Starship Asterisk*

I am wondering why all galaxies have a circular disc shape and are not sperical in shape. Also, I understand that the search for exoplanets uses the technique of watching stars for a slight dimming if/when a planet passes between the star and the observer. Does this mean that only those stars whose orbital plane is 'side-on' to us can be observed or are all galaxies/star-planet families in the same plane as us (obviously not from the APOD's). Thanks

Most photogenic galaxies are spiral galaxies. So you see a lot of pictures of them. However, they are not the most common type of galaxy at all. Elliptical galaxies are more common and those are closer to spherical in shape than they are discs but they do tend to be a little flatter on one axis.


As far as planet detection goes, you are correct. But there are other ways to detect planets. Another indirect method is to observe the wobble of a star caused by gravitational interactions and planets do not need to transit their stars for that method. Yet another option is directly observing light from a planet.
There are even more ways but I'm not familiar enough with them to describe them.
http://en.wikipedia.org/wiki/Methods_of ... ar_planets

wagonmaker wrote:I am wondering why all galaxies have a circular disc shape and are not sperical in shape.

In addition to what Geckzilla said, consider that rotating bodies are frequently disc-like. That's because of the conservation of angular momentum, and the way momentum is exchanged between particles that are drawn together by gravity. Galaxies (like stellar systems) presumably form when material is drawn together by the self-gravity of that same material. This results in all the bodies getting faster orbits, and the way momentum is transferred by collisions and other processes tends to either move them into similar inclinations, or to eject bodies that are at very different inclinations. The result is something where a lot of the mass is in the same plane- a disc. Of course, the actual dynamics are much more complex, so in a spiral galaxy there is usually a sphere-like central bulge that may contain a substantial part of the total mass of that galaxy.

Elliptical galaxies (and others that are not substantially flat) probably got that way by collisions or other disruptions over the course of their existence.

geckzilla wrote:Most photogenic galaxies are spiral galaxies. So you see a lot of pictures of them. However, they are not the most common type of galaxy at all. Elliptical galaxies are more common and those are closer to spherical in shape than they are discs but they do tend to be a little flatter on one axis.

According to Wiki Answers, which may or may not be correct, the most common galaxy is the dwarf elliptical. The most common main galaxy is the spiral galaxy. So this picture by J-C Cuillandre shows us the two most common galaxies in the world: the dwarf elliptical, in this case NGC 205, and the large or relatively large spiral galaxy, in this case the Andromeda galaxy.

Ann

That Wiki Answers article doesn't list any sources. There's no telling who wrote it or how accurate they were with their assessment so I would disregard it.

BTW, here is a paper that uses the Galaxy Zoo data and which shows ellipticals to be more common than spirals, unless I'm completely missing something or misunderstanding the presented information.
http://uk.arxiv.org/abs/0804.4483

Thanks for the info. There is enough concensus among the replies to assist my very rusty physics knowledge. I can understand what's going on.