Why are spiral galaxies flat?

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Edward Herrick-Gleason

Got a burning question about the cosmos that you just can’t find an answer to? Then this page is for you!

I am an armchair enthusiast in astronomy, and I love the enthusiastic explanations in the magazine and the superb high-quality photos. I hope you do not mind my taking this opportunity to ask why galaxies are flat? Surely the stars should fly out in all directions and galaxies should be ball-shaped, asks Dr Peter Marsden.

Spiral galaxies really are quite flat. For instance, our Milky Way Galaxy spans approximately 100,000 light years but the region outside its central bulge is only about 1,000 light years thick (the central bulge is about 10,000 light years thick). To put that into perspective: a 16" (40.7 cm) pizza with the same ratio would be a mere 4 mm thick.

This appearance is a natural outcome of how galaxies form and evolve under the laws of physics.

Galaxies begin life as vast clouds of gas in the early Universe. These clouds are not perfectly still and the tiny differences in the motions of their atoms and molecules give each cloud a small overall spin. Although the initial rotation is weak, it plays a decisive role as gravity begins to pull the material together.

Anything that spins has a property called ‘angular momentum’, which is the tendency for the object to keep rotating. Importantly, it is a conserved property that is calculated by multiplying the rotational speed of an object with its distance from the centre of rotation.

As the cloud collapses, the individual gas particles collide. This causes the gas to lose energy, mainly in the form of heat that is radiated away into space, and the collapse hastens.

While shrinking, angular momentum is conserved by the cloud rotating faster, just as a spinning ice skater speeds up when pulling in their arms.

The collisions also cause the initially turbulent gas fall in with the cloud’s overall rotation. Think of a whirlpool: throw a ball into it against the flow and repeated collisions with the water will quickly force the ball to reverse direction and move along with the spinning water.

As the rotation of the galaxy speeds up, this generates an outward force along the galaxy’s equator. This is the same force that appears to pull you outwards when driving quickly around a bend. This outward force opposes the pull of gravity along the rotating equator, slowing the collapse of the cloud in that direction.

Along the axis of rotation, no force is generated. So there is nothing to oppose gravity and over time, this allowed the gas settle into a flattened, rotating disc.

Stars then form in this disc from the cooling gas, inheriting its orderly, circular motion. Once formed, stars rarely collide, so the flattened shape of the disc is largely preserved for billions of years. This is why spiral galaxies appear as elegant pinwheels when viewed face-on, but as narrow slivers when seen edge-on.

Not all galaxies are equally flat, however. Elliptical galaxies have experienced violent mergers in their past that scramble the orderly motions of stars, puffing the galaxy up into a more spherical shape and erasing any thin disc that once existed.

Edward Herrick-Gleason served as Director of the Southworth Planetarium in Portland, Maine, USA for a quarter of a century. He now lives in St. Johns, Newfoundland.