Everything about Metallicity totally explained
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 is used differently from the usual meaning of the word 'metal', since on the grandest of scales the
universe is overwhelmingly composed of hydrogen and helium, astronomers label all the
heavier elements "metal").
Metallic bonds are impossible within stars, and all but the strongest chemical bonds are impossible in all but 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. The first
stars, referred to as
Population III, had virtually no metals at all. (Anything heavier than helium is considered a metal.) These stars were incredibly massive and, near the end of their
lives, created the first 26 elements up to
iron in the
periodic table via
nucleosynthesis. They subsequently died in spectacular
supernovae (because they've not been observed this isn't proven, but supernovae do create heavier elements) that completely dispersed their material, ejecting those elements throughout the universe. As of
2007, no Population III stars have been found; rather, their existence is inferred in current
models of the
origin of the universe. 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; 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.
Calculation
The metallicity of the Sun is approximately 1.6 percent by mass. For other stars, the metallicity is often expressed as "[Fe/H]", which represents the
logarithm of the ratio of a star's iron abundance compared to that of the Sun. (Iron isn't the most numerous heavy element, but it's among the easiest to measure with spectral data in the visible spectrum.) The formula for the logarithm is expressed thus:
The point of this notation is that if a mass of gas is diluted with pure hydrogen, then its
[Fe/H] value will decrease (since there are fewer iron atoms per hydrogen atom after the dilution), but for all other elements
X, the
[X/Fe] ratios will remain unchanged. By contrast, if a mass of gas is polluted with some amount of pure oxygen, then its
[Fe/H] will remain unchanged but its
[O/Fe] ratio will increase. In general, a given
stellar nucleosynthetic process alters the proportions of only a few elements or isotopes, so a star or gas sample with nonzero
[X/Fe] values may be showing the signature of particular nuclear processes.
Population I stars
Population I or
metal-rich stars are those young stars whose metallicity is highest. The
Earth's Sun is an example of a metal-rich star. These are common in the
spiral arms of the
Milky Way galaxy.
Generally, the youngest stars, the extreme Population I, are found farther in and intermediate Population I stars are farther out, etc. The Sun is considered an intermediate Population I star. Population I stars have regular
elliptical orbits of the galactic centre, with a low
relative velocity. The high metallicity of Population I stars makes them more likely to possess
planetary systems than the other two populations, since
planets, particularly
terrestrial planets, are formed by the
accretion of metals.
Between the intermediate populations I and II comes the intermediary
disc population.
Population II stars
Population II or
metal-poor stars are those with relatively little metal. The idea of
a relatively small amount must be kept in perspective as even metal-rich astronomical objects contain low quantities of any element other than hydrogen or helium; metals constitute only a tiny percentage of the overall chemical makeup of the universe, even 13.7 billion years after the Big Bang. However, metal-poor objects are even more primitive. These objects formed during an earlier time of the universe. They are common in the
bulge near to the centre of the galaxy, the intermediate Population II; and also, in the
galactic halo, the halo Population II, which is older and thus more metal-poor.
Globular clusters also contain high numbers of Population II stars. It is believed that Population II stars created all the other
elements in the
periodic table, except the more unstable ones.
Scientists have targeted these oldest stars in several different surveys, including the HK objective-prism survey of
Timothy C. Beers et al. and the Hamburg-
ESO survey of
Norbert Christlieb et al., originally started for faint
quasars. Thus far, they've uncovered and studied in detail about ten very metal-poor stars (as
CS22892-052,
CS31082-001,
BD +17° 3248) and two of the oldest stars known to date:
HE0107-5240 and
HE1327- 2326.
Population III stars
Population III or
metal-free stars (they contained metals at the very end of their lifetimes - we say they're metal-free because the metals exist in the core and are unobservable) are a hypothetical population of extremely massive and hot stars with virtually no metal content, except for a small quantity of metals formed in the Big Bang, such as Lithium-7. These stars are believed to have been formed in the early universe. They have not yet been observed directly, but indirect evidence for their existence has been found in a
gravitationally lensed galaxy in the very distant universe. They are also thought to be components of
faint blue galaxies. Their existence is necessary to account for the fact that heavy elements, which couldn't have been created in the Big Bang, are observed in quasar
emission spectra, as well as the existence of faint blue galaxies. It is believed that these stars triggered a period of
reionization.
Current theory is divided on whether the first stars were very massive or not. One theory, which seems to be borne out by computer models of
star formation, is that with no heavy elements from the Big Bang, it was easy to form stars with much more total mass than the ones visible today. Typical masses for Population III stars would be expected to be about several hundred
solar masses, which is much larger than the current stars. Analysis of data on low-metallicity Population II stars, which are thought to contain the metals produced by Population III stars, suggest that these metal-free stars had masses of 10 to 100 solar masses instead. This also explains why there have been no low-mass stars with zero metallicity observed. Confirmation of these theories awaits the launch of NASA's
James Webb Space Telescope. New
spectroscopic surveys, such as
SEGUE or
SDSS-II, may also locate Population III stars.
The most massive star that can form today is about 110 solar masses with a possible extreme maximum upper limit of 150 solar masses; a more massive
protostar would blow itself apart during the initial ignition of nuclear reactions. Without enough carbon,
oxygen, and nitrogen in the
core, however, the
CNO cycle couldn't begin and the star wouldn't destroy itself so readily. Direct
fusion through the
proton-proton chain doesn't proceed quickly enough to produce the copious amounts of energy such a star would need to support its immense bulk. The end result would be the star collapsing into a
black hole without ever actually shining properly. This is why astronomers consider Population III to be something of a mystery—by all rights they shouldn't exist, yet they're necessary for an explanation of the quasar observations.
If these stars were able to form properly, their lifespan would be extremely short, certainly less than one million years. As they can no longer form today, viewing one would require us to look to the very edges of the observable universe, since the time it takes light to reach Earth from great distances is extremely great, it's possible to see "back in time" by looking farther away. Seeing to this distance while still being able to resolve a star could prove difficult, even for the
James Webb Space Telescope.
Further Information
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