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
The Milky Way and OTher Galaxies Globular clusters are extremely luminous objects Their mean luminosity is the equivalent equivalent of approximately 25,000 Suns The most luminous are 50 times brighter The masses of globular clusters measured measured by determining the dispersion in the velocities of individual stars range from a few thousand to more than 1,000,000 solar masses The clusters are very large with diameters measuring from 10 to as much as 300 light-years Most globular clusters are highly concentrated at their centres having stellar distributions that resemble isothermal gas spheres with a cutoff that corresponds to the tidal effects of the Galaxy A precise model of star distribution within a cluster can be derived from stellar dynamics which takes into account the kinds of orbits that stars have in the cluster encounters between these member stars and the effects of Globular cluster M80 also known as NGC 6093 in an optical image taken by the Hubble Space Telescope M80 is located 28,000 light-years from Earth 12
The Milky Way exterior influences The American astronomer Ivan R King for instance derived dynamical models that fit observed stellar distributions very closely He finds that a cluster’s structure can be described in terms of two numbers 1 the core radius which measures the degree of concentration at the centre and 2 the tidal radius which measures the cutoff cutoff of star densities at the edge of the cluster A key distinguishing feature of globular clusters in the Galaxy is their uniformly old age Determined by comparing comparing the stellar population of globular clusters with stellar evolutionary models the ages of all those so far measured range from 11 billion to 13 billion years They are the oldest oldest objects in the Galaxy and so must have been among the first formed That this was the case is also indicated by the fact that the globular clusters tend to have much smaller amounts of heavy elements than do the stars in the plane of the Galaxy e g the Sun Composed of stars belonging to the extreme Population Population II as well as the high-latitude halo stars these nearly spherical assemblages apparently formed before the material material of the Milky Way Galaxy flattened into the present thin disk As their component stars evolved they gave up some of their gas to interstellar space This gas was enriched in the heavy elements i e elements heavier than helium produced in stars during the later stages of their evolution evolution so that the interstellar gas in the Galaxy is continually 13
