http://upload.wikimedia.org/wikipedia/commons/thumb/4/4e/Pleiades_large.jpg/800px-Pleiades_large.jpg
Pleiades, an open cluster in the constellation of Taurus are the brightest cellestial bodies than can be seen over the sky, a group of recent born stars close to Earth with less mass than the Sun.

Stars in the galactic halo (around the center of the Milky Way) have extremely low abundances of iron - less than 1/300 the Sun's abundance. These stars formed long ago, before supernovas had contributed much iron to the Milky Way. As a result, the stars acquired little iron at birth. Therefore reflects the chemical enrichement of galactic gas by the stellar life and death cycles over time.

The ancient Milky Way formed a much greater proportion of its stars with masses between 1.5 and 6 times the Sun's. About 5 percent of all modern stars are born in this mass range - but for the ancient Milky Way, the fraction was roughly 40 percent. There lay the fundamental problem: What was the mass function of the very metal-poor stars - the earliest stars to form? Scholars have only clues and circumstantial evidence.

Today, most stars are born with less mass than the Sun. But theorists have long speculated the ancient Milky Way gave birth to stars with a different distribution of masses, or initial mass function. In particular, the ancient Milky Way's initial mass function may have favored the birth of stars more massive than the Sun.

Recent compilation on the compositions of more than 2.5 million stars in the Milky Way made by the Sloan Digital Sky Survey ( SDSS-II) were described in the paper "The Milky Way Tomography with SDSS-II: Stellar Metallicity". The map survey has been able to locate metals in millions of Milky Way stars.

Astronomers use the term "metals" to describe all elements heavier than hydrogen and helium, including the oxygen we breathe, calcium in bones and the iron in blood. Although hydrogen, helium and traces of lithium were created at the beginning of the universe in the Big Bang, all other elements, such as iron and carbon, were forged in the cores of stars or during the explosive deaths of massive stars.

As a result, stars that formed early in the history of our galaxy 13-billion years ago, were made of gas that had few metals created by the generations of stars that came before. These "metal-poor stars" provide astronomers with a chemical fingerprint of the origin and evolution of the elements. As subsequent generations of stars formed and died, they returned some of their metal-enriched material to the interstellar medium, the birthplace of later generations of stars, including the sun.

According to "www.astro.utu.fi":

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As we have seen, the galaxy is thought to have collapsed from a cloud of primordial gas (whose composition was set essentially by the Big Bang) consisting of about 75% Hydrogen and 25% Helium plus a very small amount of the other elements (much less than 1%). As this cloud collapsed generations of stars were born.

The stars that explode as supernova at the end of their lives distribute back into the primordial gas a large amount of heavy elements which have been built up in the star via nucleosynthesis. As a result the metallicity of the gas from which each new generation of stars forms rises steadily, which results in correlations between the motions of stars and their metallicities.

As these stars formed there has been a generally rising metallicity, although with a great deal of scatter. There is quite good relationship between metallicity and age (called the AMR or Age-Metallicity Relation). Studies within the last decade have shown that this relationship is much weaker and contains more intrinsic scatter than previously thought.

On the relationship between metal abundance Fe/H, there is a general trend of rising abundance with decreasing stellar age, with the important caveat that at any given age, there is a large intrinsic scatter in the metallicities of stars. The youngest stars (age 2 Billion years) have metallicities close to the solar value Fe/H = 0.0, while the oldest stars in the survey have metallicities about 1/5th of the solar value.
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Source(s):
http://www.astronomy.com/asy/default.aspx?c=a&id=2755

http://www.astronomyreport.com/research/New_map_locates_metals_in_millions_...

http://www.astro.utu.fi/~cflynn/galdyn/lecture2.html

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