Why Do Galaxies Come In Their Mix Of Shapes?

If asked to draw a galaxy, you’d probably produce a spiral shape like the one above. When it comes to gaining our attention, spiral galaxies have big advantages – for one thing, most people regard them as extremely beautiful. They’re also brighter, having more hot young stars, so unless we look really closely we see more of them. Moreover, one of them is our home, creating a natural bias. 

Yet not all galaxies are spirals, and even those that are come in two main types. So why do some galaxies achieve this glorious form, while others don’t?

There is still a fair bit of uncertainty about this question, but one set of explanations seems to be winning out.

Spiral Galaxies

Spiral galaxies have several common features. All are fairly flat, with the main body of stars many times wider than it is thick. They have a central bulge packed with stars that rises far above (and below) the disk. Spiral galaxies usually have a halo that, if considered to mark their boundaries, would make them almost spherical. However, stars are so sparse in the halo we barely notice it, even when looking at a spiral side-on.

Despite all the attention paid to them, the reasons for the spiral structure are not fully understood. In part, it represents an illusion. There’s plenty going on between the spiral arms, but because star formation is most intense within the arms, they feature a lot of hot young stars. This makes the arms stand out more prominently than if we were just seeing density of material.

Spiral galaxies usually have cores that are older than their arms (although exceptions exist), suggesting the core formed first, and subsequently attracted the material that became the arms. Large spirals like the Milky Way form by cannibalizing smaller galaxies that get too close.

Some spiral galaxies are categorized by how tightly wound their arms are, but can also be divided by the number of arms, which varies.

Surprisingly, given their importance to the universe and ourselves, we don’t fully understand what causes spiral galaxy formation. Indeed, we would expect the arms to wind up tightly around the core on timescales a lot shorter than the lifetime of the spirals we know the best. Dark matter explains some of what we see, but not everything.

The disk aspect of spiral galaxies is easiest to explain, since we see something similar in the protoplanetary disks around young stars. It’s a product of the gravitational interactions between rotating particles.

The spiral arms are harder. The most popular explanation is called “density wave theory”. This suggests stars and dust are not permanently part of arms. Instead, density waves move through the galaxy, pulling material together as a crest passes and dispersing it afterward. Just as a particle of water won’t always be at the crest of a wave, stars move in and out of the arms as the wave pulls them in and pushes them out.

The full cause of these density waves is not known, but galaxies getting pulled out of a circular shape by others’ gravity could be one trigger. Magnetic fields are also suspected.

To Bar Or Not To Bar

Perhaps the main division among spiral galaxies is between those (including our own) with a distinctive central bar and those without.

Bars become more common the closer one gets to us in space, and therefore in time, suggesting they are becoming more frequent. Like the spiral shapes themselves, they are thought to be the product of density waves, in this case radiating out from the galactic center rather than circling around it.

Elliptical Galaxies

Elliptical galaxies represent the other common distinctive shape, and include the largest galaxies. People in some parts of the world may recognize the shape as resembling that of a rugby ball. Americans may prefer to think of a modestly deflated football whose ends have been pushed in somewhat.

Edwin Hubble, who first created the galaxy categories we use (with some adjustment) today, believed elliptical galaxies represented a sort of cosmic egg from which spiral galaxies formed, but this has since been rejected. Instead, elliptical galaxies are generally made up of much older stars than spiral galaxies, having largely stopped forming stars. There is evidence the jets produced by the supermassive black holes at the heart of ellipticals disrupt the gas that in other galaxies might form stars. 

Even in spiral galaxies, some stars move in directions inconsistent with their neighbors. Nevertheless, the vast bulk of spiral galaxy stars orbit in an ordered manner. In elliptical galaxies, stars are thought to wander far more randomly, undermining the cohesive shape.

It is thought that elliptical galaxies are the product of galaxy collisions. This explains their age, and lack of clear structure. Nevertheless, we know the Milky Way is made of many galaxies, yet it has retained its shape. Ellipticals are most common near the center of galaxy clusters, the places where encounters between galaxies are likely to be most frequent and violent. For all the Milky Way has been through, it’s likely ellipticals have faced more drastic encounters with others.

Irregular Messes

Many galaxies do not fall into any of these categories, and have no consistent pattern to their shapes. These are the easiest to explain, since an encounter with a major disruptive force – such as the gravity of a larger galaxy – can mess up a regular shape and leave a mess behind. Eventually, the galaxy may reform, but that can take so long that for much of its life, a galaxy will be irregular, particularly a smaller one vulnerable to bigger cosmic bullies.

Shapes of the Early Universe

Most of what we know about galaxy shapes comes from nearby galaxies as they’re easier to study. That means we are not looking far back in time, so these galaxies are fairly similar age to our own. However, the JWST has allowed us to see some galaxies from when the universe was much younger, and the shapes look significantly different.

A recent report revealed shapes astronomers compared to sporting equipment, from nearly spherical balls to those shaped like frisbees, surfboards, and pool noodles.

What happened to these galaxies as they aged, we don’t know. Nevertheless, given objects with these shapes are rare in the local universe, it seems a fair bet that they gradually transform into one of the shapes we see closer to hand.

All “explainer” articles are confirmed by fact checkers to be correct at time of publishing. Text, images, and links may be edited, removed, or added to at a later date to keep information current.  

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