An Am star or metallic-line star is a type of chemically peculiar

star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

spectral type In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...

A whose

spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors ...

has strong and often variable absorption lines of metals such as

zinc Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...

strontium Strontium is the chemical element with the symbol Sr and atomic number 38. An alkaline earth metal, strontium is a soft silver-white yellowish metallic element that is highly chemically reactive. The metal forms a dark oxide layer when it is e ...

zirconium Zirconium is a chemical element with the symbol Zr and atomic number 40. The name ''zirconium'' is taken from the name of the mineral zircon, the most important source of zirconium. The word is related to Persian '' zargun'' (zircon; ''zar-gun'' ...

, and

barium Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in group 2 and is a soft, silvery alkaline earth metal. Because of its high chemical reactivity, barium is never found in nature as a free element. Th ...

, and deficiencies of others, such as

calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar ...

and

scandium Scandium is a chemical element with the symbol Sc and atomic number 21. It is a silvery-white metallic d-block element. Historically, it has been classified as a rare-earth element, together with yttrium and the Lanthanides. It was discovered in ...

. The original definition of an Am star was one in which the star shows "an apparent surface underabundance of Ca (and/or Sc) and/or an apparent overabundance of the Fe group and heavier elements". The unusual relative abundances cause the spectral type assessed from the Calcium K lines to be systematically earlier than one assessed from other metallic lines. Typically, a spectral type judged solely from hydrogen lines is intermediate. This leads to two or three spectral types being given. For example,

Sirius Sirius is the brightest star in the night sky. Its name is derived from the Greek word , or , meaning 'glowing' or 'scorching'. The star is designated α Canis Majoris, Latinized to Alpha Canis Majoris, and abbreviated Alpha CM ...

has been given a spectral type of kA0hA0VmA1, indicating that it is A0 when judged by the Calcium k line, A0V when judged by its hydrogen lines, and A1 when judged by the lines of heavy metals. There are other formats, such as A0mA1Va, again for Sirius. The chemical abnormalities are due to some elements which absorb more light being pushed towards the surface, while others sink under the force of

gravity In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...

. This effect takes place only if the star has low rotational velocity.Am star
''The Internet Encyclopedia of Science'', David Darling. Accessed on line August 14, 2008. Normally, A-type stars rotate quickly. Most Am stars form part of a binary system in which the rotation of the stars has been slowed by tidal braking. The best-known metallic-line star is

(α Canis Majoris). The following table lists some metallic-line stars in order of descending apparent visual magnitude.

List

δ Delphini and ρ Puppis

A small number of Am stars show unusually late spectral types and particularly strong luminosity effects. Although Am stars in general show abnormal luminosity effects, stars such as ρ Puppis are believed to be more evolved and more luminous than most Am stars, lying above the

main sequence In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar He ...

. Am stars and δ Scuti variables lie in approximately the same location on the H–R diagram, but it is rare for a star to be both an Am star and a δ Scuti variable. ρ Puppis is one example and δ Delphini is another. Several authors have referred to a class of stars known as δ Delphini stars, Am stars but with relatively little difference between the calcium and other metallic lines. They have also been compared to the δ Scuti stars. Later studies showed that the group was somewhat inhomogeneous, possibly coincidental, and recommended dropping use of the δ Delphini class in favour of a narrower class of ρ Puppis stars with relatively high luminosity and late spectral types. However, there is still sometimes confusion, for example with ρ Puppis stars being considered to all be δ Scuti variables.

Notes and references

{{star Star types