NASA marks America's 250th birthday with space visuals

NASA has released four deep-space images from its Chandra X-ray Observatory to mark the 250th anniversary of the United States. The images include Cassiopeia A, a supernova remnant in the Milky Way; NGC 3603, a star-forming nebula; NGC 4736, a spiral galaxy; and a distant galaxy cluster that has helped astronomers study the distribution of dark matter.