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
- 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.
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 ( ).
|Age in billion years||Relative metal content
|Relative metal content
(part number of particles X)
|lg (X)||lg (X) -lg (X) sun||comment|
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.
- 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 ).
- ESO: A star that shouldn't exist August 31, 2011