Apparently, there is great variation among galaxies. In order to understand their properties, and ultimately their history, it is of prime importance to search carefully for common features to distinguish the typical from the incidental. The first such feature is indicated by the colour of galaxies: yellowish, approximately cor¬responding to black-body radiation with a temperature of 5000 K. At first glance, this suggests that all galaxies are stellar systems consisting mainly of ordinary stars more or less like the Sun. Especially luminous individual stars are observable in nearby galaxies. For example, the apparent magnitude of a star in M31 with absolute magnitude Mv = 0 would be about 24, so that only main-sequence stars with a spectral type hotter than B 5, or giants more luminous than KO type can be seen individually.

So a typical galaxy is primarily a vast cloud of stars, and this raises the question: of what kind of stars are galaxies made ? The observation of giant stars emphasizes a complication which makes this question a difficult one to answer: it is quite possible that the light received from a galaxy is mainly emitted by only a few, but nevertheless very luminous, giants or supergiants, whereas the bulk of the mass consists of dwarf stars whose contribution to the galaxy’s luminosity is small. At second glance, therefore, the colour alone of a galaxy gives rather poor information about the stellar make-up of the galaxy. Evidently, the whole spectrum of the system must be considered, not merely the colour indicators. There are two ways in which attempts are made to find the stellar constitution of a galaxy; theorists speak of making a POPULATION MODEL or an EVOLUTIONARY MODEL, and we shall look at these in turn.

A population model is a recipe for making a concoction of stars which looks as much like a galaxy as possible. The prescription is to observe the galaxy’s spectrum in as many narrow wavelength bands as observational constraints allow (say 30). The next step is to take a Hertzsprung-Russell diagram and divide it into the same number of boxes (also 30, in this example) as there are spectral points available. The final step is to put such a number of stars into each box that they together reproduce the galaxy’s spectrum . Although the contribution of faint dwarf stars remains highly uncertain, this method gives a reasonable amount of insight into the composition of bright galaxies . An important result obtained is that galaxies have a composition in their outskirts which differs from that in the centre. This information indicates that the abundances of heavy elements (all those heavier than helium) steadily increases towards the inner parts of the galaxy. These particular elements are formed by nuclear reactions in the cores of stars and in stellar explosions; they are subsequently thrown into the interstellar medium if a star containing them explodes. Therefore, the observed COMPOSITION GRADIENT indicates three possibilities: either (i) the original population of stars near the centre contained a larger proportion of massive stars, thus giving rise to more stellar explosions; or (ii) material rich in heavy elements hag accumulated in the galaxy’s centre, there forming new stars; or (iii) the rate of star formation (and consequently also of star explosions) increases towards the centre.

The more ambitious evolutionary model tries to produce galaxy by starting with a collection of stars all on the zero-age main sequence, that is stars newly formed, and then letting every star run along its evolutionary track. Within a few hundred million years, the most massive stars will have ended their life by explosion, those of intermediate mass (2M0, say) have evolved into giants, and the low-mass stars are still close to the main sequence By suitable adjustment of the initial stellar population, model galaxies like those observed can be obtained. This adjustment is somewhat arbitrary, of course, but evolutionary models have the advantage that they can be tested against observations of galaxies at large distances, which are presumably in an earlier stage of their evolution. Also, evolutionary models are useful for studying abundance gradients, because the birth of new stars may be included as an additional feature, so that the chemical evolution of a galaxy can be modelled.

From a determination of the stellar content of the brighter part of a galaxy by the above methods, it appears that most of the stars are dwarfs about 1010 years old. Most of the light of a galaxy is due to giant stars, whereas dwarfs make up most of the mass. A small number of massive stars with ages less than 100000 years are usually present too; these young blue stars are so luminous as to be highly conspicuous. Other spiral galaxies contain basically the same stellar populations as does our Galaxy: young (Population 1)and old (Population II). The proportions in which these populations occur vary from one galaxy to another. Their distribution is dis¬cussed below . Among the stars in other galaxies, many types of special stars known in the Milky Way are also found, such as novae, supernovae and Cepheid variables. The latter are especially important as distance indicators, but differences in composition between galaxies make a careful calibration necessary.

Not all stars are evenly distributed over a galaxy; there exist clusters and associations in addition to the general distribution. Open clusters and OB associations are Population I objects, and are usually quite prominent because of the high luminosity of their massive young member’s . Globular clusters are Population II objects, and can be seen as fuzzy patches far into the haloes of nearby galaxies. Globular clusters are useful distance indicators because they can be readily identified and their apparent magnitudes can be compared with globular clusters of known distance dose to the Milky Way. Also, they are important as probes of a galaxy’s gravitational field: their velocities are proportional to the “square root of the mass within their orbit, so that velocity measurements yield an estimate of the mass distribution in a galaxy. This method is especially instructive because it can serve to detect indirectly the presence of matter with low luminosity in a galaxy’s outskirts. Since most of the light of a galaxy comes from giant stars, enormous numbers of faint dwarfs could exist in the halo without being detected optically. However, mass determinations from the velocities of globular clusters agree with those obtained by other means, and speak against but do not quite exclude the existence of a large amount of hidden mass in galactic haloes.

As we have seen, all galaxies contain stars, and most contain gas and dust, but the total mass of the latter is small compared with the stellar mass. Also, the gas content differs enormously from one galaxy to another. Some, like M 87 , are observed to contain gas in their innermost regions only, and even that is only a tiny percentage of the central mass. Others, like NGC 4449 have gas distributed throughout the system, sometimes amounting to 25 per cent of the total mass. Dust is observable directly only in those galaxies of which about one per cent or more of the mass is in the form of gas . Indirectly, its presence can be inferred from the infrared radiation that some galaxies emit.

Gas in a galaxy is detected by means of emission lines in its spectrum. In the optical part of the spectrum, these lines are primarily due to atoms of hydrogen, helium, nitrogen, oxygen and neon. These, and carbon which is difficult to observe spectroscopically, are precisely the elements that are most abundantly generated by the nuclear reactions in stellar cores. Therefore a measurement of the relative amounts of these elements can give valuable information about the history of a galaxy. The abundances of the elements are determined from a comparison of their line strengths, just as in galactic nebulae .To radiate the emission lines, the gas must be excited by atomic collisions or by radiation. Because the gas is very tenous (typically a few million atoms per cubic metre, and often considerably less), the collision frequency is low, and excitation usually occurs near bright young stars. Therefore, abundance determinations are possible in regions of ionized hydrogen only. Because these ionized hydrogen regions occur only in a small number of places in certain special kinds of galaxy, our knowledge of the composition of galactic interstellar matter is rather incomplete. It turns out that the deduced abundances are similar to those in the neighbourhood of the Sun. For example, the ratio of the number of helium atoms to the number of hydrogen atoms in NGC 4449 is 0.088, and 0.101 in the Orion nebula. The abundance ratios for the heavier elements, carbon, oxygen, nitrogen and neon with respect to hydrogen, show larger differences : the mean value is about a thousandth, but deviations with a factor of two or three do occur. The abundance of deuterium cannot yet be determined with any confidence, because its spectral lines are too close to those of ordinary hydrogen. This is unfortunate, for the deuterium content of galaxies is a significant quantity in cosmology.

Observations of abundances in various parts of galaxies indicate that the gas, like the stars, shows an abundance gradient: in the ionized hydrogen regions nearest the centre of a galaxy, elements heavier than helium are on the average twice as abundant as they are in the outermost ionized hydrogen regions. Because the motions of the stars generally differ radically from those in the gas, it is not obvious that the abundance gradient in the one component should resemble the other. The fact that they do is a notable constraint on the possible history of galaxies.

The properties of the dust in galaxies other than ours are almost entirely unknown. Photographs of spirals that appear edge-on from our vantage point do, however, show that dust is a prominent component of some spiral galaxies. The knowledge of the size and composition of dust grains in our Galaxy is based on observations of the scattering of starlight by clouds of these grains, but the amount of light reaching Earth from stars in external galaxies is so small that these observations are impracticable. Observations of the central regions of galaxies in the infrared suggest that grains with sizes around one micron are common in these parts

Molecules are difficult to detect in external galaxies. Individual stars appear so faint that high-dispersion spectroscopy, necessary for the detection of optical interstellar absorption lines, is not feasible. But hydroxyl (OH), carbon monoxide (CO) and formaldehyde (HCOH) have been detected by their microwave lines at 18, 0.26 and 6.2 cm wavelength, respectively. The CO emission lines are most suitable^ for detection. Radio astronomers have detected absorption lines of carbon monoxide and formaldehyde against the continuum radio sources in the central parts of NGC 253.

Many galaxies emit non-thermal radio waves by the synchrotron radiation mechanism. This radiation is generated by very fast electrons diffusing through a magnetic field. From this observation theorists conclude that galaxies often contain cosmic rays and magnetic fields.

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