The oldest stars in our Galaxy occupy a different region of space, move with different speeds, and have different chemical compositions to most others. By observing these four quantities --- age, position, velocity and composition --- astronomers are discovering a wealth of information on the formation and  evolution of the Galaxy, which is the home for our Solar System in its motion through the Milky Way.

Amongst recent surprises is the discovery that some of these stars are much more primitive even than objects observed in the most distant parts of the universe, at redshifts z = 3-5. That is, the oldest astronomical finds have been made in our own neighbourhood. They are the oldest objects known, and now are beginning to yield their secrets about the first few steps the universe took in transforming matter from the Big Bang into the stars that light up the Galaxy we live in.

A spiral galaxy like ours

Even to the casual observer, the sweep of the Milky Way across a dark night sky tells much about our Galaxy. Its concentration into a band stretching around the sky tells us that most of the stars are confined to a plane --- the Galactic disk --- within which we, as Earth-bound and Solar-System-bound observers, are located. The band reaches maximum brightness in the southern constellations of Sagittarius and Scorpio, accompanied by some of the darkest contrasting patches of sky where dense intervening dust clouds obscure our view of the stars beyond. In that direction lies the heart of the Galaxy.

A wide field view of bright star fields, dark obscuring clouds, and H II regions towards Sagittarius.

The Galaxy's disk contains bright, hot, blue and white stars accompanied by exquisite gas clouds. The latter fluoresce red as ultraviolet starlight excites their hydrogen atoms. These are the tell-tale signs of astronomical youth, of stars no more than 100 million years old. A very young example in a neighbouring galaxy is the Tarantula Nebula. Other, more sedate regions of the disk contain stars up to around 6 billions years. But if you turn away from the Milky Way, and direct your eyes towards the parts of the sky where fewer stars punctuate the darkness, you are looking into the past. Those directions, towards the Galaxy's north and south poles, are where the oldest --- Population II --- stars are found, in the "halo" of the Galaxy.

What are the objects that constitute Population II? The most conspicuous are globular star clusters, concentrations of some 100 000 stars within a spherical region seldom more than 100 light years across. Each cluster contains stars covering a wide range of masses born at the same time. This makes them very suitable for measuring ages, because the lifetimes of stars depend on their mass and hence temperature; hot, massive stars burn out quickly, whereas cooler, lower mass stars last much longer. By seeing what temperature ranges of stars have burnt out, it is possible to measure the age of the system. These measurements reveal that globular clusters are around 13 billion years old, and hence must have formed very soon after the Big Bang.

Although globular clusters are visually striking, they are a minor component of Population II. Much more common, but frustratingly inconspicuous, are "field" stars, which move alone or with just one or two companions as they orbit through the Galaxy. Although it is more difficult to measure the ages of single stars than clusters, all the indicators are that the field stars have the same ages as the globular clusters, and rate likewise amongst the first objects to form after the Big Bang. In addition to there being more field stars than globular clusters, they are in many cases much closer and brighter, and therefore better able to be studied in detail.

The field stars differ in another very important respect: although the chemical compositions of globular clusters differ greatly from that of the Sun, the field stars are even more extreme. On the Earth, and over the lifetime of humans, most chemical elements retain their identity. That is to say, a box of iron atoms today will still be a box of iron atoms 1000 years from now. The box may look a little rusty if iron atoms combine with oxygen atoms to make an iron oxide (rust), but even then the atoms will still be iron (plus the oxygen ones). The alchemists' dreams of transmuting one element to another were unfulfilled until the nuclear age, when the structures in the cores of atoms could be rearranged. To stars, alchemy is trivial. Their whole existence is based on their ability to change hydrogen into helium, helium into carbon, and so forth. The trend is to build up heavier elements, so the amount of carbon, oxygen, iron and many other elements that are listed in the periodic table increase over the lifetime of the universe. Thus in stars, the Galaxy, and the Universe as a whole, chemical compositions do change with time, and studying those changes gives astronomers one of their biggest insights into the history of the Galaxy. The field of study is called "Galactic chemical evolution".

The Sun, and many other stars belonging to the disk of the Galaxy, are composed mainly of hydrogen and helium, with just 2% of the total being made of heavier atoms. (Astronomers refer to those heavier atoms as "metals", irrespective of whether iron, oxygen, or some other element is being considered.) The oldest globular clusters contain as little as 0.02% metals, a factor of one hundred less than the Sun. Because the globular clusters are older than the Sun, they formed from gas in which not many heavier elements had been made. Astronomers have found that Population II field stars are much more extreme than the globular clusters, and include objects with as little as 0.0002% metals! This shows that the field stars are not only old, but were amongst the first stars to form after the Big Bang, coming into existence before more metals were produced. Detailed studies of these objects are telling us much about the earliest epochs of the universe and about the beginnings of the formation of galaxies.

How is such a valuable yet indistinct population of stars to be found? Before they can be studied, they must be discovered! Stars with a metal content a factor of 1000 or more lower than the Sun (i.e. < 0.002%) make up only one star in 50000 in the solar neighbourhood, so dedicated searches must be undertaken.

Some searches have relied on the fact that these objects have very different orbits in the Galaxy to disk stars like the Sun. The Sun travels in a well ordered, almost circular orbit within the Galaxy, taking 200 million years to complete one circuit. Population II stars, on the other hand, formed before a well-ordered Galaxy was in place, so their orbits have both a wide range of eccentricities and no preferred direction of rotation, so that many are going around the Galaxy in the opposite direction to the Sun. Their velocities are so large as seen from the Earth that even from one year to the next there is perceptible motion across the sky. However, even when halo stars are found by this method, in many cases they are not sufficiently primitive to be terribly interesting.

Over the last decade, a new technique has been perfected that sifts out the primitive stars more efficiently. Using large prisms attached to the front of telescopes, astronomers have recorded the spectra of many objects at once. A spectrum is the rainbow of colour produced by an object when its light is split by a prism or alternatively if it is diffracted by a series of closely-spaced lines such as on a CDROM. Although to the human eye a star's spectrum usually appears as a continuous progression of colours, close inspection shows that some colours (or wavelengths) are missing. This removal (absorption) of light is unique to each chemical element, and so by seeing what colours are present, astronomers can deduce what chemicals are present in the stars and in what proportion. (This is how they have found that globular cluster stars have much lower proportions of metals (as low as 0.02%) than the Sun (2%).

The prism approach is not new, but prisms have now been used with special filters that block out unwanted light. Astronomers can then concentrate on the search for primitive objects. With the advent of high-speed computers, it is now possible to teach a computer to recognise the spectrum of a star, and in particular to find the most primitive ones. A new search of this type, based on spectra taken as part of the Hamburg/ESO (European Southern Observatory) spectral study, is now being undertaken by Dr Norbert Christlieb of the Hamburger Sternwarte and co-workers worldwide.

Large telescopes are equipped with instruments capable of studying the spectra of primitive stars in great details. Three of these are:
* the Very Large Telescope (VLT) at  the European Southern Observatory,
* the Subaru Telescope of the National Astronomical Observatory of Japan, and
* the Gemini-south Telescope operated by a consortium of countries including the USA, UK, Canada, Chile, Australia, Argentina, and Brazil.

What will these instruments discover? The best way to answer this question is to see what has been learnt from the first investigations on 4-metre diameter Anglo-Australian Telescope and William Herschel Telescope.

Although astronomers often speak about "metals" as if all of the elements heavier than hydrogen and helium were the same, in reality they form in different nuclear reactions and in different places. Carbon, for example, can be produced directly from helium and this in fact happens in the hot interiors of many giant stars. However, the nucleus of carbon is quite simple to form. Atoms whose nuclei are more complex, and therefore require more nuclear ingredients, often require more extreme conditions. The production of iron, for example, whose atoms weigh almost five times that of carbon and hence require five times as many ingredients, can occur only in the disruptive central regions of very massive stars. In fact, the conditions required to produce iron are so extreme that the star cannot survive the process, and literally explode; this is the phenomenon of a Supernova! As a result of different elements having different sites of production, astronomers can investigate the past history of the Galaxy from the debris of previous generations of stars.

One of the big surprises to have come from recent studies is that there appears to be a large gap in the metal content of the most primitive stars known and the material produced directly from the Big Bang. Although the Big Bang produced only hydrogen, helium, and a tiny fraction of lithium, no stars have yet been discovered with this composition. Instead, even the most primitive stars found to date have at least some carbon, iron, and other metals. How can this be? There are several possibilities. One likely possibility is that the population of stars made purely from Big Bang material, so-called Population III stars which existed before the Population II stars, were so few in number that they are difficult to find. Such a population would indeed have been very rare, because it takes only one supernova to produce a metal content of 0.0002% in a typical-sized star--forming region. The Population III stars which became supernovae could also have evolved and exploded so quickly that they enriched their more slowly forming neighbours, which we observe today as Population II stars. The Population III stars may also have formed before the gas which formed the Galaxy had collapsed into its current location, and hence they will be located even further away from the Sun than the Population II stars in the halo of our Galaxy. A slightly more radical explanation, but nevertheless plausible, is that Population III stars evolved very differently to Population II stars. We know that both Population I (Galactic disk) and Population II (Galactic halo) stars can be produced over a wide range of masses, from approximately 1/10 to 50 times the mass of the Sun. However, Population III stars would contain no metals at all, and it is possible that only higher mass stars can form from gas with essentially zero metal content. Theoretical studies by astronomers in Japan and elsewhere are suggesting that only stars more massive than perhaps twice the mass of the Sun would have formed in Population III. If they are right, then none of these would have survived until the present, because they would have evolved too quickly. This would account very naturally for why we cannot see them now, some 13 billion years after the Big Bang.

A second surprise, related to the elements we see in the stars, is the huge amount of carbon that was produced in the earliest stages of the Galaxy. In the more recent history of the Galaxy, carbon has built up at about the same rate as iron, even though they are produced in different sites. However, the recent studies have shown that many of the primitive stars have ten or even one hundred times more carbon (relative to iron) than stars forming later. This observation has not yet been adequately explained, and observers will be looking at the spectral signatures of other chemical elements to try to find other clues. Two possibilities are being considered at present. One is that primitive stars were better able to throw off their outer layers than more modern stars are. All massive stars go through evolutionary phases in which large amounts of matter are lost into space in a gentle wind (not a supernova explosion). It is possible that Population III stars rotated faster than ones with more metals, and that this gave an extra boost to the loss of their outermost layers. This would enrich the gas clouds with carbon, but not iron whose production requires a supernova. This could account for the higher ratios of carbon relative to iron found in the early stars. Another more radical, but possible suggestion, is that Population III stars evolve very differently to stars with more metals. Recent work by theorists in the USA and Japan have shown that freshly made carbon can be mixed to the outer layers much more readily in Population III stars, supporting the suggestion that is is produced more efficiently relative to iron in those first stars.

As a final example, we turn our attention to two of the heaviest and rarest elements, strontium and barium. The nuclei of these atoms are approximately one and a half times and two times heavier than the nucleus of iron, and are formed by another process completely. Whereas carbon and iron are built up from collisions of composite nuclei, the elements heavier than iron are built up neutron by neutron, starting with an iron seed. The additional neutrons in some cases can convert into a proton, which they do by ejecting an electron. The amount of strontium and barium found in Population I stars has generally been explained via two processes, one occurring when neutrons collide with the seed nuclei irregularly, such as in the outer layers of giant stars, and one occurring when neutrons collide with the compound nuclei much more frequently, such as during a supernova explosion. These are called the s- (slow) and r- (rapid) process respectively. It was thought that the r-process would dominate in Population II stars, and certainly there is evidence for the same r-process products being present in both Population II stars and the Sun. However, in the most primitive stars which have been discovered more recently, the strontium and barium contents are more reminiscent of the s-process, even though this is not expected to have operated so early in the history of the Galaxy, when these stars were still forming. This problem is currently under investigation by astronomers with the hope of determining whether Population III stars performed a different r-process to that seen in more recent generations of stars, or whether differences in the types of massive supernovae, associated with those leading to black holes and those leaving neutron stars as remnants, can explain the differences.

The most primitive stars known have been found not in the most remote reaches of the high redshift universe, but rather in our own back yard, the halo of our Galaxy. They are producing more surprises than imagined about the earliest stages of the formation of the Galaxy and its subsequent evolution. With the imminent availability of new large telescopes capable of obtaining detailed spectra of these stars, some answers, and no doubt more surprises, are on their way.