The nebulosity in this region is part of the Gamma Cygni Nebula. This explosion appeared in the constellation of Taurus, where it produced the Crab Nebula remnant. The asteroid belt formed from the primordial solar nebula as a group of planetesimals. It also contains the nearly spherical planetary nebula Abell 39. MWC 922 is a star surrounded by a planetary nebula. Another planetary nebula in Lyra is Abell 46. There are about 40,000 cometary knots in the Helix Nebula. Such explosions and pulsations lead to planetary nebula. The center of Corvus is home to a planetary nebula, NGC 4361. There are also supersonic "bullets" of gas piercing the hydrogen clouds of the Orion Nebula. The excitation temperature varies across the Helix nebula. The halo again predates the formation of the main nebula. Diffuse emission nebula, or H II region, cataloged as M8. They form H II regions such as the Orion Nebula. HD 93250 is one of the brightest stars in the region of the Carina Nebula.