What Is A "Metal" in Stellar Astronomy?
      When astronomers discuss the metal content of a star, they are not talking about copper, iron, gold or platinum, materials that we consider metals on Earth. They are talking about multi-electron atoms other than hydrogen and helium, which make up between 96 and 99% of the mass in most stars (in the Sun such multi-electron atoms represent about 2% of the mass, while hydrogen and helium make up about 98% of the mass). Hydrogen and helium ("non-metals") make up close to 100% of all the atoms in stars that were formed in the early Universe, as the conditions in the Cosmic Fireball that accompanied the so-called "Big Bang" origin of the Universe were only capable of creating hydrogen and helium atoms in significant numbers. A very tiny number of isotopes of those atoms, and other relatively simple atoms such as lithium and boron were also produced, but basically the earliest generations of stars formed out of material that considered almost entirely of hydrogen and helium. Since those are simple atoms, the way in which they can absorb light is very simple as well, and their spectra consist of only a relatively few absorption or emission lines widely scattered through the spectrum of electromagnetic radiation. As a result, they poorly interact with light streaming through a star.
      However, when the earliest generation of massive stars were born, lived out their very short lives and annihilated themselves in supernova explosions, virtually all the other elements were created either during their life or death. These other elements have much more complex structures, and have spectra that involve the absorption and emission of almost innumerable wavelengths. As a result, they can easily interact with light, and their presence in later generations of stars strongly affects the passage of light through their interiors. This difference in their interaction with light is very important, and as a result, all atoms significantly heavier and more complex than hydrogen and helium are referred to as "metals". In the Sun, they represent about 2% of the mass, and about one atom in a thousand. But despite their small numbers, their influence on the passage of light through a star can make a huge difference in its structure.

How Do "Metals" Affect the Heat Flow In Stars?
      If the metal content of a star is very low (as in first-generation stars, early in the history of the Universe), light passes through the gases inside the star far more easily than if the metal content is very high. This means that in the pre-Main Sequence contraction/collapse, the various stages of stellar formation occur far more rapidly. It also means that once the stars are on the Main Sequence, heat passes from their interior to their outside far more easily, and radiative zones are larger, while convective zones are smaller. For such stars to be stable, they must contract to a smaller size, so that their increased density offsets the lesser ability of their gases to block the outward passage of light. One result of this is that Main Sequence stars formed earlier in the Universe, when the metal content of stars was small, were a little smaller and fainter than stars like the Sun, which thanks to many generations of supernovae enriching the metal content of the gases that comprise them, have a higher metal content. It also means that stars now forming, which have a still higher metal content, are a little larger and brighter than the Sun when first formed.
     In the discussion of the Late-Main Sequence Life of the Sun I discuss how as the Sun runs out of hydrogen in its core it compensates for that by undergoing a slow contraction of the core, which paradoxically allows the poorer-quality fuel to actually increase the brightness of the star. At first this is very slow, and the Sun is still considered a Main Sequence star, despite being about 50% brighter than it was when first formed. However, despite that increase in its brightness, the Sun still sits right on the Main Sequence, instead of slightly above it. How is this possible?
      It is possible because subsequent generations of stars, which are now the youngest Main Sequence stars, have a higher metal content (close to 4% in our galaxy), and as a result, are little bigger and brighter than the Sun was when it first formed. As it happens, for stars with masses similar to the Sun, the increase in size and brightness due to their higher metal content is just about the same as the Sun's inrease in size and brightness because it is "aging", so it has about the same brightness as its younger compatriots. It also means that there is a small spread in the properties of Main Sequence stars of a given mass, because younger stars with metal compositions similar to the Sun's are fainter, while older stars with higher metal content are brighter.

How Do "Metals" Affect the Maximum Size and Brightness of Stars?
      For the most massive Main Sequence stars, the huge amount of heat and light pouring out of their interiors expands the stars to such a great extent that even in the cooler outer regions the density of the gas is not high enough to cause the development of a convective zone, so the entire star consists of a radiative zone. As it turns out this puts a limit on how massive stars can be. For a star's outer layers to be stable, they must have enough weight compressing them to overcome the pressure exerted on them by the flood of photons pouring out of the stellar interior. If the metal content of such a star is high, the star will block the light more effectively, but at the cost of suffering a greater outward push on its outer layers. If the metal content of such a star is low, the star will not block the outward-pouring light very effectively, and since the light passes through the gas with less interaction, it does not push it outward as hard. This strongly affects the maximum mass and brightness of stars of differing metal contents. Stars with metal content similar to the mass of the Sun cannot have masses much more than 100 solar masses. The strong interaction between their outer layers of gas with the light pouring out of the interior pushes the gas outward more strongly than the weight of the layers above the gas compresses it; so it flows out into space, reducing the mass of the star. Younger, more metal-rich stars, will eventually have even lower upper mass limits. But on the other hand, the earliest generations of stars, which had essentially no metal content at all, could have been much more massive (perhaps as much as 200 to 300 times the mass of the Sun) and still had stable outer layers. As a result, those stars would have been much larger and brighter than the brightest stars existing today (and as a result, had a much shorter lifetime).
      Of course, this theoretical result, though reasonably certain, would be considered more reliable if we could find actual examples of stars with extremely low metal content, and see if their brightest and most massive siblings really were as much brighter and more massive as suggested above. As a result, in recent years there has been a search for unusally metal-poor galaxies that are relatively close to our Milky Way galaxy -- galaxies that for one reason or another had great difficulty in creating early generations of stars, and as a result, are more like the galaxies formed in the early Universe than our own Galaxy. And as a result of that search, we have recently found a number of galaxies that do have very low metal content. In general, they are very small, so that their gravitational compression is very low, and it has taken a long time for them to compress their gases enough to form stars. A number of such galaxies have been found with metal contents as low as 3% the metal content of the Sun, and the current record holder, AGC198691, has only 1/40 the metal content of the Sun. Such galaxies should contain relatively few bright stars, since it is so difficult for them to form stars at all, but some of those stars should be close to the maximum mass of low-metal-content stars, or in other words, up to 200 to 300 times the mass of the Sun, and several millions or tens of millions of times the brightness of the Sun. Given the imporance of such galaxies for testing the theories of how the metal content of stars affects their structure, is briefly discussed immediately below.

The Extremely Metal-Poor Galaxy AGC198691
      A magnitude 18.8(?) dwarf irregular galaxy (type dIrr?) in Leo Minor (RA 09 43 32.4, Dec +33 26 58)