We like to think of the Sun as a normal star. This idea is not un¬justified, as there is not a great deal to distinguish its spectrum from those of most dwarf stars of the same temperature, and when due account is taken of the effects of temperature and luminosity differences, the solar composition seems fairly typical among stars with ages similar to the Sun’s. Nevertheless, the more stellar spectra that are examined in detail, the more peculiarities seem to emerge.

There is a general tendency for stars that are known to be old, for example by their membership in globular clusters, to have comparatively weak absorption lines in their spectra. This is interpreted as a deficiency of metals, that is. everything heavier than helium, in the material from which these stars were formed. The other main types of unusual spectra fail broadly into three groups: hot stars with extended atmospheres, the peculiar and metallic-line A stars and cool giants of anomalous composition. Each will be discussed in turn.

Hot stars with extended atmospheres
In the terminology BE STAR, the V in Be stands for emission; these stars are characterized by emission lines of hydrogen superimposed on a normal absorption spectrum. The first few members of the Balmer series, are usually affected and sometimes lines of ionized iron, titanium and silicon. The emission lines are sometimes split into two components.

SHELL STARS have sharp, deep absorption lines with emission wings, superimposed on the normal absorption spectrum. Up to 35 members of the Balmer series may be visible, whereas normally only 20 would be detected.

It turns out that Be and shell stars are identical, the difference in appearance being due simply to the direction in which the stars are viewed, as figure 2.19 makes clear. The stars are surrounded by a distended envelope of gas, drawn out into a disc around the equator by rapid rotation. This gas is emitting light, but is cooler than the surface of the star. In case I, the observer sees the absorption spectrum of the star, the emission from the two parts A and B of the shell on either side of the star, and the absorption caused by the gas in region C against the background of the hot star. In case II, the observer sees the large amount of absorption by the extended shell (E) between him and the star, together with a small amount of emission from the thin shell layer near the poles, regions D and F.

Around 15 per cent of all 0 and B stars show emission and shell spectra of this kind. They are subject to irregular spectral and light variations, presumably as the shell structure changes. Two notable examples are Pleione in the Pleiades cluster and y Cassiopeia; the latter was one of the first emission-line stars to be recognized when stellar spectroscopy started at the end of the nineteenth century, and at that time it appeared stable. In the period 1932-37 it under¬went spectral changes, reaching a maximum brightness of 1.5mag. Since 1940 it has stayed at 2.2 mag, with only minor fluctuations.

p CYGNI STARS occur in the temperature range 10 000 to 40 OOOK. Their spectra include numerous emission lines. Associated with the emission lines, and always displaced to the violet, are sharp absorption lines. These features are attributed to expanding, extended atmospheres. P Cygni stars also experience random outbursts. For example, P Cygni itself rose from being invisible to the naked eye to 3 mag in the period AD 1600-1606, after which it declined to 6mag. In 1655 it brightened again, reaching 3.5mag for 4 years, after which it faded to 5.2 mag, to remain so ever since with only minor variations. The hydrogen lines Ha and HP, and other strong lines in the spectra of 0- and B-type supergiante sometimes have profiles like P Cygni, with emission and absorption components.

WOLF-RAYET STARS, believed to be among the hottest visible stars known, were first noticed in 1867 by the French astronomers C J E Wolf and G.Rayet. Their spectra are characterized by the broad emission lines of Heo, He+ ,C+ , C2+, C3+ ,N2+, N3+, N4+, O+, O2+ , O3+, O4+. O5+ and there are very few absorption lines. They are rare objects, with only around 100 brighter than 11 mag known; about halt’ of these are known to be in binary systems. WR stars
occur as the central stars in about one-fifth of the planetary nebulae.

Wolf-Rayet stars fall roughly into two categories: WC stare that have lines of He. C. and 0. and WX stars that have the lines of He and X. There are also some intermediate between the two groups. It is difficult to determine their temperatures accurately bemuse the energy distribution in their continuous spectra does not seem to correspond well with any one temperature. Estimate put the average effective temparatures at 50000K for the WN stars and
40000K tor the WC stars.

The evolutionary status of Wolf-Rayaet stars is unknown. One plausible theory accounts for the two types by assuming they are senile stars that have lost some mass, possibly to a companion in a binary system, According to the amount of material that has been lost, the new surface layer may be either an inner layer where all the carbon has boon turned into nitrogen, or an outer layer still rich in carbon. Perhaps all Wolf-Rayet stars are binaries with undetected companions. If this supposition is right, then unless the companion were it self an O or B star. its spectrum would be too faint to see. and WR lines are so broad that a systematic velocity change would probably not be noticed either.

Certain O-type stars have some of the broad emission lines of the \Volt Ravel spectra weakly present. However, there is no clear distinction between these and O supergiants. all of which probably have extended atmospheres. These O stars are designated Of to distinguish them from the hotter Be-type that are termed Oe stars.

The peculiar and metallic -line A Stars
The term PECULIAR A STAR (Ap star) has come to encompass a whole range of oddities between spectral types B5 and F5. The chief feature that unites the group is the strong, and often variable, magnet ie field that most of the members have. This magnetism was discovered by the pioneering work of the American, H .Babcock who detected the splitting of the spectrum lines (i.e. Zeeman effect) that it causes, Those Ap stars not known to be magnetic have spectral lines too broad for any splitting to be detectable.

Ap stars also have especially strong lines of certain elements,and can be broadly classified into groups according to which lines are enhanced. There is also some correlation of the spectral peculiari¬ties with temperature. Thus, the manganese stars fall at the hot end of the range, the europium-chromium- strontium stars at the cool end, and silicon stars in the middle.

Those magnetic Ap stars that have been carefully observed seem to have cyclic variations in magnetic field, usually in a period of a few days. Associated with the magnetic variations, some of the spectrum lines change in intensity regularly, particularly those of chromium and europium. Typically, the chromium lines are at a maximum when the europium lines are at a minimum. This apparently strange behaviour of both magnetic field and spectrum can be explained reasonably well by a model called the OBLIQUE ROTATOR. The stellar magnetic poles are not close to poles of the rotation axis (unlike the situation on the Earth) and there are concentrations of different elements on different parts of the star’s .surface, perhaps near the magnetic poles. Then, as the star rotates, the N and S magnetic poles alternately come round to the observable side.

Some Ap stars have lines in their spectra that can only be explained by the excess of some very unusual elements. For example, some manganese stars have a strong spectral line for which the only identification seems to be ionized mercury. The very odd star, 3 Cen A has strong lines of phosphorus, gallium and krypton and the helium may be predominantly the isotope 3He. The star HD 101065 seems only to have lines of rare earth elements (notably holmium) in its spectrum, and virtually none of iron and the more usual elements!

The Ap stars are not distinguished from normal stars in any other way apart from their spectroscopic appearance. They occur in clusters and in the general field of stars. They are definitely on or near the main sequence in the Hertzsprung-Russell diagram. The most plausible explanation put forward for the peculiarities is a DIFFUSION PROCESS in the atmospheres which brings certain ions to the surface and makes others sink. There is no observational sup¬port for any theories involving mass exchange or peculiar atmospheric structure.

AM STARS lie in the spectral-type range from AO to FO. They are nothing like so peculiar as the Ap stars. Their chief characteristics are apparent under-abundances of calcium and scandium, slight over-abundances of the iron-peak elements, and over-abundances up to about a factor of 10 in the heavier elements and rare earths. None is known to vary or to have a measurable magnetic field. A large proportion seem to be in binary systems, sometimes with both members being Am stars, and none has so far been identified” with anything more than a moderate rotation velocity (less than 100kms-1). Like Ap stars, they are main-sequence stars, and it is difficult to explain the abundance anomalies. The diffusion theory looks the most promising explanation for the Am stars also”

Cool giants of unusual composition
The coolest stars of the spectral sequence are those classified as K and M, and their spectra are dominated by the bands of closely-spaced lines that originate from molecules rather than individual atoms. In these normal stars, the oxides of the metals titanium, scandium and vanadium are identified. However, there are some giant stars of the same temperature range, (table 2.6) whose spectra contain instead the lines of other molecules.

In the s STAR spectra, the oxides of titanium, scandium and vanadium are replaced by the oxides of the heavier metals zirconium, yttrium and barium. Also, in 1952, P.W.Merrill announced the discovery of lines of the element technetium in the spectra of some S stars. This discovery was remarkable because the longest-lived isotope of this unstable element has a half-life of only 2 x 106 years which is much shorter than the age of the stars. The implication is that we are seeing the results of nuclear reactions which are happening now or have taken place not very long ago in astronomical terms. Practically half of the known S stars are irregular long-period variables.

In the spectra of the c STARS (carbon stars), the metallic oxides are hardly observed, but instead there are strong bands of the molecules CN, C2 and CH. It has been estimated that the ratio carbon/oxygen is four or five times greater in the C stars than in normal stars of the same temperature range (table 2.7). This class of stars is sometimes divided into R types that are hotter, and N types that are cooler examples.

Among the giants of spectral type G and K are a number that nave exceptionally strong lines of the elements strontium and barium and rare earths, and also the carbon compounds, CN, CH and C2. It is noticeable that the enhanced elements in these BARIUM STARS are those which result from a particular nuclear process -called the s-process, which involves the capture of neutrons by lighter elements to form the heavier ones. It may be that the outer layers of these stars have undergone mixing with material from the interior.

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