Figure 12-19 The Crab Nebula and Pulsar (a) This nebula, named for the crablike appearance of its filamentary structure in early visible-light telescope images, is the remnant of a supernova seen in the year 1054. The distance to the nebula is about 6000 ly, and its present angular size (4 by 6 arcmin) corresponds to linear dimensions of about 7 by 10 ly. Observations at different wavelengths give astronomers information about the nebula’s chemistry, motion, history, and interactions with preexisting gas and dust. (b) The insets show the Crab pulsar in its “on” and “off” states. We can see the pulsar in visible light, too. Both its visible flashes and X-ray pulses have identical periods of 0.033 second.