Orion, the most imposing and the handsomest of the men of Greek and Roman mythology, the giant hunter and son of Poseidon, hides exceptional wonders. Right beneath his belt — and don't get any ideas — shines the brightest and most easily observed object in the sky: the great Orion Nebula. So large it spans 24 light years, and easily seen with the naked eye.
There, some 1,350 light years from Earth, among ionized gases and reflection nebulae, at a temperature of ten thousand degrees, stars and planetary systems are being born. Seven hundred stars have already been identified in the process of forming, along with more than a hundred and fifty protoplanetary discs from which new solar systems will take shape.
How a star is born
A star is born when a cloud of hydrogen and other gases contracts under its own gravity. The temperature rises rapidly through the collapse and the conversion of gravitational energy into heat. If the temperature is high enough, nuclear fusion ignites, and from that a protostar is born.
It can happen that not all the material is captured by gravity before fusion ignites. That is how a protoplanetary disc forms around the protostar, accompanying the star's birth while gathering into clumps of material destined, little by little, to take the shape of a planet.
In the field
Four very young, very hot stars form the Trapezium: their ultraviolet radiation carves a cavity in the cloud and lights the gas all around. The wider framing also shows M43, separated from M42 by a band of dark dust, and higher up the Running Man Nebula (NGC 1977), blue because it reflects the light of its stars rather than emitting its own.
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