Ellipticals are Red, Spirals are Blue, What's With These Galaxies With a Different Hue?

I'm Dr Ashley Nova: Astronomer, Cosmic Babe and Guide to the Universe, and today we're taking a journey to "The Green Valley".

No, not a London Based Deli, but a conundrum in modern science.

It's been about 100 years since Edwin Hubble first blew up our understanding of the universe, by proving that the strange clouds of stars in the sky were, in fact, distant galaxies much like our Milky Way.

(Though credit should also go to Vesto Slipher.)

Hubble also did some of the pioneering work on classifying the shapes of galaxies, and his "Tuning Fork" is still in use today as a way to describe and group galaxies. Don't be confused by the layout, however, it doesn't denote a sequence!

(img: http://astronomyonline.org/Astrophotography/GalaxyMorphology.asp)

On one end of the fork, we have the ellipticals: These galaxies are smooth with large bulges and are supported by random motion (Ex). On the other end, we have the spirals: flat disks, supported by rotation, and featuring spiral arms and bars (Sx and SBx).

In the middle of the fork are the S0 type galaxies, or "Lenticulars" as they are sometimes called, which have a disk-like structure but lack the arms and bars that make Spirals galaxies.

Remember, the Fork isn't a sequence, but we can ask "How do a galaxy's colour/shape change?"

To begin answering this question, we first have to ask what MAKES a galaxies colour and shape? WHY does it look the way it does?

The answer, of course, is in the stars.

Get it?

I mentioned that ellipticals () are supported by random motion, and spirals () by rotation, but what does that really mean?

It means, how are the stars (and gas and dust) moving in the galaxy. In , the stars all orbit in a plane in the same direction, but in they don't.

Spiral arms and bars appear because there are more stars in the arms and bars, than in the spaces in between. This can happen because of traffic jams, where all the stars bunch up and slow/speed each other up, or because of external forces.

(img https://www.spacetelescope.org/news/heic1202/ )