APOD: Messier 99 (2021 Jun 24)

Comments and questions about the APOD on the main view screen.
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VictorBorun
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Re: APOD: Messier 99 (2021 Jun 24)

Post by VictorBorun » Sun Jun 27, 2021 3:17 pm

A regular galaxy is one that is not deformed by an impact with another galaxy or has had time to relax.
A regular galaxy is one of two sorts: with viscous dense interstellar gas or without.

If you have viscousity, you minimize your inner friction; therefore your gas form a disk with circular orbits and all in one plane.
You form stars from that gas and they inherit circular orbits and all in one plane.
Any stars in odd orbits have to cross the disk every 0.2 billion years and the stellar population in the disk influence somewhat the odd stars, but not much. After 10 billion years pretty much your odd stars and stellar globular clusters are still there taking their chances every 0.2 billion years to cross your disk.

If you have no viscousity, you do not form a disk at all. Your stars orbit in every plane there is. The distant orbits may be Kepplerian ellipses and the close orbits inside the dense core are more like pendulum ellipses, centered in the center of the core.

Our galaxy cluster called the Local Group has just 3 non-dwarf galaxies: Milky Way, Andromeda and Triangulum. All three are disks.

Our immediate supercluster called the Local Sheet has just 10 more non-dwarf galaxies called the Councel of Giants. They occupy a ring and the LG is in a diameter. Two of the Counsel Giants are elliptical galaxies and they are heavier by far and each dominates in its own galaxy cluster. The two elliptical galaxies are at the ends of the diameter that contains the LG.

Maybe long ago the two between them has formed a mini-filament and then made it thinner and denser with their tidal forces, and gave a start for the forming of the LG and gave the LG a spin in line with that filament.

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Chris Peterson
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Re: APOD: Messier 99 (2021 Jun 24)

Post by Chris Peterson » Sun Jun 27, 2021 3:29 pm

Deathfleer wrote: Sun Jun 27, 2021 4:24 am Thanks..
.Formerly, I thought that there were elliptical galaxies.. Now, no more. Every galaxy is spiral and circular
"Elliptical" in this context can be confusing and ambiguous.

Every spiral galaxy is circular (assuming it hasn't been distorted by an interaction with another galaxy), and those that appear "elliptical" do so because of their tip angle with respect to us.

Not every galaxy is a spiral, however. There is an entire class of galaxies which are called "elliptical", and these take on various distorted globular shapes (ranging from spherical to substantially flattened spheroids). There are also irregular galaxies which lack any substantial symmetries along any of their axes.
Chris

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