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 […]
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 […]