Why Do Galaxies Have a Spiral Shape? Understanding Gravity's Role

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In summary, the shape of galaxies is determined by a combination of gravity and angular momentum. While a spherical shape is theoretically possible, most galaxies end up in a disk shape with features such as spokes and bars due to instabilities. However, older galaxies that have undergone mergers can become more spheroidal, known as elliptical galaxies. This is because of centrifugal forces acting on a rotating spherical mass.
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Sniperman724
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If gravity acts on all angles of a mass, then why do galaxies appear to all have the same basic spiral shape with two relatively flat areas on opposite sides of it? Shouldn't all galaxies technically be big sphere looking shapes?
 
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Spheres are tough because they would require orbits of random, different inclinations, which is unlikely for a galaxy that started as a single, rotating cloud of gas...

...just like our solar system.
 
  • #3
Sniperman724 said:
If gravity acts on all angles of a mass, then why do galaxies appear to all have the same basic spiral shape with two relatively flat areas on opposite sides of it? Shouldn't all galaxies technically be big sphere looking shapes?
Like russ said, the main problem is angular momentum. With a bunch of angular momentum, the galaxy can't turn into a spheroid; instead it will end up more like a disk, and then instabilities lead to spokes, bars, etc.

But its worth pointing out that large, old galaxies that have undergone mergers due become much more spheroidal---these are called 'elliptical galaxies' (because of their shape).
 
  • #4
A spherical mass that is rotating will naturally tend towards a disc shape due to centrifugal forces - especially with a dark matter halo.
 
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The spiral shape of galaxies is a result of the interplay between gravity and rotational motion. While gravity does act on all angles of a mass, it is not the only force at play in the universe. In the case of galaxies, the rotation of the stars and gas within them creates a centrifugal force that balances out the pull of gravity. This results in a disc-like shape, with a bulging center and flattened edges.

Additionally, the distribution of matter within a galaxy also plays a role in its shape. The majority of the matter in a galaxy is concentrated in the center, with less dense regions towards the edges. This concentration of matter further reinforces the disc-like shape of a galaxy.

It is also important to note that not all galaxies have a perfect spiral shape. Some may have irregular or distorted shapes due to interactions with other galaxies or the presence of dark matter. The overall shape of a galaxy is determined by a complex interplay of various forces and factors, not just gravity alone.

In conclusion, while gravity does play a significant role in shaping galaxies, it is not the sole factor responsible for their spiral shape. The rotational motion, distribution of matter, and other external influences all contribute to the diverse shapes we observe in the universe.
 

1. What is a galaxy?

A galaxy is a large system of stars, gas, and dust held together by gravity. It typically contains millions to billions of stars and can range in size from a few thousand light-years to hundreds of thousands of light-years.

2. How many galaxies are there in the universe?

It is estimated that there are around 2 trillion galaxies in the observable universe. However, this number may continue to increase as technology advances and allows us to see further into the universe.

3. Can we see galaxies with the naked eye?

Yes, there are a few galaxies that can be seen with the naked eye, such as the Andromeda Galaxy and the Large and Small Magellanic Clouds. However, most galaxies are too far away and too faint to be seen without the aid of a telescope.

4. How are galaxies formed?

Galaxies are formed through a process of gravitational collapse. As gas and dust clump together, they form pockets of higher density which then collapse under their own gravity. This process continues until a galaxy is formed.

5. Are there different types of galaxies?

Yes, there are three main types of galaxies: spiral, elliptical, and irregular. Spiral galaxies have a distinct spiral shape and contain a central bulge surrounded by arms of stars. Elliptical galaxies are more spherical in shape and have a smooth, featureless appearance. Irregular galaxies do not have a defined shape and often contain young, hot stars.

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