Sometimes I think it's a good idea to start off the week with a gorgeous spiral galaxy. So here's a fantastic example of a flocculent (fluffy or patchy) spiral: NGC 3521 in the constellation of Leo, care of the Very Large Telescope:
[Click to enflocculenate.]
NGC 3521 is a mere 35 million light years away (350 quintillion kilometers, a comfy airplane ride of just 50 trillion years or so; ask for an extra bag of peanuts), which is outside our local area but still close as the Universe goes. It's half the size of our Milky Way home, about 50,000 light years across. [Note that it has that same effect I mentioned in an earlier post where the dust on the side of the galaxy closer to us appears darker; the light from intervening stars in that galaxy appear to "fill in" the dust on the other side.]
A large fraction of spiral galaxies have these patchy, ill-defined arms, so nature is telling us something: these things are easy to make. Grand design spirals -- ones like ours, with splashy well-defined spiral arms -- appear to be due to some global effect creating the arms; stars near the galaxy's center orbit more quickly than ones farther out, so spiral arms should get wound up relative quickly. The fact that so many grand design spirals are seen means that this differential rotation does not destroy the spiral pattern: something most be maintaining it (we think it's a traffic jam-like effect).
Flocculent spirals, on the other hand (arm?) are more likely to have some sort of local effect in the disk creating the patchiness -- if it were some galaxy-spanning effect then we'd see better defined arms! Perhaps regions of local star formation from dense clouds are being stretched and pulled apart by differential rotation, for example, or, rather more likely, combination of several factors working in concert.
But the contrast between the two types of spirals is striking. And the differences between spirals don't stop there: there are barred spirals, ones with small nuclei, ones with big nuclei, arms that are wound tightly, others loosely... the variety in nature on how to make a colossal structure 500 quadrillion kilometers across containing hundreds of billions of stars is pretty amazing. And we have a long way to go to understanding why they're different! The math and physics of the behavior of galaxies is fierce, to say the least. They may look fluffy, but the science underlying them is anything but.
Image credit: ESO/O. Maliy