The Milky Way as a GalaxyThe Earth is orbiting around the Sun, which itself isorbiting around the center of the Milky Way. Our MilkyWay, the Galaxy, is the only galaxy in which we are ableto study astrophysical processes in detail. Therefore, ourjourney through extragalactic astronomy will begin inour home Galaxy, with which we first need to becomefamiliar before we are ready to take off into the depthsof the Universe. Knowing the properties of the MilkyWay is indispensable for understanding other galaxies.2.1 Galactic CoordinatesOn a clear night, and sufficiently far away from cities,one can see the magnificent band of the Milky Wayon the sky (Fig. 2.1). This observation suggests that thedistribution of light, i.e., that of the stars in the Galaxy,is predominantly that of a thin disk. A detailed analysisof the geometry of the distribution of stars and gasconfirms this impression. This geometry of the Galaxysuggests the introduction of two specially adapted coordinatesystems which are particularly convenient forquantitative descriptions.Spherical Galactic Coordinates (!, b). We considera spherical coordinate system, with its center being“here”, at the location of the Sun (see Fig. 2.2). TheGalactic plane is the plane of the Galactic disk, i.e., itis parallel to the band of the Milky Way. The two Galacticcoordinates * and b are angular coordinates onthe sphere. Here, b denotes the Galactic latitude, theFig. 2.1. An unusual opticalimage of the MilkyWay. This total view ofthe Galaxy is composedof a large number ofindividual imagesangular distance of a source from the Galactic plane,with b[−90◦, +90◦]. The great circle b = 0◦ is then∈located in the plane of the Galactic disk. The directionb = 90◦ is perpendicular to the disk and denotesthe North Galactic Pole (NGP), while b = −90◦ marksthe direction to the South Galactic Pole (SGP). Thesecond angular coordinate is the Galactic longitude *,with *[0◦, 360◦]. It measures the angular separation∈between the position of a source, projected perpendicularlyonto the Galactic disk (see Fig. 2.2), and theGalactic center, which itself has angular coordinatesb = 0◦ and * = 0◦. Given * and b for a source, its locationon the sky is fully specified. In order to specify itsthree-dimensional location, the distance of that sourcefrom us is also needed.The conversion of the positions of sources given inGalactic coordinates (b, *) to that in equatorial coordinates(α, δ) and vice versa is obtained from the rotationbetween these two coordinate systems, and is describedby spherical trigonometry.1 The necessary formulae canbe found in numerous standard texts. We will not reproducethem here, since nowadays this transformationis done nearly exclusively using computer programs.