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The metallicity , i.e. H. the metal abundance is a term used in astrophysics for the abundance of heavy chemical elements in stars .

In contrast to the chemical meaning of this term, the term “ metals ” usually refers to all elements except hydrogen and helium , less often the elements from carbon , i.e. from an atomic number of six.

Formation of heavy elements

The heavy elements in the universe were only formed by nuclear reactions in stars ( nucleosynthesis ), which is why the metallicity is closely related to the time a star was formed:

  • Stars with low metallicity ( Population II ) were formed in an earlier stage of development of the universe, when only a few “metals” were present.
  • Stars with high metallicity ( Population I ) emerged at a later point in time from the “ashes” of earlier generations of stars, which were enriched with heavy elements.

The elements lithium , beryllium and boron , between helium and carbon, occur in very low concentrations in stellar atmospheres. They cannot come from stars, because much faster synthesis steps destroy them again immediately. Apart from the cosmological part of lithium-7, they come from the spallation of heavier elements by cosmic radiation in the interstellar gas .


Relative values: related to the sun

As a measure of the metallicity of a star, it is often not the mass, but the number of particles in its heavy elements that is related to that of hydrogen; this relative element abundance can be determined from the measured strengths of the absorption lines of iron and hydrogen. For normal main sequence stars the relative abundance of elements is then compared (normalized) as a logarithmic ratio with the corresponding abundance of the sun , since the elements accumulate uniformly in the universe :

According to this formula

  • the sun (index ) has a metallicity of 0 by definition,
  • Stars with a positive metallicity contain relatively more iron than the sun and are therefore younger,
  • Stars with a negative metallicity contain relatively less iron than the sun and are therefore older.

Absolute values

The ratio of the number of particles used above between iron and hydrogen atoms in the sun is:

, which corresponds to a logarithmic value of −4.5 ( ).

Therefore the mass fraction of iron in the solar mass is approx. 0.16%.

Clues to the ages of the main sequence stars
Age in billion years Relative metal content
(mass fraction)
Relative metal content
(part number of particles X)
lg (X) lg (X) -lg (X) sun comment
11.75 0.04% 0.0008% −5.114 −0.619
04.57 0.16% 0.0032% −4.5 0 Sun
02.40 0.40% 0.0077% −4.114 0.381
01.45 0.80% 0.0154% −3.813 0.682
00.90 2.00% 0.0385% −3.415 1.080
00.55 5.00% 0.0962% −3.017 1.478

The general frequency pattern no longer applies to chemically peculiar stars or stars that have already developed away from the main sequence.


The metallicity of stars in our galaxy is roughly between −5.6 and +1 (indicated in each case as , i.e. not related to the sun), with only the oldest stars in population II reaching a value in the range −5 and only a few of them known are:

  • The long-term front runner was the star CD − 38 ° 245 , the metallicity of which was determined to be −4.0 in 1984. This means that its iron content is 10,000 times less than that of the sun.
  • In 2002, HE 0107-5240, a star with a metallicity of −5.2 was discovered,
  • soon afterwards the star HE 1327−2326 with a value of −5.4, which means an iron content of 250,000th of the solar value. However, this star surprisingly contains a very large proportion of other elements such as sodium , magnesium , titanium and, above all, strontium .
  • The star SDSS J102915 + 172927 ( relative magnitude  16.9) appears to be almost metal-free. The lack of lithium is explained by the high temperature of the star.
  • The metallicity of iron in SMSS J031300.36-670839.3 is less than −7.1.

Usually the abundances of other elements such as thorium , uranium , iridium and carbon are also determined for such stars to determine the age and categorization .


  • Bradley W. Carroll, Dale A. Ostlie: An Introduction to Modern Astrophysics. Addison-Wesley, Reading MA et al. 1996, ISBN 0-201-54730-9 , pp. 920f. (International Edition. Reprint. Ibid 2005, ISBN 0-321-21030-1 ).

Web links

Individual evidence

  1. Iron is about 56 times as heavy as hydrogen
  3. The Star That Should Not Exist
  4. Anna Frebel : On the trail of the star old. In: Spectrum of Science. September 2008, pp. 24-32