# Why isn't the Milky Way spherical? **URL:** \ **Category:** Factual Questions\ **Created:** [January 19, 2023, 5:52am UTC](https://boards.straightdope.com/t/why-isnt-the-milky-way-spherical/978439 "2023-01-19T05:52:40Z")\ **Posts on this page:** 9\ **Page:** 2
**Author:** ![nightshadea](https://avatars.discourse-cdn.com/v4/letter/n/7ba0ec/32.png) [@nightshadea](https://boards.straightdope.com/u/nightshadea)\ **Post date:** [January 19, 2023, 11:29am UTC](https://boards.straightdope.com/t/why-isnt-the-milky-way-spherical/978439/21 "2023-01-19T11:29:47Z")
becuase york peppermint patties and reeses has the round candy bar market covered? :ducks and runs: ---
**Author:** ![Charlie\_Tan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/charlie_tan/32/12582_2.png) [@Charlie\_Tan](https://boards.straightdope.com/u/Charlie_Tan)\ **Post date:** [January 19, 2023, 2:12pm UTC](https://boards.straightdope.com/t/why-isnt-the-milky-way-spherical/978439/22 "2023-01-19T14:12:42Z")
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. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [January 20, 2023, 12:30am UTC](https://boards.straightdope.com/t/why-isnt-the-milky-way-spherical/978439/23 "2023-01-20T00:30:49Z")
> [@AHunter3](#): > > Also — incidentally — some galaxies are indeed spherical. Or close to it. The roundest of elliptical galaxies, E0, are basically huge fuzzy spheres. 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. ---
**Author:** ![AHunter3](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ahunter3/32/368_2.png) [@AHunter3](https://boards.straightdope.com/u/AHunter3)\ **Post date:** [January 20, 2023, 1:39am UTC](https://boards.straightdope.com/t/why-isnt-the-milky-way-spherical/978439/24 "2023-01-20T01:39:22Z")
> [@dtilque](#): > > Elliptical galaxies are the result of two roughly equally-sized spiral galaxies colliding. 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. ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [January 20, 2023, 1:46am UTC](https://boards.straightdope.com/t/why-isnt-the-milky-way-spherical/978439/25 "2023-01-20T01:46:55Z")
The center of the Milky Way is in fact spherical. > **[The Spherical Structure at the Core of the Milky Way Formed in a Single Burst...](https://www.universetoday.com/148668/the-spherical-structure-at-the-core-of-the-milky-way-formed-in-a-single-burst-of-star-formation/)** > > Where did all the stars in the Milky Way's central bulge come from? A new survey says they came from one epic burst 10 billion years ago. 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. > **[The Milky Way's Halo of Stars Isn't The Neat Sphere Astronomers Expected It...](https://www.sciencealert.com/the-milky-ways-halo-of-stars-isnt-the-neat-sphere-astronomers-expected-it-to-be)** > > Step outside of the Milky Way for a moment and you might notice the bright disc of stars we call home has a weird warp to it. 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. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [January 20, 2023, 6:55am UTC](https://boards.straightdope.com/t/why-isnt-the-milky-way-spherical/978439/26 "2023-01-20T06:55:51Z")
> [@AHunter3](#): > > > [@dtilque](#): > > > > Elliptical galaxies are the result of two roughly equally-sized spiral galaxies colliding. > > Cite? I found all kinds of pages that said ellipticals are the result of galaxy collisions. For example: > **[What happens when galaxies collide?](https://phys.org/news/2016-10-galaxies-collide.html)** > > We don't want to scare you, but our own Milky Way is on a collision course with Andromeda, the closest spiral galaxy to our own. At some point during the next few billion years, our galaxy and Andromeda – which also happen to be the two largest... > [@](#): > > And the delicate spiral structure of both galaxies will be destroyed as they become a **single, giant, elliptical galaxy**. But as cataclysmic as this sounds, this sort of process is actually a natural part of galactic evolution. (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). ---
**Author:** ![xtenkfarpl](https://avatars.discourse-cdn.com/v4/letter/x/c2a13f/32.png) [@xtenkfarpl](https://boards.straightdope.com/u/xtenkfarpl)\ **Post date:** [January 20, 2023, 12:47pm UTC](https://boards.straightdope.com/t/why-isnt-the-milky-way-spherical/978439/27 "2023-01-20T12:47:51Z")
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). ---
**Author:** ![AHunter3](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ahunter3/32/368_2.png) [@AHunter3](https://boards.straightdope.com/u/AHunter3)\ **Post date:** [January 20, 2023, 2:41pm UTC](https://boards.straightdope.com/t/why-isnt-the-milky-way-spherical/978439/28 "2023-01-20T14:41:33Z")
> [@dtilque](#): > > I found all kinds of pages that said ellipticals are the result of galaxy collisions 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=elliptical+galaxies+old+stars) [https://www.google.com/search?q=galactic+collisions+new+stars](https://www.google.com/search?q=galactic+collisions+new+stars) ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [January 20, 2023, 8:17pm UTC](https://boards.straightdope.com/t/why-isnt-the-milky-way-spherical/978439/29 "2023-01-20T20:17:22Z")
OK, I guess I’ll have to quote a bunch more. From the same page I cited above: > [@](#): > > This is why giant elliptical galaxies, the results of galaxy collisions, have so many old red stars and very little active star formation. > **[Why do colliding spiral galaxies form elliptical galaxies?](https://www.astronomy.com/science/why-do-colliding-spiral-galaxies-form-elliptical-galaxies/)** > > categories:Galaxies | tags:Galaxies, Magazine > [@](#): > > It is also what ultimately destroys the initial spiral patterns of the two galaxies and results in an elliptical galaxy once the merger is complete. [https://www.astronomy.ohio-state.edu/ryden.1/ast162\_8/notes35.html](https://www.astronomy.ohio-state.edu/ryden.1/ast162_8/notes35.html) > [@](#): > > Low-speed, head-on, encounters between galaxies result in **mergers**. The individual stars in the two galaxies pass harmlessly by each other, without slamming into each other. A few stars attain high speed, and are lost to intergalactic space. Most of the stars, however, are traveling slowly enough to settle back down to form a single large elliptical galaxy. > **[What Happens When Two Galaxies Collide? | Facts & Impact](https://nineplanets.org/questions/happens-two-galaxies-collide/)** > > People wonder what happens when two galaxies collide? The truth is, what could happen when galaxies interact largely depends on the composition and size. > [@](#): > > The combination of the two galaxies then forms what appears to be an elliptical galaxy as the arms begin to disappear. Responding to your other point > [@AHunter3](#): > > 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. 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. [Previous page](https://boards.straightdope.com/t/why-isnt-the-milky-way-spherical/978439.md?page=1)