Sally Schoon, of Wellington, asks :-

I read somewhere that gold is not a naturally occurring mineral on earth, but instead has come from debris after certain planets collided. If that is so, are any further visitations possible or was it a one-off event during earlier stages of the Big Bang?

Alan Gilmore, an astronomer at the University of Canterbury's Mt John Observatory, responded.

There are two possible sources of your memory of gold being the result of collisions. Recent studies have shown that the universe's supply of gold and some other heavy elements may come from the merging (collisions) of pairs of neutron stars. Gold and other heavy elements in the earth's crust may have come from late-arriving asteroids during earth's formation around four billion years ago.

The universe started off with only hydrogen and helium. Most of the other chemical elements have been made in stars and supernova explosions. Stars like the sun get their energy by joining up hydrogen atoms (strictly hydrogen nuclei) to make helium nuclei. Helium nuclei can be fused together to make carbon and similar elements. Stars heavier than the sun are hotter in the centre. The higher temperature allows bigger nuclei to be made. However, even in the most massive stars, this process stops at iron. There isn't any energy to be had from making iron into heavier stuff like gold, platinum or uranium.

Supernova explosions make the really heavy elements. A supernova, strictly a type II supernova, is the explosion of a massive star. It happens when the star's iron core collapses. For a short time there are many neutrons zipping around. These collide with iron nuclei and build up big nuclei. Unfortunately, precise modelling of supernova explosions shows that they won't produce all the heavy elements that we find in nature.

Merging neutron stars fill these gaps. Neutron stars are the remnants of type II supernova explosions. The core of the star is squashed down to pure neutrons. A mass about one and a half times the mass of the sun is squeezed into a ball only 20-30 km across.

Occasionally two massive stars orbit around each other. After each becomes a supernova two neutron stars are left. These eventually spiral into one another and merge. During the merger tidal forces throw out clumps of neutrons. These become heavy nuclei. The calculated ratios of heavy elements neatly matches the ratios found in nature, supporting this theory. The heavy elements are thrown into space where they mix with the clouds of dust and gas that eventually form new stars and planets. From the number of neutron stars ("pulsars") known, it is estimated that a pair would merge every 100,000 years in a galaxy like the Milky Way.

The problem with gold and other heavy elements appearing in the Earth's crust is why didn't they sink to Earth's core when the world was forming? The best explanation is that the stuff in the crust arrived late, after most of the mixing had been done. It arrived on big asteroids. Big asteroids mostly stopped bombarding Earth and the other planets about 3.8 billion years ago.