B33 - Horsehead Nebula
M 42, the Great Orion Nebula, can be seen with the naked eye very easily. Nevertheless, astronomers did not become aware of this object before the first telescopes were constructed. Therefore, it is believed that the nebula was fainter in the last centuries than it is today. M 42 was discovered in 1611 by Nicholas-Claude Fabri de Peiresc and independently by Johann Baptist Cysatus. M 43 is just a part of the nebula.
The Orion nebula is a showcase of a galactic nebula just 1,300 light-years away. M 42 is the visible end of the almost dark molecular cloud OMC-1 spanning 10° on the sky. M 78 as well as the Horeshead nebula are also part of this structure. A star cluster is borne in the inner part of M42. Most of the stars are still hidden in dense dust clouds. Only the four bright stars of the Trapezium have blown away this dust with their intense radiation. The brightest star Φ2 Orionis C delivers most of the energy to ionize the complete nebula. M 42 is a mixture of emission and reflection nebulas and dark dust clouds. In the inner parts of the nebula very young stars become visible surrounded by proto-planetary disks. Most of these disks will be destroyed by the intense radiation of the bright stars in the next future.