sentences:  At the center of the Crab Nebula are two faint stars, one of which is the star responsible for the existence of the nebula.It was identified as such in 1942, when Rudolf Minkowski found that its optical spectrum was extremely unusual.The region around the star was found to be a strong source of radio waves in 1949 and X-rays in 1963, and was identified as one of the brightest objects in the sky in gamma rays in 1967.