Rare Earth hypothesis the atmosphere is not stripped over time by solar winds. A terrestrial planet of the right size is needed to retain an atmosphere, like Earth and Venus. On Earth, once the giant impact of Theia thinned Earth's atmosphere, other events were needed to make the atmosphere capable of sustaining life. The Late Heavy Bombardment reseeded Earth with water lost after the impact of Theia. The development of an ozone layer formed protection from ultraviolet (UV) sunlight. Nitrogen and carbon dioxide are needed in a correct ratio for life to form. Lightning is needed for nitrogen fixation. The carbon dioxide gas
Messier 78 two B-type stars, and , are responsible for making the cloud of dust in M78 visible by reflecting their light. The M78 cloud contains a cluster of stars that is visible in the infrared. Due to gravity, the molecular gas in the nebula has fragmented into a heirarchy of clumps, the denser cores of which about to form stars with masses of up to . About 45 variable stars of the "T Tauri" type, young stars still in the process of formation as well as some 17 Herbig–Haro objects are known in M78. Messier 78 Messier 78 or M 78,