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Why are galaxies flat?

Scientific discovery and discussion
AsleepInYorkshire
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Why are galaxies flat?

#343604

Postby AsleepInYorkshire » September 28th, 2020, 10:48 pm

Presumably gravity makes planets, stars and moons round.

Why is our galaxy flat in comparison?

AiY

JohnB
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Re: Why are galaxies flat?

#343607

Postby JohnB » September 28th, 2020, 11:29 pm

Conservation of angular momentum. When the gas cloud that formed the Milky Way collapsed, it was spinning. Gravity allowed collapse along the spin axis to form the disk, but the disk itself cannot collapse without shedding angular momentum, and there is no good mechanism for that.

Many galaxies are not disks, but blobs. This can be because their initial clouds spun less, but normally its because they were formed by 2 galaxies colliding, and the collision process allows material to be thrown out taking angular momentum away, leaving a spherical core.

And our planets lie in the plane of the ecliptic analogous to a galactic disk. Some angular momentum was lost, but much is locked up in Jupiter's orbit round the Sun.

Spet0789
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Re: Why are galaxies flat?

#343612

Postby Spet0789 » September 29th, 2020, 12:09 am

JohnB wrote:Conservation of angular momentum. When the gas cloud that formed the Milky Way collapsed, it was spinning. Gravity allowed collapse along the spin axis to form the disk, but the disk itself cannot collapse without shedding angular momentum, and there is no good mechanism for that.

Many galaxies are not disks, but blobs. This can be because their initial clouds spun less, but normally its because they were formed by 2 galaxies colliding, and the collision process allows material to be thrown out taking angular momentum away, leaving a spherical core.

And our planets lie in the plane of the ecliptic analogous to a galactic disk. Some angular momentum was lost, but much is locked up in Jupiter's orbit round the Sun.


To add to this excellent answer, consider that mass with no spin will agglomerate as a sphere. Any spin (angular momentum) will then cause the sphere to flatten.

Planets (when molten and still forming) were so dense that this flattening is minor. The Earth looks pretty spherical but it's well known that it has a greater circumference round the equator than around the poles.

Galaxies are mainly empty space, so the relative pull of gravity acts far more gently and the spin dominates. Hence they generally form discs or spirals.


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