The Local Group of Galaxies
摘要
Moving outwards on our route through the universe, we come to the galaxies in the Local Group, a small group bound together by its own gravity. We first come to the smaller satellites of the Milky Way, dwarf satellites, and learn that they can be classified in two groups: one which formed all its stars more than ten thousand million years ago (“fast” dwarfs) and the other (slow dwarfs which started forming stars at the same time, but much more slowly, and have continued star formation until the present time. After taking a detailed look at some of these dwarfs, we turn our attention to our nearest neighbour galaxies, the two Magellanic Clouds: the SMC and the LMC. We see how observations in different wavelength ranges give us information about their stars, and their gas clouds, picking out binary stars which produce X-rays in the SMC, and ionized gas around a huge star forming region, the Tarantula nebula in the LMC. We also take a look at Supernova 1987a in the LMC, the nearest supernova to explode in the modern age of telescopes and instruments. Looking further away we find and inspect our two main companion galaxies in the Local Group: the largest member, Andromeda with its two principal satellites, and M33 the Tiangulum Galaxy, which appears not to harbour a central supermassive black hole. Our visit to the Local Group ends at NGC 6822, “Barnard’ Galaxy” which was the first in which Hubble used Cepheid variable stars to show that is lies well outside the Milky Way, before his more famous work on Andrómeda.