One of the most fascinating things about astronomy is the incredibly rich diversity of objects we can study in the universe. Take galaxies, for example. These collections of stars fall into so many types, based on the shapes they have, that astronomers are continually sorting them into ever-finer “bins.” When I was first in school, we learned there were three basic galaxy types: spirals, ellipticals, and irregulars. Today, as we study more and more galaxies, we find that these gross distinctions do hold up, but within each category there are many variations, and what appear to be some transitional states between different types.
Spirals are pretty much what their name sounds like: roughly circular-shaped collections of stars, gas, and dust with arms spiraling in to a central core. We live in a spiral galaxy, and more than half of all galaxies observed are spirals. One of the most notable things (among many) about spirals is that they are studded with stellar birthplaces. Some spirals, including our Milky Way, also have black holes at their hearts.
When you look at a spiral galaxy in a full view, you can see the arm structure. In this image of M101, a spiral that lies about 26 million light-years away from us, you can also see soft, fuzzy looking regions in the spiral arms. These are clouds of gas and dust where stars form.
The other main type of galaxy is the elliptical. These are the ones that don’t have spiral arms and not a lot of other structure.They are simply large, oblong collections of stars with densely packed central bulges. Some have supermassive black holes at their hearts.
The big question, which does not have a simple answer, is how do galaxies get to be spirals or ellipticals? (We’ll leave out irregulars for now. They need their own entry sometime.) Does one type become another type in a sort of galactic evolution scenario? Is there a kind of galaxy that is somewhere between spirals and ellipticals?
The answers are far from complete. One scenario has spirals merging over billions of years to become ellipticals. It’s a complex one, and observations are giving support to the idea, because astronomers are finding galaxies in many stages of merger, and some of the resulting interactions seem to be making galaxies that look just like ellipticals. And, mergers are an important way that galaxies are assembled. Our own Milky Way is still “collecting” other galaxies smaller than itself in a case of cosmic cannibalism.
There ARE galaxies that look like they’re stuck somewhere between being a spiral or an elliptical. These are the lenticulars. They have disks like spirals, but their central bulges are more like ellipticals. If you view one edge-on, as we see in this Hubble Space Telescope image, it looks like a spiral viewed from the edge.
There is an intriguing element to this galaxy, and that’s the dust lane that seems to divide it in half. NGC 5866 also has a substantial collection of young blue stars in its disk, with older, redder stars at its heart. The only thing it doesn’t seem to have is spiral arms. But, those blue stars tell us that the galaxy has recently undergone episodes of star birth. Coupled with the fact that the disk is slightly warped, we have the possibility that this galaxy might have had some sort of gravitational interaction with another one (or more) a long time ago. That would explain the bursts of starbirth and the warped disk.
But why no arms? Did it once have them, along with rich regions of starbirth? If so, what happened to the gas and stars that used to be in the galaxy disk? If an interaction is the culprit, then it would have stripped out the gas and stars and somehow affected the structure of the galaxy. And that would give us with the edgy galaxy we see today. Stay tuned!
