Am star
Encyclopedia
An Am star or metallic-line star is a type of chemically peculiar star
of spectral type A whose spectrum
has strong and often variable absorption lines of metals such as zinc
, strontium
, zirconium
, and barium
, and deficiencies of others, such as calcium
and scandium
. These abnormalities are due to some elements which absorb more light being pushed towards the surface, while others sink under the force of gravity. This effect takes place only if the star has low rotational velocity.
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 Sirius
(α Canis Majoris.) The following table lists some metallic-line stars in order of descending apparent visual magnitude.
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
of spectral type A whose spectrum
Spectrum
A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...
has strong and often variable absorption lines of metals such as zinc
Zinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...
, strontium
Strontium
Strontium is a chemical element with the symbol Sr and the atomic number 38. An alkaline earth metal, strontium is a soft silver-white or yellowish metallic element that is highly reactive chemically. The metal turns yellow when exposed to air. It occurs naturally in the minerals celestine and...
, zirconium
Zirconium
Zirconium is a chemical element with the symbol Zr and atomic number 40. The name of zirconium is taken from the mineral zircon. Its atomic mass is 91.224. It is a lustrous, grey-white, strong transition metal that resembles titanium...
, and barium
Barium
Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in Group 2, a soft silvery metallic alkaline earth metal. Barium is never found in nature in its pure form due to its reactivity with air. Its oxide is historically known as baryta but it reacts with...
, and deficiencies of others, such as calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
and scandium
Scandium
Scandium is a chemical element with symbol Sc and atomic number 21. A silvery-white metallic transition metal, it has historically been sometimes classified as a rare earth element, together with yttrium and the lanthanoids...
. These abnormalities are due to some elements which absorb more light being pushed towards the surface, while others sink under the force of gravity. This effect takes place only if the star has low rotational velocity.
Normally, A-type stars rotate quickly. Most Am stars form part of a binary system
Binary star
A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...
in which the rotation of the stars has been slowed by tidal braking
Tide
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the moon and the sun and the rotation of the Earth....
.
The best-known metallic-line star is Sirius
Sirius
Sirius is the brightest star in the night sky. With a visual apparent magnitude of −1.46, it is almost twice as bright as Canopus, the next brightest star. The name "Sirius" is derived from the Ancient Greek: Seirios . The star has the Bayer designation Alpha Canis Majoris...
(α Canis Majoris.) The following table lists some metallic-line stars in order of descending apparent visual magnitude.
| Name | Bayer Bayer designation A Bayer designation is a stellar designation in which a specific star is identified by a Greek letter, followed by the genitive form of its parent constellation's Latin name... or other designation | Apparent visual magnitude |
|---|---|---|
| Sirius A | α Canis Majoris A | −1.47 |
| Castor Ba | α Geminorum Ba | 2.96 |
| α Volantis | 4.00 | |
| Acubens A | α Cancri A | 4.26 |
| Kurhah | ξ Cephei | 4.29 |
| θ1 Crucis | 4.30 | |
| 2 Ursae Majoris | 5.46 | |
| τ3 Gruis | 5.72 | |
| WW Aurigae WW Aurigae WW Aurigae is an eclipsing binary in the constellation Auriga. It is approximately 275 light-years from Earth.Both components, WW Aurigae A and WW Aurigae B, are detached white A-type main sequence dwarfs with a mean apparent magnitude of +5.82... |
5.82 |