The bigger NGC4631 on the left is nicknamed “Whale Galaxy”. NGC4631
is probably rather typical spiral galaxy, but visible from the side.
It shows yellowish central bulge, composed mainly of old stars, as
well as dark interstellar dust lanes and reddish hydrogen clouds
around the galaxy disk. Light blue portions indicate presence of many
bright, young blue stars.
The smaller NGC4656 on the right side is nicknamed “Hockey Stick
Galaxy”. Its color is mainly blue because of the large amount of young
blue stars. Typically, abundance of star birth activity indicates some
violent event, which triggered the condensation of gas clouds into
proto-stars, which lead to occurrence of many new young stars. Such
event is very often merger of two galaxies and the slightly irregular
shape of NGC4656 and also streams of stars reaching far from the
galaxy spiral arms hint such merger occurred only recently