Christmas is less than a week away, and even the night sky is spreading some holiday cheer with what could be the largest Christmas tree in the universe. On Tuesday, NASA released an image of NGC 2264, also known as the "Christmas Tree Cluster," a group of young stars located around 2,500 light-years away from Earth. The image was captured by the Chandra X-ray Observatory, and the green nebula surrounding the stars resembled a festive Christmas tree. Some stars in the cosmic Christmas tree are only one-tenth the size of our sun, while others are several times larger. NASA added...