[...]... The study of the motions of stars has played an essential role in unravelling the shape and determining the mass of the Milky Way we live in Whenever you see a stellar system, you are always asking: `Why does this thing not just collapse in on itself? What is holding it up?' A system of stars like the Milky Way stays inflated only because of the motions of the stars within it This motion comes in two... attention to the theory behind just why the Universe might be expanding As Hubble and Slipher studied the receding galaxies, it was left to several theorists ± including Albert Einstein ± who were developing a theoretical framework that involved such an expanding Universe An important implication of an expanding Universe was the beginning of all things at some finite time in the past This event has... Universe only in visible light In the intervening seventy years or so, the Universe has been studied in a myriad of new wavelengths, each revealing new forms of previously invisible matter, including interstellar gas and dust, neutron stars, radio galaxies and black holes But the addition of these previously unseen sources of matter falls a long way short of accounting for the effects of dark matter... time of his death at the age of 92, and was still ice-skating well into his eighties The Netherlands has a long and strong tradition of astronomy, and Oort continued (and led) this tradition He became Professor of Astronomy at Leiden Observatory in 1935, around the time that radio astronomy was emerging in Australia and the United States He became increasingly interested in radio astronomy, realising... the vast majority of stars in the Milky Way Therefore, by adding the masses of all the stars of different brightness in the proportions seen in the Milky Way, it is possible to make an intelligent estimate of the Milky Way's `luminous mass' This was originally done in the 1920s, and although more modern estimates are produced in a more theoretical way (such as by making populations of virtual stars... If you were sitting near the Sun looking at those stars going by, you would notice that some of them are oscillating inwards at this point and some outward at this point, while others are at the turning point in their oscillation Overall it is a sort of average circular motion, which is why the outline of the Milky Way is circular plus some random motion Let us look at these two types of motion ± circular... balance the motions of the stars, and so is called gravitational mass Now, the amount of gravitational mass is just fine until compared with the amount of mass implied by the luminous matter contained within the Milky Way ± the so-called luminous mass MASS±LUMINOSITY RELATIONSHIP In the early decades of the twentieth century, determination of the amount of visible matter was a pretty indirect procedure... explained by the sum of various types of forces When different forces act, the resultant motion is the sum of the effect of each different force Especially in the case of gravity, we have a law which epitomises the concept of laws in physics: Newton's laws apply equally everywhere in the Universe The balance between motion and gravity is often obvious and beautiful, perhaps best visualised by thinking in. .. galaxies of immense proportions lying at fantastic distances from our own Further observations using techniques similar to the spectral line observations described in the last chapter were made by astronomers including Vesto Melvin Slipher and, later, Edwin Hubble By far the majority of galaxies studied showed a redshift, indicating that they are moving away from our own Milky Way But rather than implying... immensely larger scale, and in far greater quantities This contrast with Oort's findings is matched only by the contrast between the two men themselves 3 Seeing the invisible INTRODUCING ZWICKY Soon after Oort had announced the existence of dark matter in the disk of the Milky Way, another astronomer, Fritz Zwicky, announced its existence not just in individual galaxies but in clusters of galaxies At a time . alt="" In Search of Dark Matter Ken Freeman and Geoff McNamara In Search of Dark Matter Published in association with Praxis Publishing Chichester, UK Professor Ken Freeman Mr Geoff McNamara Research. only in visible light. In the intervening seventy years or so, the Universe has been studied in a myriad of new wavelengths, each revealing new forms of previously invisible matter, including interstellar. lensing 72 The Einstein radius 72 Probability of lensing events 73 Using gravitational lensing to measure dark matter 75 Strong lensing and the Hubble constant 75 vi Table of contents Weak lensing