Soon after the establishment of spiral nebulae as galaxies beyond our own, practical astronomers put a major effort into the determination of their optical apparent luminosities, colours, angular sizes, radial velocities, distances and shapes. From these data, galaxies appear to be the most massive single concentrations of stars known in the Universe. This deduction should not be taken to mean that they are in fact the most massive, or in any sense, the most important, for there are many ways in which matter can escape detection. Matter that is not of itself sufficiently luminous can be found only with difficulty, for example by the scattering or absorption it causes to radiation from elsewhere. Furthermore, luminous matter can be virtually undetectable when it is spread out over such a large area as to yield a low surface brightness. In the latter case the light from the object may be drowned by the night sky, by the zodiacal light or by the galactic background. Only gravitation reveals the presence of all matter in the Universe, but this very indirect method of detection gives as yet highly uncertain results. Consequently all one may say is that galaxies are very conspicuous components of the Universe, but equally impressive amounts of matter could be completely invisible to our telescopes.

In order to get some feeling for the appearance of these components, let us inspect a part of a typical photograph of the night sky . With a little practice, images that are starlike can be distinguished from fuzzy ones. The latter are almost exclusively galaxies. Most of them look rather small and faint because of the great distances separating them from us: very faint galaxies have been recognized out to ten billion parsecs. Nearby ones, however, usually appear as spectacular disc-like systems of stars, gas and dust or large spheroidal concentrations of stars.

Apart from occasional patches radiating the blue continuum and red hydrogen-alpha line emission, characteristic of massive young stars and their attendant regions of ionized hydrogen, the predominant colour of galaxies is yellowish. The colour index B-V lies usually between 0.6 and 1.0, and most probably near 0.8 corresponding to stars of late G type. (These stars are slightly yellower than the Sun.)

The sizes of galaxies have a much larger range than do their colours: diameters lie between approximately two and 50kpc. The smallest galaxies are probably the most numerous, although our knowledge of these midget systems is severely limited by the selection effects (difficulty of detection) mentioned above. But the total amount of mass contained in these DWARF GALAXIES is quite negligible, since their individual mass is only a few hundred million solar masses. At the other end of the mass range, the GIANT GALAXIES with masses up to 1013MO are extremely rare (less than one per million cubic megaparsec). Galaxies of intermediate size, having a diameter of about 20kpc and a mass of about 50 billion solar masses, are sufficiently numerous (one per 50 cubic megaparsec) to make up most of the luminous matter in the Universe. Although individual galaxies are very massive, the mean density of visible matter in the Universe is almost incredibly tiny. If all the matter in galaxies were smeared out uniformly in space, the resulting mass density would be 2 x 10-28kgm-3; that is equivalent to one atom per ten cubic metres. This is ten million times less than the density of the interstellar medium. If the Moon were expanded to the average cosmic density, it would fill a sphere around the Sun encompassing the nearest 10stars!

The moan distance between two intermediate-sized galaxies is about three megaparsecs, or 150 galaxy diameters. Therefore, galaxies are rather near neighbours: in everyday terms we can picture them, on the average, as being about as far apart as are the inhabitants of a country village. For comparison, the mean distance between stars in the plane of our (Galaxy is 30 million stellar dia¬meters. Because of their relatively small mutual distances, we may expect to find a sizeable number of galaxies near ours.

Let us, therefore, look at a sample of neighbouring galaxies. As it turns out, the nearest ones, the Magellanic Clouds, have less than average masses and are so close (about 60kpc) as to be satellites of our Galaxy. The nearest truly external large galaxy is the Andromeda Nebula : its distance is only 690kpc. Our Galaxy is part of a local concentration of galaxies, the Local Group.

Another nearby galaxy is M 87, near the northernmost boundary of Virgo. This one looks entirely different from M 81. The angular diameter is four minutes of arc, apparent magnitude 10.7, colour index 0.97 (early K-type), distance 13Mpc, linear diameter 15kpc. Outside this average diameter, it has an extensive halo, which contains so many stars that M 87 is the most massive galaxy known: about 1013M0. Its shape might lead one to believe that it is an outsized globular cluster, but in reality the galaxy itself contains a very large number of true globular clusters .

The galaxy M 104, near the southern edge of Virgo, is different again . The central part looks like a spheroidal cloud of stars with a colour index 0.98; but the galaxy also contains a disc terminating in a prominent dust ring with a diameter of seven minutes, which at its distance of 12Mpc corresponds to 24kpc. The central bulge contains numerous resolved patches, which are globular clusters caught in snapshot on their 100 million-year orbit
in the galaxy’s halo. The mass of M 104 is probably 5 x 1011M0, slightly more than M 31.

The galaxy known as NGC 3992. near the star ?Ursae Majoris. presents yet another structural variety. Spiral arms are seen our to a diameter of seven minutes, winding inwards until, at about 1.5 arc minutes from the centre, they abruptly stop. Within that region a bar-like structure extends on opposite sides of the galaxy’s nucleus. This central region has a linear diameter of 15kpc, for the distance of the galaxy is probably about 17Mpc. NGC 3992 is slightly whiter than any of the preceding galaxies (B-V = 0.85, like a late G-type star), whereas its mass is roughly equal to that of our Galaxy. The last galaxy in this series of examples is NGC4449 near the eastern border of the constellation Canes Venatici. Although its size is quite ordinary (apparent magnitude 10.3. mass 5 X 1010MO, diameter 12kpc for a distance of 8Mpc), its shape is nowhere near as regular as any of the other examples. Also, its colour is very much bluer; the B-V index is 0.38, like an early F-type star. This is due to the blue light contributed by the relatively large number of regions of ionized hydrogen.

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