Science stories of the week

on that note....this time spiral galaxy formation.


What causes spiral galaxies to maintain their “structure”? Gravity of its stars? Dark matter?
 
What causes spiral galaxies to maintain their “structure”? Gravity of its stars? Dark matter?
Whoohoo, that'a tough one.

From Wiki:https://en.wikipedia.org/wiki/Spiral_galaxy

The pioneer of studies of the rotation of the Galaxy and the formation of the spiral arms was Bertil Lindblad in 1925. He realized that the idea of stars arranged permanently in a spiral shape was untenable. Since the angular speed of rotation of the galactic disk varies with distance from the centre of the galaxy (via a standard solar system type of gravitational model), a radial arm (like a spoke) would quickly become curved as the galaxy rotates. The arm would, after a few galactic rotations, become increasingly curved and wind around the galaxy ever tighter. This is called the winding problem. Measurements in the late 1960s showed that the orbital velocity of stars in spiral galaxies with respect to their distance from the galactic center is indeed higher than expected from Newtonian dynamics but still cannot explain the stability of the spiral structure.

The article goes on to explain, but it's bloody complicated to understand. I've read it 3 times and can't say I've got it yet.
 
Whoohoo, that'a tough one.

From Wiki:https://en.wikipedia.org/wiki/Spiral_galaxy

The pioneer of studies of the rotation of the Galaxy and the formation of the spiral arms was Bertil Lindblad in 1925. He realized that the idea of stars arranged permanently in a spiral shape was untenable. Since the angular speed of rotation of the galactic disk varies with distance from the centre of the galaxy (via a standard solar system type of gravitational model), a radial arm (like a spoke) would quickly become curved as the galaxy rotates. The arm would, after a few galactic rotations, become increasingly curved and wind around the galaxy ever tighter. This is called the winding problem. Measurements in the late 1960s showed that the orbital velocity of stars in spiral galaxies with respect to their distance from the galactic center is indeed higher than expected from Newtonian dynamics but still cannot explain the stability of the spiral structure.

The article goes on to explain, but it's bloody complicated to understand. I've read it 3 times and can't say I've got it yet.
I ask because it’s curious to me how spiral galaxies remain spiral galaxies for a billion years. Hmm…

I found this on YouTube in researching a little on the subject. It’s just a cool visual of how these types of galaxies form.

 
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The article goes on to explain, but it's bloody complicated to understand. I've read it 3 times and can't say I've got it yet.
I have vague memories of something to do with dynamic density waves in the spiral arm pattern. That is, individual stars actually cycle in and out of the spiral arms over long periods of time. The pattern is always there, but it is not consistently made of the same stars,
 
I have vague memories of something to do with dynamic density waves in the spiral arm pattern. That is, individual stars actually cycle in and out of the spiral arms over long periods of time. The pattern is always there, but it is not consistently made of the same stars,
That's what the Wiki article explains. There is even an animation showing the process. But I still can't quite visualise what is going on.
 
I read somewhere that spiral and vortices are a natural feature of our universe (link to some features below). Galaxies are complicated and come in several shapes. Spherical, spiral, irregular, tubular (images below) As to how they maintain their shape? If that was truly understood there would not be the Kerfuffle regarding flat rotation curves.

Newton does not explain it, Einstein’s GR does not explain it either. Based on the mass we observe (stars, gas) the galaxy should fly apart, there is not enough mass and therefore gravity holding it together.

The outside should be rotating very slowly compared to the inside but it isn’t, the velocity should fall off so the distance velocity relationship should yield a line that falls off also.

It doesn’t, it is more flat, hence flat rotation curve and the need for something there that does not emit or interact with light, Dark matter.


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Elliptical galaxies are the most abundant type of galaxies found in the universe but because of their age and dim qualities, they're frequently outshone by younger, brighter collections of stars.

Elliptical galaxies lack the swirling arms of their more well-known siblings, spiral galaxies. Instead, they bear the rounded shape of an ellipse, a stretched-out circle.


More here: https://www.space.com/22395-elliptical-galaxies.html

I'm under the impression that elliptical galaxies can never ''become'' spiral galaxies. But can spiral galaxies become elliptical ones? It sounds like it can only go one way - spiral to elliptical (but it's not a given). I think this has to do with the fundamental differences of how they're both formed and elliptical galaxies contain less gas and older stars.
 
I read somewhere that spiral and vortices are a natural feature of our universe (link to some features below). Galaxies are complicated and come in several shapes. Spherical, spiral, irregular, tubular (images below) As to how they maintain their shape? If that was truly understood there would not be the Kerfuffle regarding flat rotation curves.

Newton does not explain it, Einstein’s GR does not explain it either. Based on the mass we observe (stars, gas) the galaxy should fly apart, there is not enough mass and therefore gravity holding it together.

The outside should be rotating very slowly compared to the inside but it isn’t, the velocity should fall off so the distance velocity relationship should yield a line that falls off also.

It doesn’t, it is more flat, hence flat rotation curve and the need for something there that does not emit or interact with light, Dark matter.


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I don't think much of the link. For a start, a helix is quite different from a spiral, yet the article treats the two as if they are the same.
 
I don't think much of the link. For a start, a helix is quite different from a spiral, yet the article treats the two as if they are the same.
That is only a small part of my post. I googled "spirals occur naturally in the universe," that is what I got.
There will be better links, the point is they do occur.
The focus was the shape and dynamics that is not yet explained.
Galaxy evolution is pretty complicated and whatever models they had are not explaining these early ones.
 
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