ASTROPHYSICS, that branch of astronomy which deals with the physical constitution of the heavenly bodies or involves the use of instruments and methods specially dependent on physics. It is contrasted with "astrometry," which deals with the positions and motions of the heavenly bodies. There is no strict line of demarcation, but in a general way we can divide astronomical methods into those making use of general types of equipment (the telescope, camera, micrometer, etc.) and those involving distinctively physical apparatus (the spectroscope, photo-electric cell, thermo-couple, etc.). Similarly, on the theo retical side we distinguish between conclusions based on geom etry or on the law of gravitation (spherical astronomy and celes tial mechanics) and those depending on advanced knowledge of atomic physics and thermodynamics. But even if it were desirable to divide astronomy in this way into two separate branches, the attempt is frustrated by the fact that astrometrical data are commonly found by astrophysical methods, and astro physical data by astrometrical methods.
Astrophysics came into prominence through the application of the spectroscope in the third quarter of the 19th century; and it is mainly in its limited meaning of celestial spectroscopy that we shall give an introduction to it here. The spectroscope, like a glorified prism, takes the light of a body, separates it into its different constituents (different wave lengths) and lays them out side by side for examination. Primarily this spectrum tells us what chemical elements are present in the source of light, each element having its own characteristic set of lines. (See SPECTRO SCOPY.) The lines may appear either as bright emission lines, or as relatively dark lines on a background of continuous light. In either case they are a sign of the presence of the corresponding element, either shining on its own account or robbing the light that comes from lower down in the star of these particular constitu ents. In this way 57 terrestrial elements have been recognized in the sun certainly and nine doubtfully. But absence of the characteristic lines does not necessarily mean that the element is absent or scarce; it may often happen that the temperature and density of the source are not suitable for exciting the spec trum, so that the element, although abundant, does not disclose itself. In any case the spectroscope, like the telescope, reaches only the outermost layers or atmosphere of the star and cannot indicate the chemical composition of the interior.
The first results obtained with the spectroscope related to the chemistry of the stars and nebulae ; but later a much wider field of application was found in relation to the physics of the heavenly bodies. It is just because it is an erratic tool for the chemist that the spectroscope is so valuable for the physicist. It will not show the spectrum of an element unless the physical conditions are suitable; conversely, if it does show the spectrum we can infer that the physical conditions in the star are suitable. For example, we see very prominently in the spectrum of Sirius a series of lines due to hydrogen, and very little besides. We have to ask ourselves, what are the physical conditions which would account for so great a stimulation of this hydrogen spectrum? The answer, given partly by laboratory experience and partly by general physical theory, goes a long way towards settling the temperature and density in the outer layers of Sirius.
At high temperature an atom may become ionized, that is to say one of the electrons in the system of the atom breaks loose. The element then emits an entirely different spectrum. Or two, three, four electrons may break loose; a different spectrum being shown in each case. Stars of fairly low temperature show the spectrum of the complete calcium atom ; those of higher tempera ture show the spectrum of the atom deprived of one electron. At still higher temperature there is no indication of calcium and we infer that it has all become doubly ionized, the calcium atom with two electrons missing being known to give no lines in the part of the spectrum which astronomers can observe.
In the sun and stars, the lines of which we have been speaking appear as dark gaps in the band of light forming the spectrum. But in some of the nebulae they appear as isolated bright lines with little or no continuous background. It is commonly said that continuous spectrum indicates a solid or liquid or highly compressed gas; whilst a bright line spectrum indicates rarefied gas. This is not quite accurate, because a rarefied gas will show a continuous instead of bright line spectrum if we look at a sufficient thickness of it. It is a question of transparency. Light which is strongly emitted by any kind of atom is also strongly absorbed by it; and the internal absorption in a deep layer of material tends to even out the emission in different wave lengths. Thus, if the light is strongly emitted and absorbed, we receive only the emission from a few atoms in the forefront, these forming an opaque screen to the radiation behind ; if the emission is weak we see down to a greater depth, and so the weakness is compensated by the greater number of atoms visible. For a deep layer at uniform temperature this compensation is so com plete that the resulting spectrum is a continuous band inde pendent of the nature of the material and depending only on the temperature; this is known as the "black-body" spectrum. The continuous spectrum from a star is not very different from a black-body spectrum; in fact, it is much closer than we should expect, seeing that the observed layer of the star is by no means at a uniform temperature, the upper part being considerably cooler than the lower part. By measuring the distribution of energy in the spectrum we can determine the temperature; for, as the temperature of a black body rises, the radiation comes more and more from the blue end of the spectrum. This tem perature is commonly called the "effective temperature" or sur face temperature of the star; it is strictly the mean tempera ture of the layers which we actually see. Deep down in the interior the temperature is, of course, far greater.
Bright line spectra are shown by the gaseous nebulae, by tails of comets, and by the uppermost layers (chromosphere and corona) of the sun when viewed transversely at the edge of the disc. We are then looking at extremely rarefied gas, and the layer, although enormously thick compared with terrestrial stand ards, is still thin enough to be transparent. Occasional bright lines are also found in the spectra of some stars superposed on the continuous spectrum. These probably indicate either specially disturbed conditions or that the star is surrounded by an extended nebulous envelope.
We commonly judge stars by their light, but it is quite prac ticable to measure the heat which they send to us across inter stellar space. This is done by placing a thermocouple at the focus of the telescope where the star's rays are concentrated. The chief difficulty is that a great deal of the heat is absorbed in our atmosphere, so that large and sometimes uncertain cor rections must be applied in order to obtain the true output of heat by the star. (A. S. E.)