaxy

Spiral type B (Sb) galaxies are spiral galaxies that have a moderately-sized central bulge and moderately-well-defined spiral arms. The galaxy shown on screen right now is a Sb galaxy where the arms aren't as tightly wound as the arms of Sa galaxies.

An Sb galaxy
An Sb galaxy

Spiral type C (Sc) galaxies are spiral galaxies that have a very small central bulge as well as narrow and well-defined central arms. NGC 4321, which you see now, is a type of Sc galaxy. Note how the central bulge is very small and the arms are very loosely wound in comparison to the Sa galaxy.

An Sc galaxy
An Sc galaxy

Like cars have a reason for their shape and size, so do these galaxies. I mean, Hummers are made to lug a lot of stuff around (at least the original ones were) and Porsches have been built for speed. The way they look is representative of how they function or what's going on inside of them.

Firstly, the formation of stars occurs mainly in the disc of the galaxy. Thus, in places like the spiral arms, you'll find a lot of star formation and a lot of massive hot and young blue stars. However, the central bulge of the galaxy contains older stars and has little star formation. Massive blue stars die off quickly. Thus, the stars that can live the longest are of low mass and are red in color. This helps to explain why the central bulge of a galaxy is yellower or redder in color than the disc.

Now, the difference in the shapes of these three types of galaxies can be partially explained by one interesting observation. Astronomers know that interstellar dust and gas are like the two ingredients from which stars form. The more interstellar dust and gas there is in a galaxy, the more active the star formation and the larger a galaxy's disc, and therefore the smaller the central bulge.

Observation tells us that 25% of the mass of a Sc galaxy is made up of interstellar dust and gas compared to only 4% for a Sa galaxy, which has a big central bulge and a little disc. Because there is so much dust and gas in a Sc galaxy, there is a lot of star formation going on. Therefore, its disc, where star formation occurs, is large, and its central bulge, where little star formation occurs, is small compared to the Sa galaxy.

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