At optical wavelengths, interstellar matter has proved to be more a hindrance than a help for the determination of galactic structure. Happily, in the radio domain (this is the region of the electromagnetic spectrum where the wavelengths are longer than one millimeter), the converse is the case: firstly because the galactic atmosphere is almost everywhere transparent to waves of nearly all radio frequencies, and secondly because the matter itself is a source of weak radio radiation. And just as in the optical case, this radiation shows a continuous spectrum (continuum) with spectral lines in absorption and emission superimposed on it. The radio continuum intensity is highest in a band coinciding with the Milky Way . Within this diffuse band, numerous compact radio sources can be seen. The majority of these are thermal sources: clouds of ionized hydrogen (H+ regions) with temperatures up to 10000K. Because of their strong concentration towards the galactic plane, these regions belong to the extreme Population I, the most youthful component of the Galaxy. Their distribution indicates the presence of spiral arms, which is not surprising since they receive their energy from 0 and B stars, which are tracers of the spiral structure. The majority of the remaining compact radio sources are supernova remnants.

Apart from the individual thermal sources in the Galaxy there is also the continuum associated with the diffuse background. This shows a non -thermal spectrum, which indicates that this radiation is not caused by hot gas, but instead is synchrotron radiation, emitted by cosmic-ray electrons as they diffuse through the galactic magnetic field. Because the kinetic energy of the electrons is very large, they ascend to heights exceeding 2kpc above the galactic plane and thereby form a GALACTIC HALO. Just how far this galactic halo of faint radio emission extends is still a matter of controversy. Possibly it is so extended as to coincide with the enormous volume through which the globular clusters are distributed. The halo is extremely tenuous: at heights beyond 2 kpc, its density is less than 10-25kgm-3, which is a ten-thousandth of the density of interstellar matter in the plane.

The non-thermal radio emission from the Galaxy is by no means featureless, even on a large scale. Outside the galactic plane, numerous ridges of enhanced radio emission can be seen to extend into the halo. These SPURS are probably remnants of the shells of nearby supernovae which exploded more than 100 000 years ago. Such ancient shells have diameters up to 150pc which means that at close range, radio astronomers observe them as circles with very large angular radius projected on the sky. For example, the NORTH POLAR SPUR traces a gigantic loop with a radius of 50°. Its radio waves are strongly polarized, which indicates that the object must be near by. We can deduce this interesting fact because if it were far away, the blurring caused by the galactic atmosphere (Faraday rotation) would largely have destroyed the polarization, which is imposed on the radiation by the magnetic field of the blast wave. Objects like these are too small to be of direct importance for galactic structure as a whole. They look spectacular to us merely because they are not very far away. Indirectly, however, they help to maintain the galactic atmosphere by the injection of fast particles, probably even cosmic rays, which replenish and heat the interstellar medium.
In the galactic plane, regions of enhanced radiation occur in places where the line of sight happens to run along a local spiral arm (e.g. in the direction of Cygnus). But the most conspicuous extended source in the galactic plane lies in Sagittarius . In this region, most of the stars are obscured by interstellar dimming, but some can be seen through low-absorption WINDOWS. Because these windows occur at somewhat higher galactic latitudes, the stars seen through them must lie a kiloparsec or so from the plane. These distant stars form the CENTRAL BULGE of our Galaxy, which contains the Sagittarius radio source complex . This is about l0 kpc from the Sun, at the very centre of the Galaxy. The central region consists of an oblate spheroidal cloud of stars, with a radius of 1 kpc, a thickness of 400 pc.

In the equatorial plane of this spheroid lies a rotating disc of gas with radius 750 pc, and a varying thickness that decreases from 250pc on the periphery to 100 pc at the centre. The edge of the disc spins at 230 km s-1. The density is about 10-21kgm-3; this amounts to 8 million solar masses, half of which is in the form of neutral hydrogen, half is molecular hydrogen, and a small admixture (about one per cent) dust.

Inside the central region of the Galaxy, a number of small radio sources are observed. The brightest of these are called Sagittarius A and B . The source Sgr A is only a few parsecs in extent. This object is the galactic centre, situated at right ascension 17hr 42mm 37 sec and decimation —28° 57′ (for the equinox and epoch of 1950.0). It forms a reference point for the HELIO¬CENTRIC GALACTIC COORDINATES. Heliocentric galactic coordinates are defined as a right-handed spherical coordinate system centered on the Sun. Its longitude 1 is measured in the plane of the Galaxy from 0 (on the galactic centre at ra 17hr 42min 37 sec. dec — 280 57′) to 360°. Its latitude b ranges from -90° to +90°, the latter value being reached at the GALACTIC NORTH POLE, which is situated at ra 12hr 49min Osec, dec 27°4′ (1950.0). In projections., it is customary to look upon the galactic plane from the north pole.

The central region of the galaxy has many unique features. It contains the galactic dynamical centre, coinciding with Sgr A. The region centered on this point is the most rapidly rotating section of the Galaxy: it turns around its axis once every 50 000 years. From this high speed we can deduce that it has the highest average density: within one parsec of the centre are contained four million solar masses. The central density is therefore a million times higher than the density near the Sun. If all of this is stars, the average distance between the stars is only 20 light days! A resident of a hypothetical planet orbiting a star in the galactic centre would be able to see its neighboring stars resolved in an optical telescope: the apparent angular diameters would be upwards of 2 seconds of arc. Planets around these stars could easily be seen, from our hypothetical planet, since a planet like Jupiter would have an apparent visual brightness of 6 mag. Another unique object is the source Sgr B2, which is part of the Sgr B complex. It is probably the most massive cloud in the Galaxy: three million solar masses concentrated within a radius of 3pc. More species of interstellar molecules have been detected in Sgr B2 than in any other galactic cloud. Infrared observations reveal a number of similar, though less massive, molecular clouds. From information about their velocities it has been deduced that they form an expanding ring (or at least a large sector of a ring) around the centre of the Galaxy.

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