sentences:  Changes in the cloud, suggesting its small extent, were discovered by Carl Lampland in 1921.That same year, John Charles Duncan demonstrated that the remnant is expanding, while Knut Lundmark noted its proximity to the guest star of 1054.In 1928, Edwin Hubble proposed associating the cloud to the star of 1054, an idea which remained controversial until the nature of supernovae was understood, and it was Nicholas Mayall who indicated that the star of 1054 was undoubtedly the supernova whose explosion produced the Crab Nebula.