Chile: Summer and Winter

I've now spent over a year in Chile and have taken pictures from time to time. Here I present two pairs of pictures of approximately the same location, but shifted in time by several months. That is, one picture is taken around Summer, while the other was taken around Winter (or close enough). This showcases how the seasons change here in Santiago and it's a good opportunity to talk about how the seasons work on Earth.

As always, remember that you can click the pictures to see them larger.
More details, and the second picture, after the jump.

Astronomy: Young Stellar Moving Groups

Open cluster M25. Credit:  J.-C. Cuillandre (CFHT), G. Anselmi (Coelum Astronomia), Hawaiian Starlight

Stars are born in groups, as clusters of stars. Some groups stay well-knit and the members remain together many hundreds of millions of years later. Others, however, are loosely bound to each other, and, after traveling a bit through the Galaxy, get dispersed. However, the initial bulk motion of the stars in these groups is preserved. So, if you search carefully, you can find groups of widely separated stars throughout the sky all moving in approximately the same direction and with the same properties like age and composition. These are stellar moving groups, and here I'm going to tell you why astronomers love them.