M63: Hubble image of the Sunflower Galaxy.

What Makes This Beautiful Spiral Galaxy So Patchy?

What Makes This Beautiful Spiral Galaxy So Patchy?

Bad Astronomy
The entire universe in blog form
Sept. 14 2015 10:00 AM

Pardon My Flocculence

A galactic sunflower
Flocculent galaxies are gassy.

Photo by ESA/Hubble & NASA

Spiral galaxies are just so danged pretty.

Seriously, look at that! It’s an image of the nearby galaxy M63, aka the Sunflower Galaxy, taken using the Hubble Space Telescope. M63 is relatively close to us, a mere 37 million light-years, easily visible in small telescopes. I’ve seen it a few times myself through my own ’scope—I vaguely remember finding it by accident when I was in high school, looking for the more famous M51 Whirlpool Galaxy. They’re not far from each other in the sky, and in fact they aren’t far from each other in space; both are part of a small group of galaxies.

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  


M63 is what we call a flocculent galaxy—the arms are patchy, like tufts of cotton. Roughly a third of all spirals are flocculent, and apparently it’s still not well understood what causes it. One idea is that local gas clouds get stretched and sheared by the galaxy’s differential rotation (that is, stuff closer to the center makes an orbit around the galactic center in less time than stuff farther out).

Another is that star formation is localized, starting randomly in spots in the disk of the galaxy, triggering more star formation around it, creating that patchwork. In galaxies like the Milky Way, the spiral arms themselves trigger star formation, so you get more of a sweeping, grand design to the spiral arms.

M63 Spitzer
Dusty, too.

Photo by NASA/JPL-Caltech

Spitzer detects infrared light emitted by warm objects, so what you’re seeing here is mostly dust gently heated by nearby stars. The arms look a little more organized here, but you can still see they’re patchy. 

I love big, sweeping spiral galaxies, but there’s something to be said for flocculent ones as well (like NGC 2841, NGC 3521, and NGC 1398). And besides being just pretty, they also show us that there’s more than one way to make a spiral arm, or to spawn millions of stars.

Nature’s fairly creative. And it has a broad canvas upon which to work.