This Diagram is Better than 183,487 Images
"So what does the diagram tell us? Evidently, there are two peaks: one near the bluish end on the left, one near the reddish end on the right. That indicates two distinct types of galaxies. Galaxies of the first kind are, on average, of a bluish-white color, with some specimens a little more and some a little less blue (which is why the peak is a little broad). Galaxies of the other kind are, on average, much redder."
"The existence of two distinct classes of galaxies — star-forming vs. “red and dead” — is a driving force behind current research on galaxy evolution in much the same way the HR diagram was for stellar evolution. Why are there two distinct kinds? What makes the bluish galaxies produce stars, and what prevents the reddish ones? Do galaxies move from one camp to the other over time? And if yes, how and in which direction? When you read an article like this about the care and feeding of teenage galaxies, or this one about galaxies recycling their gas, it’s all about astronomers trying to find pieces of the puzzle of why there are these two populations."
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