A faint luminous band can be observed in the clear night sky spanning a great circle on the celestial hemisphere. This band varies in width around an average of some 20°. Dark patches are seen in it, but it is clearly brighter than the sky background and hence it was long ago named the MILKY WAY on account of its appearance to the naked eye . Inspection with a moderately powerful telescope reveals that the light comes from a vast number of faint stars. This observation of the stellar nature of the Milky Way gives a first indication of the structure of the world beyond our Solar System: an enormous mass of stars, distributed over a flattened region of space. The Sun is one among these stars. In this section we survey the space around us to summarize its principal contents and their distribution.

When a radio signal is sent forth from Earth, as was done in 1975 with the Arecibo radio telescope to indicate our presence to possible alien civilizations, the wave carrying the message expands into space with the velocity of light, almost 300 000 km-1. This is the highest velocity known to experimental physics, and no material particle or radiation can travel any faster. On its journey outward, the signal passes the outermost planet of the Solar System after about 5.5 hours. Thereafter it will traverse space for 4.3 years before the next sizeable object is reached. This is the star a Centauri C, the third member of the triple-star system a Centauri, one of the brightest stars in southern skies. Because it is the star nearest to us, a Centauri C is also called Proxima Centauri Its distance of 4.3 light years corresponds to 30 million times the diameter of the Sun, or 7000 times the diameter of the Solar System. To visualize this immensity, imagine what would happen if people were separated by 30 million human diameters; one’s nearest neighbour would then be as far away as one tenth of the distance to the Moon! Evidently stars are very sparsely distributed in space: the average distance between stars is about five light years. Within 17 ly (5 pc) from the Sun, only 45 stars have been found .

The nearest stars appear to be distributed at random in space. At larger distances, however, there exist many loosely-bound groups of some 100 stars each. They are called associations and open clusters. The nearest of these is reached by our signal after 68 years. This is the Ursa Major star cluster, which is so close that its 100 or so members spread over a region on the sky of 20° diameter; several of the visible stars in the constellation Ursa Major, though not all of them, are members of this association. Further well-known groups are the Hyades (at 140 light years or 43 pc) and the Pleiades (125 pc). When the positions in space of these open clusters are plotted, it becomes clear that their distribution is not random at all. They lie in the Milky Way: of the nearest 30 open clusters, half lie within 5° from the middle of that luminous band. This distribution defines a plane in space, analogous to the plane of the ecliptic which is approximately defined by the positions of the planets. This GALACTIC PLANE, hi projection on the sky, forms the central line of the Milky Way. The mean distance of the open clusters from the plane is only 70 pc. Because such clusters are, on average, seen to distances of about 3 kpc, the, thickness/diameter ratio of the galactic plane is smaller than 0.002 (i.e. 70/3000). The corresponding ratio of our Solar System is 0.034; therefore the open clusters are at least 15 times more narrowly confined to their plane than are the planets.

The open clusters and associations which define the galactic plane are very young. Their ages are found by two independent methods. Firstly, most of them contain stars of certain types, especially T Tauri variables and 0-type stars, which are known from studies of stellar evolution to be no older than between 106 and 106 years. (For comparison, the age of the Sun exceeds five billion years). Secondly, the clusters have a low star density: some thousands of stars at most, within a diameter of about 5 pc. This means that they are loosely bound, that is to say the mutual gravitational attraction of the stars in the cluster, which holds the group together, is so small that a star manages to escape about once in every 100000 years. This evaporation effect limits the age of open clusters to around 100 million years.

There exists another distinctive type of local condensation in the stellar field: the groups called globular clusters because of their spherical appearance. Among the nearest of these to be passed by our signal, after 9000 years, is the object Messier 4 (M4), which contains about a million stars within a diameter of 10 pc. The distance of M4 indicates that globular clusters are much sparser than open clusters. In fact, whereas the mean distance between globulars is some 3 kpc, the open clusters are on the average only 100 pc apart. Another important distinction is that globulars are not concentrated towards the galactic plane. Precise determinations of their distances show that they are distributed hi a roughly spherical volume of some 20 kpc radius. They are concentrated towards the sphere’s centre, which is a point at a distance of 10 kpc from the Sun in the direction of the constellation Sagittarius. This point lies exactly in the galactic plane. The globular clusters are old systems that contain stars which are known to have existed for billions of years.

So, we now see the following rough picture of our starry surroundings emerge. The Sun lies in a vast stellar system, our Galaxy. The Galaxy may be divided into two sub-systems. One sub-system consists of stars confined to a narrow plane in space. This galactic plane coincides with the central line of the Milky Way and is outlined by young stars in associations and open clusters. The planar component has been named POPULATION I. The other system fills a spherical volume of space centred upon a point in the galactic plane. This spherical component contains old objects, such as globular clusters, and it has been named POPULATION n. It should be noted that there is no sharp distinction be¬tween the two populations. Rather, this classification is a simplified form of a scheme in which the stars are arranged continuously according to properties such as age, content of heavy elements, and mean height above the galactic plane. For practical purposes, this continuum is subdivided into the five population classes listed . The total matter density of the entire assembly shows a strong concentration towards the galactic plane.
We now have to sketch into the above schematic picture some important details of the stellar distribution. As has been said observation of the stellar component with the naked eye readily indicates the disc-like shape of the Galaxy. A similar, but more refined approach to the determination of galactic structure is to count the number, per square degree on the sky, of stars in a given interval of apparent brightness. One may try an attempt to find the spatial distribution of the stars by such a statistical method is graphically explained. The reconstruction of third dimension (depth) can never be achieved satisfactorily because some information is always lacking. One source ambiguity is the fact that not all stars have the same intrinsic luminosity, so the apparent brightness is not an honest measure of distance. Worse still, starlight is dimmed by interstellar matter.

Whereas the Population II objects show a smooth distribution in the plane, the extreme Population I exhibits a clear inhomogeneity. The distances to 0 and B stars can be determined by spectroscopic means, and the distances to the Cepheids are found using the relationship between their period and luminosity. When their positions in the galactic plane are plotted, these stars appear to be concentrated in clumpy bands . These are thought to be parts of spiral structure extending throughout the plane and so 0 and B stars and Cepheids are called SPIRAL TRACERS. In the solar neighbourhood, a section at right angles across a spiral arm as delineated by these stars appears like a very flat ellipse with axes of about 1000 and 150 pc.

Throughout the Galaxy, many objects are found which are not ordinary stars (like the Sun or Sirius, say), but which have none¬theless a stellar-ancestry. Among these are the planetary nebulae, novae, supernova remnants, pulsars, X-ray binaries, and possibly black holes. There is no evidence that they are of great importance in the overall structure of the Galaxy.

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