When the cores collapse to form dense stellar objects called neutron stars, they blast off the outer layers of the star in a supernova. In higher-mass red supergiants, however, carbon doesn't burn convectively; this limits neutrino losses and leads to a more extended core with dense material around it.
While the core collapses, the outer layers of material in the star to expand outward. The star expands to larger than it has ever been – a few hundred times bigger! At this point the star is called a red giant. What happens next depends on how the mass of the star.