The least massive members of the Local Group cluster around the dominant galaxies. At a distance of 50 kpc from the centre of a 1011Mo galaxy, the escape velocity is about 140kms-1. Because the dwarf elliptical galaxies in the Local Group move more slowly than this, if, appears fairly certain that they are satellite galaxies […]
The distance to external galaxies makes it difficult to resolve them into stars, and hence the motions of their stars have been but poorly studied. In the nearby Magellanic Clouds, such studies are quite possible in principle; in fact, however, in important regions the stars are so close together in the sky that it becomes […]
Only the Magellanic Clouds are observed to contain a few globular clusters, presumably because the Clouds are more NGC6822 and 1C 1613. Open clusters and association are very patchily distributed through all these galaxies, tracing no discernible structure. Some associations are rather large and the LMC contains a vast clumping of H+ regions, called the […]
The ANDROMEDA NEBULA, M31, is a GIANT SPIRAL galaxy. With a mass of 300 billion solar masses (M0) it is the most massive member of the Local Group; it is twice the mass of the Galaxy. Its total absolute magnitude Mv = —21.1 makes it also the brightest member. In the sky, the optical image […]
To an observer in the southern hemisphere, the existence Of galaxies beyond our own should be as evident as the existence of stars beyond the Sun: the two nearest external galaxies, the LARGE and the SMALL MAGELLANIC CLOUD, (LMC and SMC) are readily visible to the naked eye . At their distances of 50 and […]
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 […]