The Distribution of Galaxy Properties Along the Hubble Tuning Fork

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  • 0:02 The Hubble Sequence
  • 1:00 Elliptical Galaxies
  • 3:26 Spirals and Barred Spirals
  • 4:51 Simplifying the Fork
  • 5:55 Irregular Galaxies
  • 6:30 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

This lesson will explore Edwin Hubble's famous classification scheme. We'll go over the major morphological differences between the categories of galaxies and the differences within them.

The Hubble Sequence

Galaxies, like just about anything around us, come about in different shapes and sizes. Take, for instance, organisms on Earth, like parasitic eggs. Some are very round, others are ovoid, and others still have very asymmetrical shapes. Similarly, some galaxies have circular shapes, while others are devoid of any really recognizable form.

Not only that, but there are also some slight deviations within each type of shape. You know, like some circles are kind of perfect looking, while others, like the ones I draw, look like I was drawing them with a blindfold on or something.

To help classify these types of morphological differences between galaxies, the famous American astronomer Edwin Hubble came up with the Hubble tuning fork diagram, more technically known as the Hubble classification scheme, a diagram which groups galaxies into four major categories along a specific sequence.

We'll be pulling it apart in this lesson.

Elliptical Galaxies

Okay then. Take a look at the diagram on your screen:

Hubble Classification Scheme
hubble classification diagram

Since English speakers read from left to right, we'll start all the way at the left and work our way to the right in this lesson.

The very first thing you'll notice is a straight line where a bunch of galaxies, called elliptical galaxies, are located. 'E'lliptical galaxies are, well, 'e'lliptical or 'e'gg shaped! But there are a couple of catches here; it's not as straightforward as it seems.

First of all, elliptical galaxies do vary a bit in their shape; some are rounder than others. Secondly, and perhaps more importantly, is another point I'd like for you to remember. The classifications of the shapes of the elliptical galaxies are not necessarily actually representative of their true shape! I'll explain what I mean right after we go over the classifications.

Note how the elliptical galaxies are classified using an 'E' for elliptical and then a number ranging from zero to seven. The roundest, most spherical, of the elliptical galaxies are called E0 galaxies, and the flattest, the most elliptical of them all, are the E7 galaxies.

Now that you know this, let's go back to my second point I just mentioned. The way these elliptical galaxies are classified may not reflect what their true shape actually is. We classify them based on what they look like to us here on Earth.

For example, an E0 galaxy might actually be a flat disk of stars that just happens to be seen face-on here on Earth. It's like taking a thin, flat, wafer and turning its broad circular side to you. Similarly, an E7 might actually look a spherical galaxy if we saw it end-on. That would be like taking a cigar and turning it so our point of view only saw the circular end. It's all about perspective.

Properties of elliptical galaxies that are less about perspective and more about their true nature include:

  • A wide range of sizes, ranging from small dwarf elliptical galaxies to giant elliptical galaxies
  • Little to no visible gas and dust for new star formation
  • A makeup that consists of old stars
  • No disk and no spiral arms

Spirals and Barred Spirals

Galaxies that have a disk and central bulge but very little to no gas and dust and no spiral arms are known as S0, or lenticular galaxies, and they are a middle ground kind of galaxy between elliptical galaxies and spiral galaxies.

Spiral galaxies contain a disk, a central bulge, spiral arms, and a lot more interstellar gas and dust for new star formation. The disk in a spiral galaxy is where active star formation actually occurs. Nevertheless, spiral galaxies also contain middle-aged and old stars as well.

Furthermore, spiral galaxies are actually split up into two different paths in our fork. They can either be normal spiral galaxies (S) or barred-spiral galaxies (SB), the latter of which have a more elongated central area (the nucleus) than the former.

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