A Stars: Definition

The Stars of spectral type A are characterized by the great strength of the hydrogen absorption lines in their spectra. On the main sequence they range in temperature from 7500K at A9 to 9900K at A0. Their corresponding masses and radii, in terms of the Sun, are 1.8 and 1.4 respectively at A9, increasing to 3.2 and 2.5 at A0.

A Stars: Description

Whereas in the other principle spectral types peculiarities are uncommon, among the A stars abnormalities are the rule rather than the exception. This as a result of the most incomplete absence of convection near the surface of A stars: minor chemical composition anomalies that are mixed into the interiors of the stars are able to remain at the surface of A stars. In stars of other spectral types emission lines from their chromospheres can be observed in the far ultraviolet spectra, but in the A stars such chromospheres, which derive their high temperatures from the energy in the convective zones, are absent and emission line A stars are consequently extremely rare.

Roughly 10per cent of the A stars have strong magnetic fields (from a few hundred to more than 20,000 gauss.) In these certain chemical elements become concentrated in their atmospheres and produce unusually strong spectrum lines. Such stars were classified as peculiar A (Ap) stars long before it was realized that they were magnetic. There are two principle types of peculiarities in spectral Ap stars: the strontium-chromium-europium stars and the silicon stars. As the overabundances of these elements are concentrated in patches of the surface of the stars (often at the magnetic poles) the strengths of the anomalous spectrum lines in the Ap stars change as the stars rotate. Therefor most Ap stars are spectrum variables.

Another kind of anomaly produces the metallic line (Am) stars. In these, the hydrogen line strengths and temperatures are those of normal A stars but the spectrum lines of the metals are usually strong – more characteristic of those seen in F stars. The brightest star visible at night, Sirius is an Am star.

The Ap and Am stars lie on or near the main sequence, but the metallic line phenomena is also seen among the sub-giants and giant A stars. These are called, after the prototype Delta Delphini stars.

There are other, rare kinds of anomalous spectra among the A stars, include the Lambda Bootis stars, which have very weak metal lines without belonging to population II. The main peculiarities in the Ap and Am stars are currently thought to be a result of diffusion in their atmospheres. In an atmosphere that is not mixed by convection, separation of chemical elements cam occur because of imbalance between gravitational force and pressure among the minor constituents of the atmosphere. An element that absorbs light efficiently experiences more radiation pressure that the one that does not. Thus, some elements are concentrated in the upper atmosphere of a star and the other sinks out of sight.

Even without convection, currents set up by rapid rotation can mix the outer regions of a star. Normal A stars rotate quite rapidly, preventing diffusive separation of the elements, but the Ap anAm stars are all found to be relatively slow rotators, in accord with the diffusion explanation of their peculiarities. The slow rotation rates of Ap stars probably results from magnetic breaking. But the Am stars (which do not have magnetic fields) rotate slowly because they generally are A stars in binary systems with orbital period less than 100 days.

A class main sequence stars burn much more brightly than the Sun. They exhaust their hydrogen in around 100 million years.
Like F and G stars, they pass through a series of Red Giant phases. Their spent cores become White Dwarfs.

A Stars: Variability/Peculiarity

Four type of pulsational variability appear among the A stars: Delta Scuti stars, which are population I stars near the main sequence; Dwarf Cepheids, which resemble the Delta Scuti stars but with larger amplitude; the RR Lyrae stars of population II and; the cooler Ap stars, which oscillate with periods of 5 to 15 minutes. No variability is known among the Am stars. Bright examples (A stars appear white to the neaked eye); Sirius Alm V; Vega A0; Alair A7 V; Deneb A2Ia.