Even the philosophers of antiquity noted that some stars are grouped together form clusters like the Pleiades. Photographs of the sky show that many stars are found arranged into a number of different groups. In addition to isolated stars in space, known as FIELD STARS , and to the binary stars and multiple stars, there […]

The spectral lines of a rapidly rotating star are broadened by the Doppler effect, because one half of the disc is coming towards the observer, while the other half is moving away from him. The centre of the disc is travelling across the line of sight, so light from there suffers neither blue nor red […]

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 […]

The absorption-line spectra of stars allow astronomers to deduce the relative proportions or the ABUNDANCES of the various chemical elements in the stellar atmospheric layers where the absorption lines are formed. If the physical conditions in a stellar atmosphere can be discovered, then their effect on the appearance of the spec-trum lines of an element […]

Formation of the spectrum Most of the direct evidence we have on the physical conditions in stars is obtained from their spectra. The energy that finally reaches us from a star was originally created in the nuclear reactions in the star’s deepest interior. This energy gradually filters out through the gas of which the star […]

The mass of a star governs the physical nature of its interior, nut masses are hard to determine: they can only be measured for stars in binary systems, and only then under favourable conditions. The two stars in a binary system orbit around their common centre of mass. The properties of the orbit are connected […]

The oldest quantitative observations of stars are connected with the determination of the apparent brightness of a star as soon from Earth. AN unfortunate consequence of this is that the subject is even now Union with historical ballast. In particular, the unit given in antiquity, the MAGNITUDE, is still in regular use. Furthermore there is […]

If we are to understand the nature of heavenly bodies, it is essential to know how far away they are; only then can we calculate the sizes and energy outputs of planets, stars, and galaxies, and ultimately determine the scale of the Universe itself. So far as we can tell, mankind has always considered the […]

This chapter describes the properties of stars that can be derived from observations. Stars may be the most important component of the Universe, and to illustrate this point we may note that life on Earth is sustained by light and heat from the nearest star, our Sun. The stare themselves make up the great galaxies, […]