becuase york peppermint patties and reeses has the round candy bar market covered? :ducks and runs:
Thanks guys.
I’ve known about rotating since I was a wee space crazy lad in the Apollo era. The thought just struck me that we take the rotation as a given, but I’ve never read the reason why.
The shape of an elliptical may be more dependent on the angle we see it at than anything else. Elliptical galaxies are the result of two roughly equally-sized spiral galaxies colliding. The resulting galaxy will have half the stars rotating at one angle and the half rotating at another. The cross section of this as seen from most angles will usually be an ellipse, although from the right direction it will be circular.
Cite?
Spiral galaxies certainly do collide, but the result (or so I thought) was most often an irregular or distorted galaxy, or a spiral with odd asymmetries. And unless new discoveries have changed things, elliptical galaxies are not considered to be the consequence of galactic collisions.
The center of the Milky Way is in fact spherical.
They call the bulge “kind of like two fried eggs with a yolk sticking out from the top and bottom.” This fits the answer that gravity is dominant. The dense center forms a ball. The arms of the spiral are thrown out from rotation. And then farther out the very diffuse halo of gas, stars, and clusters is mostly spherical again.
But not completely spherical, because of other disturbances, probably a collision with another galaxy.
Accidents of history, plus other factors we don’t fully understand, like dark matter, probably explain the hugely varying appearance of large galaxies. But gravity, density, and angular momentum explain a lot.
I found all kinds of pages that said ellipticals are the result of galaxy collisions. For example:
(Emphasis mine)
Note that this happens only when the two galaxies colliding are roughly the same size. When a small galaxy “hits” a big one, the small one is absorbed, although the effects can often be seen for a very long time. For instance, there’ve been several small galaxies absorbed by the Milky Way that astronomers have deduced/detected. If the small one has a dense core, that core may end up as a globular cluster (there’s at least one such and others suspected). The rest of the small galaxy may leave a star stream orbiting at an angle to the disk (there’s some of these known, too).
Um, to be pedantic, the electromagnetic force is also long range. It doesn’t have much effect at long ranges in practice since most matter is electrically neutral (protons & electrons balance out).
That one doesn’t say ellipticals are the result of galaxy collisions. It says a galaxy collision will result in a giant elliptical.
If the current thinking is that all ellipticals had their origin in previous galactic collisions, I’d like to update my awareness of cosmological theory to include that. I’m only an armchair follower and don’t claim any expertise or anything, but my sense of the conventional wisdom on ellipticals is that they are mostly old galaxies with old stars. Galactic collisions tend to produce a wave of new star formation. For all the ellipticals to be the result of old galactic collisions, they’d most of them have to be so old that first they were spiral galaxies, then they collided, then the resultant rise of collision-generated new stars faded, and the stars of the original spiral galaxies all aged to the point that the entire elliptical was a bit of a stellar retirement home.
My own cites:
https://www.google.com/search?q=elliptical+galaxies+old+stars
https://www.google.com/search?q=galactic+collisions+new+stars
OK, I guess I’ll have to quote a bunch more.
From the same page I cited above:
https://www.astronomy.ohio-state.edu/ryden.1/ast162_8/notes35.html
Responding to your other point
That’s exactly what happens. The reason is that the collision consumes pretty much all of the gas and dust of the two spirals. Spirals are bluish (and not red) because they have ongoing stellar creation, which is fed by the gas and dust. The brightest stars have very short lifespans relative to the age of the galaxy, as short as half a million years for the brightest. So there has to be continual star births to make up for that. If there is no gas in a galaxy, as is the case of ellipticals, there’s no more stellar births. So the brightest stars have all died (supernovaed) or are in the red Supergiant phase.