Solar system formation. Artwork of the early Sun when it was still a protostar about 4.7 billion years ago. The Sun is surrounded by an inter- stellar cloud (nebula) of glowing dust and gas. The Sun was created by the gravitational collapse of this nebula which was possibly triggered by shock waves from a nearby exploding star (supernova). As the nebula condensed, its parti- cles converted their gravitational potential energy into heat. The proto-Sun became so hot that nuclear reactions started in its core and it became a true star. The planets formed in the outer areas of the nebula.