The tightness of winding is the feature that is most commonly used to characterize the overall shape of a spiral galaxy. Because the pitch angle is not always the same at every distance from the nucleus, spiral types are not simply distinguished by a number giving that angle. Instead they are indicated by S followed by a, b, c or d, in order of decreasing tightness, where ‘a’ corresponds to a pitch angle of about 10° (tightly wound), ‘b’ to about 15°, ‘c’ to 20° and ‘d’ to 25° (loose structure). Galaxies with more open patterns than Sd are sometimes called Sm. The borderlines between these classes are necessarily somewhat vague, but the sequence is still quite recognizable .

Formerly, it was thought that this sequence of shapes was due to a real evolution in the course of time. A galaxy was then expected to start as an Sm type with very open arms, much gas and young stars, to evolve through a winding up of the arms and the dying of the massive stars, towards an Sa type, eventually leaving an elliptical galaxy behind. This notion is now known to be completely wrong. Still, many classification schemes for the appearance of spiral galaxies have been proposed in the hope that they might lead to an understanding of the causes of these shapes. Only the above classification by pitch angle corresponds reasonably well with other physical properties, which will be mentioned below.

Spiral galaxies are made of the same basic ingredients as ellipticals, but in rather different proportions. The greatest similarity is found between ellipticals and the NUCLEAR BTTLGE of a spiral — the somewhat extended, elliptical-looking cloud of stars centred on the nucleus of a spiral galaxy, which can be most clearly seen in edge-on systems.

The great difference between spirals and ellipticals is to be found in the Population I component (young stars, gas and dust) In elliptical galaxies it is absent outside the nucleus itself, but it shows up as a disc with some gas and dust in Sa galaxies, increasing its prominence through the Sb and Sc types, until its light completely dominates the Sd galaxies. The fraction of the mass of a galaxy in the form of neutral hydrogen accordingly increases from about two per cent in Sa types to 15 per cent in the open-armed Sd galaxies . Another difference is that spiral galaxies are much bluer than are ellipticals: whereas the latter typically have a B-V colour index around 0.9, this parameter for Sc types lies between 0.5 and 0.6. This difference in colour is caused by the presence of young massive stars in spiral galaxies. The average number of such stars and their attendant regions of ionized hydrogen increases with increasing pitch angle, so that Sa galaxies contain a few ionized hydrogen regions whereas Sc and Sd types are completely dominated by them. This has two important implications. Firstly, it shows once more that where there is much gas, there are many new stars being formed. Secondly, it shows that in spiral galaxies the relative importance of the Population I component is correlated with the spiral type . This is an extremely significant conclusion, because it shows that spiral patterns are not random, but have some fundamental connection with the other properties of spiral galaxies. In general the ionized hydrogen lies along the spiral arms . These intriguing clues to the origin of spiral structure are expounded below. It is dangerous to jump to conclusions from such correlations but they are certainly relevant.

The Population I component in spiral galaxies also shows its presence at radio wavelengths. The ionized hydrogen in the vicinity of bright young stars emits a thermal continuum which is detectable at centimetre wavelengths, if the source is bright and relatively nearby. For example, the giant cloud 30 Doradus, in the Large Magellanic Cloud would still be detected at a distance of 5Mpc, that is 170 galaxy diameters from the Sun. The most distant H+ region known lies in the galaxy NGC 3310, at 20Mpc from our Galaxy. But at decimetre wavelengths these sources are rather small and weak compared to the non-thermal continuum from a galaxy. If such radiation is detected, it comes either from the galactic nucleus or from the disc, or both. Apparently the cosmic rays and the magnetic fields, which together generate this radiation, are confined to the disc, anchored somehow to the gas in the plane of the galaxy. In some cases, the continuum shows ridges that follow the spiral arms , which is evidence that the density of fast electrons and magnetic fields is larger inside than it is outside the spiral arms. Also, the maximum intensity of the radio emission coincides with the dust lanes in the optical spiral arms.

Apart from the continuum, the radio spectrum of spiral galaxies often contains spectral lines. Molecular lines like those of hydroxyl and formaldehyde, have been found in absorption against the continuum of some nuclei, and carbon monoxide has been seen in emission in a score of galaxies. Most information to date has come
from the 21-cm line of neutral atomic hydrogen. This radiation is emitted from the plane of spiral galaxies, and in many cases shows ridges that beautifully follow the optical spiral arms .

Radio astronomers have discovered that the total extent of the neutral hydrogen plane is often considerably larger than the optical image of the galaxy. Its radius may be 1.6-2 times the optical radius. These outermost regions of the disc are often warped or otherwise distorted, as is true for both our Galaxy and M 33.

The total mass detected from the 21-cm line is thus far the only practical indicator of the gas content of a spiral galaxy . Regions that are too cold to emit the 21-cm line could be analysed from the 2.6-mm carbon monoxide line, whereas those regions that are too hot could be observed by means of hydrogen radio recombination lines. But these emissions are too faint to be of practical value as yet.

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