2008-08-09T08:44:00.000-07:00

The globular cluster M80. Stars in globular clusters are mainly older metal-poor members of Population II.

In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium. This terminology should not be confused with the usual meaning of the word "metal". Since on the grandest of scales normal (i.e. atomic) matter in the universe is composed overwhelmingly of hydrogen and helium, astronomers label all the heavier elements "metal".^[1] Metallic bonds are impossible within stars, and only the strongest chemical bonds are possible at all in the coolest layers of "cool" K and M stars, so normal chemistry has little or no relevance in stellar interiors. A nebula rich in carbon, nitrogen, oxygen, and neon would be "metal rich" in astrophysical terms even though those elements are nonmetals in conventional chemistry.

The metallicity of an astronomical object may provide an indication of its age. When the universe first formed, according to the Big Bang theory, it consisted almost entirely of hydrogen which, through primordial nucleosynthesis, created a sizeable proportion of helium and only trace amounts of lithium and beryllium and no heavier elements.

While older stars do have fewer heavy elements, the fact that all stars observed have some heavier elements poses something of a puzzle, and the current explanation for this involves proposing the existence of Population III stars. Without metals, it is believed that only incredibly massive stars could be formed, and near the end of their lives, created the first 26 elements up to iron in the periodic table via nucleosynthesis. Because of their high mass, the current model is that they subsequently all died in spectacular supernovae that completely dispersed their material, ejecting those elements throughout the universe, and forming later generations of stars, with heavy elements, that we see today. The high mass of the first stars is used to explain why, as of 2007, no Population III stars have been observed. Because they were all destroyed in supernovae in the early universe, Population III stars should only be seen in far away galaxies whose light originated much earlier in the history of the universe, and searching for these stars or establishing their nonexistence (thereby invalidating the current model) is an active area of research in astronomy.

The next generation of stars was born out of those materials left by the death of the first. The oldest observed stars, known as Population II, have very low metallicities;^[2] as subsequent generations of stars were born they became more metal-enriched, as the gaseous clouds from which they formed received the metal-rich dust manufactured by previous generations. As those stars died, they returned metal-enriched material to the interstellar medium via planetary nebulae and supernovae, enriching the nebulae out of which the newer stars formed ever further. These youngest stars, including the Sun, therefore have the highest metal content, and are known as Population I stars.

Across the Milky Way, metallicity is higher in the galactic centre and decreases as one moves outwards. The gradient in metallicity is attributed to the density of stars in the galactic centre: there are more stars in the centre of the galaxy and so, over time, more metals have been returned to the interstellar medium and incorporated into new stars. By a similar mechanism, larger galaxies tend to have a higher metallicity than their smaller counterparts. In the case of the Magellanic Clouds, two small irregular galaxies orbiting the Milky Way, the Large Magellanic Cloud has a metallicity of about forty per cent of the Milky Way, while the Small Magellanic Cloud has a metallicity of about ten per cent of the Milky Way.

Metallicity