This chapter concerns the smallest of all galaxies. Examined more closely, they reveal, apart from their small dimensions, further properties which distinguish them strongly from their larger siblings.

THE MORPHOLOGY OF DWARF GALAXIES

The classification of irregular galaxies often contains a separate group of especially small, dim galaxies. These are dwarf galaxies. Dwarf galaxies are differentiated according to their appearance: blue compact dwarfs (BCDs), dwarf spheroidals (dSph), dwarf ellipticals (dE), and dwarf irregulars (dIrr), which also include tidal dwarfs. The terms are not used uniformly in the literature, which is due to the fact that because of the difficulty of detecting them, studies of these galaxies began in earnest only in the last two decades. A further type is ultra-compact dwarf galaxies (UCDs), which were discovered in nearby galaxy clusters in 1999. The UCDs are more compact than other dwarf galaxies and their appearance is similar to that of bright globular clusters.

The term “d” for dwarfs was introduced to extend the “ regular” galaxies in the Hubble sequence to dwarf morphologies. Thus, a “d” was placed before the “E” or “Irr” to denote an elliptical or irregular galaxy as a dwarf. The BCDs have the highest surface brightness among dwarfs. Their prominent blue colour is due to a high star-formation rate at the centre. Much dimmer are dwarf spheroidal galaxies. These have less or even almost no concentrated gas and, along with the UCDs, are the smallest type of dwarf galaxy.

The classification of dwarf galaxies also includes so-called tidal dwarf galaxies which can form from the material of tidal tails which are common in the interaction of large galaxies. These tails extend from a few ten thousand to a few hundred thousand light years into space. As the interacting galaxies orbit each other, twisted tracks of gas, dust, and stars form which surround the pair. A local gravitational potential far enough away from the centre of the galaxy can collect and concentrate material, which can lead to a tidal dwarf galaxy forming in the tidal tail.

SPECTRAL LINES

In 1835 a French philosopher, Auguste Comte (1798–1857), considered the limits of human knowledge. In his book Positive Philosophy, Comte wrote of the stars, “We see how we may determine their forms, their distances, their bulk, their motions, but we can never know anything of their chemical or mineralogical structure.” Thirty-three years earlier, however, WilliamWollaston (1766–1828), like Newton before him, passed sunlight through a prism to produce a rainbow-like spectrum. He discovered that a number of dark spectral lines were superimposed on the continuous spectrum where the Sun's light had been absorbed at certain discrete wavelengths. By 1814, the German optician Joseph von Fraunhofer (1787–1826) had cataloged 475 of these dark lines (today called Fraunhofer lines) in the solar spectrum. While measuring the wavelengths of these lines, Fraunhofer made the first observation capable of proving Comte wrong. Fraunhofer determined that the wavelength of one prominent dark line in the Sun's spectrum corresponds to the wavelength of the yellow light emitted when salt is sprinkled in a flame. The new science of spectroscopy was born with the identification of this sodium line.

Kirchhoff's Laws

The foundations of spectroscopy were established by Robert Bunsen (1811–1899), a German chemist, and by Gustav Kirchhoff (1824–1887), a Prussian theoretical physicist. Bunsen's burner produced a colorless flame that was ideal for studying the spectra of heated substances. He and Kirchhoff then designed a spectroscope that passed the light of a flame spectrum through a prism to be analyzed. The wavelengths of light absorbed and emitted by an element were found to be the same; Kirchhoff determined that 70 dark lines in the solar spectrum correspond to 70 bright lines emitted by iron vapor. In 1860 Kirchhoff and Bunsen published their classic work Chemical Analysis by Spectral Observations, in which they developed the idea that every element produces its own pattern of spectral lines and thus may be identified by its unique spectral line “fingerprint.” Kirchhoff summarized the production of spectral lines in three laws, which are now known as Kirchhoff's laws: