A gap in the rainbow
Floating in a party balloon, helium seems like harmless fun – but you also find it at the furthest extremes of heat and cold. Helium is the second-simplest chemical element: each atom is built from just two protons, two neutrons and two electrons. It can be created when four atoms of the very simplest element, hydrogen, which has one proton and one electron, fuse together.Hydrogen transforms like this in the hot, dense conditions we find in stars like the Sun. The hydrogen-to-helium nuclear fusion process creates the awesome power that takes the Sun’s core temperature to around 15 million degrees Celsius.