The hierarchical model of galaxy formation holds that the Milky Way and other such large galaxies are conglomerations of matter from older, smaller galaxies, themselves the result of mergers of even older and even smaller things. Star clusters give rise to dwarf galaxies that then give rise to bigger ones and so on.
We know mergers happen, we see the Large and Small Magellanic clouds hanging onto our galaxy and the signature of residual tidal disruption in the edges of the Andromeda galaxy. We also see possible mergers in pictures that contain many galaxies (such as those used in the Galaxy Zoo – Mergers project, which allows you to run simulations of merging events and identify which is the closest to the image on screen). But there is a problem. Up to now, the observations of dwarf galaxies show them to be stuffed full of young stars. Nothing that matches the age of some of the stars in the Milky Way. This would seem to suggest the opposite – top down hierarchy, whereby massive structures are formed that then spin off smaller galaxies.
A group of astronomers decided the problem wasn’t that the galaxies were young, but that previous astronomers had used too few of the available stars to find an old one, or the wrong heavy element to identify them. Old, very old stars tend to be small and faint objects. But their major feature, the real clincher in stellar evolution, is the iron content of their spectrum.
During the course of a star’s life, it will fuse together light elements to make heavier and heavier elements. The process releases energy as the heavier elements tend to have lower binding energies (they are more stable and require less force to hold them together). But this trend holds only up to a point – iron. Above this most stable of elements, the trend reverses, with heavier elements now less stable than lighter ones. This is why really heavy things like Uranium split apart in nuclear fission events. All atoms would like to be iron and they expend energy when they move closer and absorb it when moving further away. So if you want to create elements heavier than iron through fusion, you must supply energy, which is why stars come to an end the moment they have substantial amounts of iron in their core. They blast apart and the energy of the explosion allows some heavier elements to be formed. It also means the amount of heavy elements in the gas that goes into creating new stars is richer in ‘metals’ ie heavy elements.
We generally can’t see into the core of a star, though in some cases stars can dredge up material from close to the core, exposing it. We can only see the outer shells. This envelope of gas is mostly made up of the stuff the star was formed from, so contains a signature of the ‘metalicity’ of the star. If it is made of stuff that is pure hydrogen, there’s little or no iron in the spectrum. If some of the stuff it’s made of exploded out from a previous generation of stars, it will contain a little iron. If it’s gone through two generations, it will hold more iron.
Using a new spectroscopic technique, astronomers have been able to look through the relatively nearby (only 300,000 light years away!) Sculptor Galaxy for stars that seem to be old. Having spotted a couple, they looked closer and found their goal. One of the stars was old, very old. Perhaps close to the age of the Universe. Its spectrum had four hundred times less iron than the Sun’s spectrum. This was similar to ancient stars found in the halo of the Milky Way – a sphere of star clusters centred on the nucleus of the galaxy.