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'''Metallicity''' is a term in astronomy that refers to the proportion of elements in an astronomical object (usually a star) that are other than [[hydrogen]] or [[helium]]. In astronomy, all elements heavier then hydrogen and helium are collectively referred to with the blanket term "metals"<ref name="metal">https://edocs.uis.edu/jmart5/www/rrlyrae/metals.htm</ref>. Therefore a star or a nebula that contained significant amounts of elements such as [[carbon]], [[oxygen]], or [[nitrogen]] would be considered "metal rich" even though those same elements are classified as nonmetals in chemistry<ref>http://www.wisegeek.com/what-is-metallicity.htm</ref>.
'''Metallicity''' is a term in [[astronomy]] that refers to the proportion of elements in an astronomical object (usually a [[star]]) that are other than [[hydrogen]] or [[helium]]. In astronomy, all elements heavier then hydrogen and helium are collectively referred to with the blanket term "metals"<ref name="metal">https://edocs.uis.edu/jmart5/www/rrlyrae/metals.htm</ref>. Therefore a star or a [[nebula]] that contained significant amounts of elements such as [[carbon]], [[oxygen]], or [[nitrogen]] would be considered "metal rich" even though those same elements are classified as nonmetals in chemistry<ref>http://www.wisegeek.com/what-is-metallicity.htm</ref>.
The metallicity of such astronomical objects may offer clues to those objects ages. According to the [[Big Bang|Big Bang theory]], when the universe first formed, it consisted mostly of hydrogen with a sizable proportion of helium; only trace amounts of lithium and beryllium believed created with no heavier elements forming. Therefore, older stars will have lower metallicity than younger stars, as all other elements were created through Stellar and Supernova nucleosynthesis.
The metallicity of such astronomical objects may offer clues to those objects' ages. According to the [[Big Bang|Big Bang theory]], when the universe first formed, it consisted mostly of hydrogen with a sizable proportion of helium; only trace amounts of [[lithium]] and [[beryllium]] are believed to have been created, with no heavier elements forming. Therefore, older stars will have lower metallicity than younger stars, as all other elements were created through Stellar and Supernova nucleosynthesis.
Usually, metallicity is expressed in terms of the relative amount of iron and hydrogen present in a star, since iron is among the easiest elements to measure with spectral data in the visible spectrum. This is determined by analyzing absorption lines in a stellar spectrum using a spectrometer, compared to the star's solar value. The ratio of the amount of iron to the amount of hydrogen in the object is divided by the ratio of the amount of iron to the amount of hydrogen in the Sun (which has a metallicity of 1.6 percent by mass). This value, expressed as [Fe/H], is calculated using the following logarithmic formula<ref name="metal">https://edocs.uis.edu/jmart5/www/rrlyrae/metals.htm</ref>.
Usually, metallicity is expressed in terms of the relative amounts of [[iron]] and hydrogen present in a star, since iron is among the easiest elements to measure with spectral data in the visible spectrum. This is determined by analyzing absorption lines in a stellar spectrum using a spectrometer, compared to the star's solar value. The ratio of the amount of iron to the amount of hydrogen in the object is divided by the ratio of the amount of iron to the amount of hydrogen in the Sun (which has a metallicity of 1.6 percent by mass). This value, expressed as [Fe/H], is calculated using the following logarithmic formula<ref name="metal">https://edocs.uis.edu/jmart5/www/rrlyrae/metals.htm</ref>.
<center><math> [\mathrm{Fe}/\mathrm{H}] = \log_{10}{\left(\frac{N_{\mathrm{Fe}}}{N_{\mathrm{H}}}\right)_{star}} - \log_{10}{\left(\frac{N_{\mathrm{Fe}}}{N_{\mathrm{H}}}\right)_{sun}} </math></center>
<center><math> [\mathrm{Fe}/\mathrm{H}] = \log_{10}{\left(\frac{N_{\mathrm{Fe}}}{N_{\mathrm{H}}}\right)_{star}} - \log_{10}{\left(\frac{N_{\mathrm{Fe}}}{N_{\mathrm{H}}}\right)_{sun}} </math></center>
Where ''N''<sub>Fe</sub> and ''N''<sub>H</sub> is the number of iron and hydrogen atoms per unit of volume respectively. Using this formula, a star that has a higher metallicity then our Sun will have a positive logarithmic value, while any star with a lower metallicity then our sun will result in a negative value. Because the logarithm is based on powers of ten; a star with a value of [Fe/H] = -1 will have an abundance of heavy elements (metallicity) one tenth of that of our sun, while a value of [Fe/H] =+1 represents a star with ten times the abundance of heavy elements of our sun. Measurements of thousands of stars have resulted in a generally established range from -4 (extremely metal-poor) to +1 (very metal-rich).
Where ''N''<sub>Fe</sub> and ''N''<sub>H</sub> is the number of iron and hydrogen atoms per unit of volume respectively. Using this formula, a star that has a higher metallicity then our [[Sun]] will have a positive logarithmic value, while any star with a lower metallicity then our sun will result in a negative value. Because the logarithm is based on powers of ten; a star with a value of [Fe/H] = -1 will have an abundance of heavy elements (metallicity) one tenth of that of our sun, while a value of [Fe/H] =+1 represents a star with ten times the abundance of heavy elements of our sun. Measurements of thousands of stars have resulted in a generally established range from -4 (extremely metal-poor) to +1 (very metal-rich).
==Types of stars according to Metallicity==
==Types of stars according to Metallicity==
==References==
==References==
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