Atmosphere of Earth years ago. About 3.4 billion years ago, nitrogen formed the major part of the then stable "second atmosphere". The influence of life has to be taken into account rather soon in the history of the atmosphere, because hints of early life-forms appear as early as 3.5 billion years ago. How Earth at that time maintained a climate warm enough for liquid water and life, if the early Sun put out 30% lower solar radiance than today, is a puzzle known as the "faint young Sun paradox". The geological record however shows a continuous relatively warm surface during the complete early
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,