Grasslands Observatory - Essays - The Milky Way versus M31

The Milky Way versus M31

By Tim Hunter

Introduction

M31, the Andromeda Galaxy, NGC224, is the nearest spiral galaxy to our own galaxy, the Milky Way. It is often said to be the most distant object one can see with the unaided eye, because it is readily visible to the naked eye from a dark sky location. The nature of the Milky Way and M31 were not realized until the 1920’s and 1930’s when work by Hubble and others showed that M31 is a vast stellar system similar to yet quite distinct from the Milky Way. Both are large, luminous spiral galaxies which are the dominate members of a small cluster of nearby galaxies known collectively as The Local Group. M31 besides being the closest large galaxy to the Milky Way is a very important “laboratory” for observation of galaxy dynamics and evolution. Baade’s work on the concept of Population I and Population II stars was based on his observations of M31 (Walterbos 2000). Study of M31 has yielded fundamental knowledge about star formation and evolution, the distance scale, and evolution of the Universe (Moore 2002). Indeed, about 30 novae can be detected in M31 each year, though the only supernova detected in M31 was in 1885.

For much of the last century M31 and the Milky Way have vied for the title of the largest galaxy in the Local Group (Sky & Telescope 2000). This paper will present an overview of both galaxies and contrast their features and attempt to draw a conclusion as to their relative sizes, luminosities, and masses. In other words, which one is the first among equals- primus inter pares? This is not an easy question to answer. The mass, size, and luminosity of the Milky Way are particularly hard to measure since we are imbedded in the Milky Way and can never hope to see it with the same perspective with which we view M31. Unfortunately, M31 is significantly inclined to our line of sight, which complicates our ability to measure many of its parameters.

The Milky Way is a large spiral galaxy which probably has a bar. It often is classified as a type Sbc galaxy, but if it has a bar, then it probably should be classified as a type SBbc galaxy. The Solar System is 8 kpc from the Galactic center. The structure of the Milky Way remains uncertain due to our immersion within it. Radio studies of hydrogen atom radiation at 21 cm can delineate the gas clouds associated with the spiral arms in the Milky Way. This work has also been supplemented by CO observations which suggest there are four spiral arms in the Milky Way (Moore 2002).