Our Galaxy is a vast system of 100 billion stars, occupying a volume of space with a diameter of 60 kpc. The youngest stars form a disc some 500 pc thick. Half the stars are contained within 1 kpc from the disc, and the oldest stars pervade a roughly spherical halo with 30 kpc radius […]

In the optical spectrum of some bright nearby stars, spectral lines are observed which have not formed by absorption in the stellar atmosphere, but which arose by the interception of the starlight by interstellar matter. These interstellar absorption lines appear to occur at wavelengths which differ from those at which the cor¬responding atoms in the […]

The order and regularity of the planetary motions provided the key to the structure of the Solar System and the discovery of the law of gravitational attraction. Likewise, the study of galactic dynamics has been, and is, essential to a determination of the structure of the Galaxy. Careful observations of stellar radial velocities (motion of […]

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 […]

Two simple observations with the naked eye show that the Milky Way contains gas. Firstly, certain nebulae such as the orion Nebula have a fuzzy appearance . Secondly the presence of obscuring matter may be inferred from the existence f the dark patches in the Milky Way that were mentioned . This matter is much […]

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 […]