Galaxies display a great variety of appearances and in studying this, diversity, it is difficult to distinguish the typical from the incidental The contents of a galaxy do not readily change, because of the slowness of evolution of the low-mass dwarf stars that make up most of a galaxy’s mass. But what about the evolution of a galaxy’s shape ? This could conceivably change as rapidly as it takes a star to cross from one end of the galaxy to another. Since this crossing time is at most a few hundred million years, it is possible in principle that a galaxy’s shape is ephemeral in its lifetime of 10 billion years.

Many galaxies are remarkable for not showing any special features. These look like symmetrical clouds of stars, usually denser in the middle and containing a nucleus, but sometimes without such a condensation. These are called ELLIPTICAL GALAXIES. They closely resemble one another, the main variation being the degree of flattening and the rate at which the surface brightness decreases outward. The colours are virtually the same in all ellipticals; the spectrum typically resembles that of a KO-K5 star. Population models show that the bulk of the light from ellipticals comes from K and M giant stars, whereas most of the mass is in dwarfs smaller than the Sun. Towards the centre, the colour becomes redder; this may be in part due to slight changes in population. However, spectroscopic indications, notably the strength of the absorption bands of titanium monoxide in the stellar spectra, show that the colour changes mainly because the amount of elements heavier than helium decreases from the centre outwards.

For dynamical reasons, the stars in elliptical galaxies are thought to occupy a volume in space known as an oblate spheroid, which is the figure obtained by spinning an ellipse about its minor axis. The flattening of the spheroid is expressed by the ELLIPTICITY n, which equals 10 times the difference between the apparent lengths of the major and minor axes, divided by the apparent length of the major axis. The type of an elliptical galaxy is then indicated as En. In this system, a spherical galaxy such as M 87 has type EO, whereas M 32 is an E2 galaxy. Most ellipticals have type EO, a few have type E7 and none are flatter than this. A complicating factor is that the ellipticity of a galaxy image depends on the distance from the centre of the galaxy. In consequence of this, a short-exposure photograph of an elliptical galaxy shows a different ellipticity to that displayed in a long exposure of the same object, usually in the sense that the central parts look more nearly circular than the outskirts. The measured distribution of ellipticities is only apparent, because we see galaxies as projected in the sky. For example, many EO and El types are really flatter (E4, nay), but are seen almost pole-on instead of edge-on. If one makes the obvious assumption that galaxies are oriented at random in space, the true distribution of ellipticities can be deduced from the apparent one.

The mass of a galaxy is determined by several dynamical methods: (i) by analysis of the rotation curve; (ii) by using the motion of its globular clusters; (iii) by means of the velocity dispersion of the stars in the galaxy; or (iv) by measuring the orbital parameters of interacting galaxies .The range of masses of elliptical galaxies is tremendous: from dwarf ellipticals like the Draco system with a mass of only 100000 M0 to SUPER-GIANT ELLIPTICALS like M 87, with almost 10« M0; this latter colossus is the most massive galaxy known. In terms of the entire Universe, the dwarf ellipticals are probably the most numerous, whereas the giants and supergiants are extremely rare. Most of the mass locked in the elliptical galaxies actually resides in the inter¬mediate population, which is characterized by a mass of 1011 M0. These determinations show that ellipticals are much more massive than would be expected on the basis of their total luminosity. For if these galaxies did consist of stars of about 0.6 M0, as the galaxies’ effective temperatures would suggest, the ratio of the mass of the galaxy to its luminosity would be 4 (taking the solar mass/luminosity ratio to be unity). But most mass determinations show that M/L is about 50 in an elliptical galaxy, which is fully 12 times the expected value. These elliptical galaxies contain much matter in a form which is not especially luminous.

Stars shed mass quietly in the form of stellar wind, or sometimes more violently by generating planetary nebulae, or nova or super¬nova explosions. In cool stellar winds, certain kinds of dust particles are formed in the outer layers of stars. In a galaxy consisting of 1011 stars, a few solar masses per year are returned to the interstellar medium by these processes. Considering that the age of a galaxy is about 1010 years, we might reasonably expect that any galaxy should contain a large amount of gas and dust, but ellipticals typically do not. Furthermore those that do are always peculiar for other reasons (for example, NGC 5128 corresponds to the intense radio source Centaurus A and it is swathed in dust). Only in the nuclei of some elliptical galaxies do we find traces of gas, and even then it is less than a few tenths of a per cent of the nuclear mass. Clearly, the gas we expect to find in ellipticals is somehow removed, a point that we return to below.

The elliptical galaxies are rather faceless, but there are galaxies with salient features (or personalities!). Those like M 81 and M 104 are called SPIRAL GALAXIES, NGC 1300 is called a barred spiral , and asymmetric, more or less chaotic ones like NGC 4449 are called irregular .

Spiral galaxies are recognizable by either luminous or absorbing lanes, or both, winding outwards from their central parts. These lanes are called SPIRAL ARMS, and many photographs of spiral galaxies give the distinct impression that these arms lie in a rather thin plane. An attempt to reconstruct M 81 as it would be seen when looking directly onto this plane i. The fact that spiral galaxies are rather flat is emphasized by the existence of many of these systems seen apparently edge-on . The spiral arms are clearly inclined with respect to a circle about the centre of the galaxy. This inclination, usually called the pitch angle, varies from one spiral galaxy to another. When the pitch angle is small, the spiral arms are said to be tightly wound, whereas loosely-wound spirals have a large pitch angle.

Comments are closed.