The Christmas Tree Cluster was named for its triangular shape, formed by a cluster of very young stars, that looks like a tree in visible light. The bright variable star S Monocerotis (near the bottom) marks the base of the Christmas tree and lights up the nearby nebulosity.
(This cluster was one of my favorite objects to look at with my first 4 1/2 inch telescope back in the mid 1950s.)
Since the images in this blog are relatively large and contain a lot of detail, if you are using a phone or a small tablet, you might want to consider returning sometime when you can use a computer with its larger screen.