C. RR Lyrae variables
RR Lyrae variables are variable stars that have pulsation periods of 1.5 hours to 1.2 days.
Nebula is the cloud of interstellar dust and gas that forms a star. It is collborately shaped during colliding and collapsing of the interstellar mediums this is influenced by the gravitational attraction of the atoms and particles in the entites. Hence, there are three types of nebular namely, are classical nebula, diffuse nebula, planetary nebular and supernova remnants.
