What is it about this terrestrial world’s atmosphere that gives it its unusual colour?

What is it about this terrestrial world’s atmosphere that gives it its unusual colour?
O, B, A, F and so on… What do they all mean?
A new look at the debris from an exploded star in our galaxy has astronomers re-examining when it actually happened
The ‘little lion’ galaxy contains the lowest level of heavy elements ever detected in a gravitationally bound system of stars
Recent colour images reveal more about the dwarf planet