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The galaxies with the lowest metallicity (extremely metal poor galaxies) are of interest at high redshift for representing an early step toward the typical chemical abundances of stars and galaxies found today, and those nearer are of interest as a means to study what the very early galaxies must have been like. A common cut-off for selecting such galaxies is 1/10 solar metallicity, often using oxygen abundance as an indicator using the absolute 12 scale (rather than the bracket notation that compares abundances to those of the Sun), εO defined as:
εO = 12 + log10(NO/NH)
where NO is the number of oxygen atoms, etc. By this scale the solar abundance of oxygen is about 8.7 so less than 7.7 is 1/10 solar or less.
Such galaxies naturally include extremely metal poor stars, which are of interest whether within such galaxies or within Milky Way or any particular galaxy. Of similar interest are galaxies' related groups of stars (such as stellar clusters) if they are similarly extremely metal poor.