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thomasxin t1_j813hgr wrote

Reply to comment by r2k-in-the-vortex in Why is the Oort cloud spherical? by Outliver

On the topic of spherical vs disk... do elliptical galaxies eventually collapse into spiral disks? It's a question that's always had confusion about it since spiral galaxies form by themselves but ellipticals form from collisions; if undisturbed, would spherical orbits always eventually collapse into disks (albeit perhaps taking vast amounts of time), or is it actually physically possible and statistically likely to have perfectly stable spheres that never decay?

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Player_KK t1_j81edoc wrote

Originally, it was believed that elliptical galaxies would turn into spiral galaxies, but we have since found this to not be true. Elliptical galaxies..... in short, are not active. They are characterized by a distinct lack of star forming gases, and little to no new star formation. Existing stars seldom interact with one another, so much of the shape comes from those star forming gases, via angular momentum.

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At least this is how I understand it.

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thomasxin t1_j81gnu6 wrote

Gravity does interact over long distances over long periods of time though, is there a mechanism that acts against that compared to smaller accretion disks? What prevents an elliptical galaxy collapsing into a spiral, and how do we know that won't happen even if given a trillion years?

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Cheshire-Kate t1_j81i8j2 wrote

If the only thing acting on stars is gravity, then they will stay in their elliptical orbits undisturbed and the overall shape of the galaxy won't change.

It's only once you start adding drag and friction into the mix from gas clouds and nebulas that the forces necessary to collapse the shape into a disc are present. With those gases, a galaxy will eventually collapse into a disc, but without them, there's no reason for them to do so.

Most elliptical galaxies are very old, and no longer have any new star formation. This means far less drag and friction, meaning no forces to flatten the orbits of stars within the galaxy, meaning it will remain an elliptical galaxy forever

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willun t1_j81up2b wrote

Would the dark matter provide drag? I wonder what the movements of dark matter would be in galaxies like that. I assume for the Milky Way it just rotates in line with the movement of stars (or more accurately, the stars move in line with the dark matter)

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gladfelter t1_j82x2uy wrote

The electrical field of an atom is many orders of magnitude larger than the radius of a theoretical WIMP. So interactions and friction would be orders of magnitude less for dark matter.

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willun t1_j82xgoa wrote

Not drag from friction, but gravity is what i meant. The same effect that causes the moon to face earth.

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SkoomaDentist t1_j864qop wrote

> Would the dark matter provide drag?

The way it was explained to me, dark matter only interacts via gravity, including on itself.

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UnarmedSnail t1_j82y5ly wrote

I suspect elliptical galaxies are the merger of galaxies where the collision canceled out both galaxies rotational energy.

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Client_Hello t1_j836l12 wrote

You having it backwards, disks expand to ellipticals as they use up their gas.

Forming a disk requires collisions, and stars are too far apart. The gas in young galaxies does collide, which bleeds off angular momentum, allowing the gas to form a disc, then stars form in the gas.

Those blue stars you see in spiral galaxy arms will not survive a single trip around the core. They light up the region of compressed gas from density waves.

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thomasxin t1_j839eyv wrote

There are smaller blue stars that can still last a billion years or so, right? That would be at least 4 orbits around the milky way for instance. Though larger ones would obviously not last as long

This is an interesting take though, I've not seen sources explaining spiral galaxies evolving into elliptical ones even without disturbance. What would be the cause of the opposite for galaxies, when normal cloud/sphere orbits collapse into rings?

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