Metallicity

The metallicity, that is, the metal abundance, is a common name in astrophysics for the frequency of heavy chemical elements in stars.

As "Metals " will be, otherwise, usually referred to by the chemical meaning of that term all elements except hydrogen and helium, the rare elements from carbon, ie from an atomic number of six.

Formation of heavy elements

The heavy elements were formed in the universe only by nuclear reactions in stars ( nucleosynthesis ), so the metallicity is closely related with the formation time of a star:

• Stars with low metallicity ( Population II ) were created in an earlier stage of development of the universe, as a few " metals " were only available.
• Stars with high metallicity ( Population I ) have arisen at a later date from the enriched with heavy elements " ashes" of previous generations of stars.

The elements lithium, beryllium and boron, between helium and carbon are present in stellar atmospheres at very low concentrations. You can not come from stars, because much faster synthesis steps destroy them again. They come - except for the cosmological share of lithium -7 - from the spallation of heavier elements by cosmic rays in the interstellar gas.

Units

The mass fraction of heavy elements in the Sun's mass is about 1.8%. For comparison with other stars, but not the mass, but the number of atoms is often chosen as a reference; they are the element frequency of all the heavy elements relative to hydrogen relative to the frequencies of the sun again. Since the elements uniformly accumulate in the universe, this is sufficient for normal main sequence stars. In the sun, the ratio between iron and hydrogen atoms:

Often a normalized to the sun, logarithmic ratio of the strength of the absorption lines of iron and hydrogen is given:

According to this formula is the metallicity of the sun just 0

Populations

The metallicity stars in our galaxy is approximately between -5.6 and 1, with only the oldest stars of Population II reach a value between -5 and few of them are known. In chemically peculiar or already developed away from the main sequence stars of this general frequency pattern, however, is no longer correct. In addition, the sun is shining, compared with the stars of the immediate neighborhood, have almost twice as many " metals ".

Longtime leader was the star CD -38 ° 245, whose metallicity was determined in 1984 with -4.0. This means that its content of iron is less than 10,000 times that of the sun. 2002 was discovered with HE 0107-5240 a star with the value of -5.2, and soon after the star HE 1327-2326 with a value of -5.4, which means an iron content of a 250.000stel of the solar value. However, this star surprisingly contains a very large amount of other elements, such as sodium, magnesium, titanium, and especially strontium. SDSS J102915 172927 The star (relative magnitude 16.9 ) appears to be virtually free of metal. The absence of lithium is explained by the high temperature of the star.

Usually, in such stars, the abundances of other elements such as thorium, uranium, iridium and carbon are determined, which can be used for age determination and categorization of the stars.

As a guide for the age of the main sequence stars following values ​​are used: