- Horse (Wikipedia)
The horse (Equus ferus caballus) is a domesticated, one-toed, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of Equus ferus. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature, Eohippus, into the large, single-toed animal of today. Humans began domesticating horses around 4000 BCE, and their domestication is believed to have been widespread by 3000 BCE. Horses in the subspecies caballus are domesticated, although some domesticated populations live in the wild as feral horses. These feral populations are not true wild horses, as this term is used to describe horses that have never been domesticated. There is an extensive, specialized vocabulary used to describe equine-related concepts, covering everything from anatomy to life stages, size, colors, markings, breeds, locomotion, and behavior.
- The Puzzle of the Metallic Line Stars
In the puzzle of the metallic line (Am) stars, there still seem to be missing pieces. While the “normal” A stars have elemental abundances close to solar, the classical Am stars show stronger absorption lines for most heavy elements in their spectra. Elements with ionization potentials that nearly agree with those of hydrogen or helium have reduced abundances. The Ca ii and Sc ii lines are especially weak. The Am stars have no ultraviolet emission lines. They are binaries that, with very few exceptions, have rotational velocities vsin i lower than 100 km s−1. Of the main‐sequence A stars, 20% to 30% are Am stars. Here we rediscuss previous suggestions that tried to explain the peculiar line strengths in the Am star spectra. In particular, we compare the well‐studied properties of Hyades A and Am stars in order to identify reasons that can or cannot explain the differences. We find that accretion of interstellar material by A stars with distorted magnetic fields, which are weaker than those in peculiar A (Ap) stars, has the best chance of explaining the main characteristics of the peculiar heavy‐element abundances in Am star photospheres. Charge‐exchange reactions also seem to be important.
- Am star (Wikipedia)
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. 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”.