Galaxies display a great variety of appearances and in studying this, diversity, it is difficult to distinguish the typical from the incidental The contents of a galaxy do not readily change, because of the slowness of evolution of the low-mass dwarf stars that make up most of a galaxy’s mass. But what about the evolution […]
Short-exposure photographs of the central regions of galaxies usually show a bright spot which is star-like in appearance, or just resolved (one second of arc diameter which, although frequently absent in some small galaxies, is a characteristic feature of the larger ones. The angular diameter of one second corresponds to a linear size of 3.3pc […]
Apparently, there is great variation among galaxies. In order to understand their properties, and ultimately their history, it is of prime importance to search carefully for common features to distinguish the typical from the incidental. The first such feature is indicated by the colour of galaxies: yellowish, approximately cor¬responding to black-body radiation with a temperature […]
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 […]
Just as Earth is not the only planet in the Solar System, and just as our Sun is not the only star in the Galaxy, so our Galaxy is not the only one in the Universe. But it was only 50 years ago that this im¬portant fart was established, even although SPIRAL NEBULAE -distant galaxies […]
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 […]