Look at most introductory books on astronomy and you’re bound to find a picture of the Milky Way galaxy. This is our local ‘island in space’, a vast agglomeration of some 200 billion stars isolated from the rest of the universe in all ways except through that most feeble yet far reaching of forces, gravity. Often you will see a photograph of the nearest large galaxy to our own, the great Andromeda galaxy, presented as an analog of ours. But the Andromeda galaxy differs from ours in two ways. Firstly, it is roughly twice the size of the Milky Way. Secondly, it is a typical spiral galaxy, whereas the Milky Way is a barred spiral, these names taken from the most obvious patterns traced out by the brightest stars. In any case, the overall shape of the Milky Way galaxy is typically presented as a disk with a central bulge, like two fried eggs back to back. The yolk of eggs represents the central bulge of the Galaxy, while the surrounding disk is laced with beautiful luminous swirls that trace out the spiral arms. It is within these spiral arms that the vast majority of current star formation takes place, fed by an enormous supply of hydrogen gas. Star formation itself is triggered by ‘density waves’, the galactic equivalent of a traffic jam, that drift through the disk of the Galaxy bunching up the hydrogen gas clouds and causing them to collapse and fragment, eventually spawning clusters of fledgling stars. The true shape of the Milky Way, however, is not a disk but a sphere. Surrounding the visible disk is a spherical halo of dark matter that controls its bulk behavior.
KeywordsDark Matter Neutron Star White Dwarf Globular Cluster Dwarf Galaxy
Unable to display preview. Download preview PDF.
- 1.By studying the motion of globular clusters, astronomers can trace out the structure of the Milky Way itself.Google Scholar
- 2.By measuring the motion of globular cluster stars, astronomers can study the cluster’s dynamics, including its mass. This has revealed a major puzzle concerning the nature of globular clusters: why they contain so little dark matter. The Milky Way is surrounded by the stuff, and in fact globular clusters drift through this dark matter halo continuously, yet apparently oblivious to its existence. Even more puzzling is the fact that there are other collections of stars similar in size to globular clusters that are saturated with dark matter. The Milky Way is attended to by an entourage of dwarf galaxies whose masses are comparable with the largest of the globular clusters. Dwarf galaxies show a trend of inverse proportion of dark matter with size: the smaller the dwarf, the greater the proportion of dark matter. In defiance of this, globular clusters contain virtually no dark matter at all. The reasons for this are unclear and the matter is further confused by the fact that the globular clusters and dwarf galaxies formed at around the same time. For some reason, the formation of globular clusters involved only ordinary matter, while dwarf galaxies are dominated by dark matter.Google Scholar
- 3.The brightest are 47 Tucanae, Omega Centauri in the southern hemisphere, and M5 and Ml3 in the northern hemisphere.Google Scholar
- 4.We will look more closely at the contributions of this magnificent instrument to pulsar astronomy in Chapter 15.Google Scholar