Universe Puzzle No. 11

How did you do in last week’s Universe Puzzle?

Do you enjoy these puzzles? What do you particularly like? Dislike? Would like to see changed? Would like to see more of? Let me know please!

Once again, this week’s puzzle requires you to cudgel your brains a bit and do some lateral thinking (five minutes spent googling likely won’t be enough). But, as with all Universe Puzzles, this is a puzzle on a “Universal” topic – astronomy and astronomers; space, satellites, missions, and astronauts; planets, moons, telescopes, and so on.

Where is the green valley? What are the hills/mountain range(s)/ridges which border it? How did it get its name?

UPDATE: Answer has been posted below.

The green valley I had in mind is indeed the region in a color-magnitude diagram of galaxies, between the red sequence and blue cloud. In this case five minutes spent googling would have given you this answer (so not really a Universe Puzzle).

It got its name because, first, between red and blue comes green, and second because one way of representing the number of galaxies per unit area in a chart (or graph) of their color vs luminosity is to draw contours (if the chart is representing hundreds of thousands of galaxies then plotting them all as points becomes visually bland, shall we say). And what does a chart with contour lines on it remind you of? A topographic map! So the red sequence and blue cloud would be ridges or mountains, and in between would be a …. valley.

At the time I accessed it, the Wikipedia entry is a good summary (with Wikipedia you have to be very careful that the contents of an entry haven’t changed!)

Check back next week for another Universe Puzzle!