The tightness of winding is the feature that is most commonly used to characterize the overall shape of a spiral galaxy. Because the pitch angle is not always the same at every distance from the nucleus, spiral types are not simply distinguished by a number giving that angle. Instead they are indicated by S followed […]
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 […]