# Chapter 1)Introduction - Welcome to the Milky Way!

## Not For Tourists Guide to the Milky Way – An Introduction

What Greek and Roman mythologies describe as a splash of milk on the night sky, coming from the Gods, the Milky Way is the home galaxy of our Solar System. The “milky” part of its name was given due to its “misty patch” appearance as seen from Earth at night – the region of the sky containing the galactic plane (which in projection on our celestial sphere approximates a big circle) is seen as a dim glowing ring in powerful contrast with its dark surroundings, for which the human eye is unable to resolve individual stars.
Understanding our galaxy and our place in the universe has been a long pursued challenge for humans across the world – there are different legends describing the creation of the Milky Way in multiple cultures, and scientists worldwide have long been fascinated with the study of our galaxy. Through his telescopic observations, Galileo Galilei first suggested that the Milky Way is a large composite of individual stars, which can be observed even with the naked eye by looking at the night sky at an angle of approximately $$62.6\degree$$ above the celestial equator. Immanuel Kant and Thomas Wright have later argued that our galaxy is a disk-shaped stellar collection, and that our Sun and Solar System lie inside the plane of this disk. William Herschel first created a map of the Milky Way in $$1785$$ (see Fig. [1]), by counting all the stars and analyzing their spatial distribution as he could observe from Earth, to conclude that stars are located in a big disk formation. The 20th century then brought additional discoveries to the field of astronomy, with Edward Shapley calculating our location inside the Milky Way in $$1920$$. By using Cepheid variable stars as distance indicators and applying the Period-Luminosity Law discovered by Henrietta Leavitt, Edward Shapley estimated distances to nearby globular clusters, which he has tracked in 683 regions of the sky. By plotting the found distances, he calculated that the Sun lies approximately two-thirds of the way out from the galactic center.

Map of the Milky Way Galaxy, as designed by William Herschel (Yerkes Observatory)

Today we believe the Milky Way is 13.21 billion years old, and that we are located at a distance of  8kpc (calculated using different methods, see summary in Table [1] below) from the Galactic Center. A wealth of new techniques and data analysis methods for space exploration have been developed since the early scientific efforts mentioned previously, yet we still do not have a precise 3D map of our galaxy. There are multiple advantages to building one, ranging from the large spatial resolution of observations conducted in our galactic neighborhood (which allows us to collect detailed information about specific types of objects), to the possibility of discovering and testing theories on the Milky Way which we could expand for the future study of other galaxies.

 RR Lyrae Variables 8.0 0.5 Globular Clusters 8.0 0.8 Miras 7.9 1.0 Cepheids 8.0 0.5 OB Stars 9.1 1.0 HI and HII regions 8.1 0.5 H2O proper motion 7.2 0.7 OH/ IR stars 8.1 1.1 Nearby stars/ Oort constants 8.9 1.0 X-Ray Sources 7.4 1.0 Planetary Nebulae 7.6 0.7 Red Giants 7.9 1.0 Sgr A* Proper Motions 7.7 0.9 Weighted Average 8.0 0.5