78
Viewing the Constellations with Binoculars
emits the most light at the wavelength of 830 nm and therefore has a lower effective temperature, about 3,000 K. A star surface is not like the surface of a solid planet. Stars are gaseous, so their surface is composed of a layer of gas several hundreds kilometers thick and becomes thinner with increasing height. It is from the surface that most of the light is emitted and eventually reaches us. Inner layers of the surface emit a continuous spectrum. Atoms in more rarefied outer layers absorb certain wavelengths from the continuous spectrum. Dark absorption lines appear in the stellar spectrum. From these, astronomers determine the chemical composition and even abundances of certain chemical elements. And this is by far not all! We can learn a lot about emitting gas also from the shape of the spectral line. If it is narrow then the gas is rare and has a moderately low temperature. If the line becomes broader at high temperatures, it means the atoms are moving faster, toward and away from the observer, and because of the Doppler effect the observed wavelength is shorter and longer. The line also broadens during the rotation of the star around its own axis, due to the collisions in the dense gas between atoms and to a strong magnetic field, if there is one. The width and shape of the spectral lines can therefore be used to deduce the density of gas in a stellar atmosphere or in interstellar nebulae; it can also be used to determine the rotation velocity of a star and strength of the magnetic fields. Regular monitoring of spectral lines and their eventual periodic oscillation due to the motion of the star toward and away from us can reveal spectroscopic binaries as well as more massive cold companions – brown dwarfs and planets.
Classification of Stars Toward the end of nineteenth century, astronomers at Harvard Observatory began to systematically take photographs of the stellar spectra. Taking into account detailed spectral characteristics, Edward C. Pickering (1846–1919) classified stars into classes, which he labeled alphabetically from A to Q. The basis for his classification system lay in the absorption lines of hydrogen and helium, with the addition of some other elements, for instance, iron in the case of colder stars. His coworker Annie Cannon, who classified almost a quarter of a million stars by their spectra, established that certain classes were unnecessary. This is why nowadays only the fundamental spectral classes O, B, A, F, G, K, and M remain. Spectral class O corresponds to the highest surface temperatures of stars, while M corresponds to the lowest. We can easily remember the classes with a help of an old mnemonic, ‘‘Oh, Be a Fine Girl, Kiss Me.’’ With the advance of spectroscopy, which soon became one of most important branches of astronomical observation, it was discovered that stars within the same spectral class may differ. So each main class was further divided into subclasses, designated by Arabic numbers, for example, G2 for our Sun. For an even more detailed definition of the stellar spectrum, prefixes and endings were added, and we write these with a small or capital letter of the alphabet. Here’s an illustration: normal dwarf stars obtained the prefix ‘‘d,’’ while white dwarfs obtained ‘‘D.’’ For the purposes of designating special groups of stars, a new and expanded classification system was introduced. Fundamental classes were joined by class W for Wolf–Rayet-type stars, L for red and brown dwarfs, T for cold methane dwarfs, Y for very cold dwarfs, C for carbon stars (the older classes R and N were joined to form this class), P for planetary nebulae, Q for novae, and so on. In addition to this, it was soon discovered that the stars within the same spectral class can substantially differ in their luminosities. Because of this, new luminosity classes were introduced, and these are designated by a Roman numeral: Ia are super giants with intense luminosities, Ib are super giants with less intense luminosities (compared to class Ia, of course), II are bright giants, III are