For many years, astronomers have tried to map out the structure of the Milky Way Galaxy (MWG). Since we reside inside the galaxy, obtaining the relevant observations is no small task. After many years of difficult work, astronomers finally have a good idea of what the Milky Way would look like if seen from outside. Here is a rendition.
The beautiful spiral arm structure jumps off the page! While many galaxies exhibit something akin to the Milky Way’s spiral structure, far more appear elliptical and have much older stars (The color indicates star ages—blue means new stars, red means old stars). Astronomers believe that in order for a galaxy to maintain a spiral arm structure, new star formation must take place. When star formation ceases, the spiral structure then collapses and the galaxy takes on a more elliptical appearance. Consequently, observations should show galaxies making this transition.
The discovery of these galaxies and the confirmation of astronomers’ picture of galaxy development emphasizes one aspect of fine-tuning for a life-support star and planet. In order for a star to contain enough carbon, nitrogen, and oxygen for life (and enough uranium and thorium for plate tectonics on the life-support planet), at least two generations of stars must form, burn, and explode to enrich the galaxy with these life-essential elements. Then a third generation of stars must form from these previous stars’ ashes. Too-large galaxies form stars quickly and exhaust their gas supply. Consequently, star formation ceases before sufficient life-essential element production. On the other hand, too-small galaxies form stars so slowly that not enough of the heavy elements, like uranium and thorium, form (remember, these elements decay also).
Only in galaxies like the Milky Way does star formation occur quickly enough to produce the life-essential elements and last long enough to incorporate those elements into future stars. Additionally, the ongoing star formation ensures that the spiral structure lasts, thereby providing a suitable environment for a life-supporting planet.