The Milky Way and OTher Galaxies some 27,000 light-years from the nucleus or centre centre of the Galaxy The Gal axy’s disk and spiral arms have a diameter of roughly 100,000 light-years The Sun and all the other stars of the Galaxy take about 200 million years to orbit about the nucleus This image the result of 10 days observation by the Hubble Space Telescope shows 1,500 galaxies in different stages of their evolution Galaxies vary greatly in size and grow by colliding colliding and merging with other galaxies Some extreme dwarf galaxies found near the Milky Way Galaxy are only 100 light-years across while large spiral galaxies typically have diameters of 100,000 to 500,000 light-years On the other hand giant radio galaxies which are very strong sources of radio waves extend more than 3,000,000 light-years Most galaxies move through the universe in clusters sometimes in small groups and sometimes in enormous complexes Isolated galaxies are quite rare The Milky Way Galaxy for example belongs to the Local Group which 8
inTrOducTiOn also includes the Magellanic Clouds the Andromeda Galaxy Galaxy and about 50 other galaxies Evidence suggests that at the centre of many galaxies galaxies lies a supermassive black hole a region of such intense gravity that nothing not even light can escape Additionally Additionally when two galaxies merge their nuclei and black holes collide generating an immense amount of energy At the centre of the Galaxy is a massive black hole named Sagittarius A and it has a mass about 4 3 million times larger than the Sun’s In the late 20th century a surprising finding was made most of the mass in galaxies is not in the form of stars or other visible matter Most galaxies have much more mass than can be accounted for by their stars This means that they must contain some dark matter from which no light is seen The nature of this dark matter is not yet known This text studies the nature of the various types of galaxies including our own Scientists have only begun to understand the complexities of these immense celestial networks 9
CHAPTER 1 The Milky Way The Milky Way Galaxy is the galaxy that we call home Although Earth lies well within the Milky Way astronomers do not have as complete an understanding of its nature as they do of some external star systems A thick layer of interstellar dust obscures much of Artist’s rendition of the Milky Way Galaxy
The Milky Way the Galaxy from scrutiny by optical telescopes and astronomers astronomers can determine its large-scale structure only with the aid of radio and infrared telescopes which can detect the forms of radiation that penetrate the obscuring matter Although most stars in the Milky Way Galaxy exist either as single stars like the Sun or as double stars there are many conspicuous groups and clusters of stars that contain tens to thousands of members These objects can be subdivided into three types globular clusters open clusters and stellar associations They differ primarily in age and in the number of member stars GLOBULAR CLUSTERS The largest and most massive star clusters are the globular clusters so called because of their roughly spherical appearance appearance The Galaxy contains more than 150 globular clusters the exact number is uncertain because of obscuration by dust in the Milky Way band which probably prevents some globular clusters from being seen They are arranged in a nearly spherical halo around the Milky Way with relatively few toward the galactic plane but a heavy concentration toward the centre The radial distribution when plotted as a function of distance from the galactic centre fits a mathematical mathematical expression of a form identical to the one describing the star distribution in elliptical galaxies 11
