Two weeks ago, I introduced my blog by describing my issue with how each of the Star Wars movies references civilizations in “a galaxy far, far away.” We astronomers have observed thousands of far, far away galaxies and so far none possess features similar enough to our Milky Way Galaxy (MWG) to be possible candidates to host an advanced civilization. In this post I describe yet another feature, namely that a galaxy’s spiral arms (where most new stars are formed) must be neither too tight nor too loose to harbor life.
Spiral Arms Must Be Just-Right
To demonstrate some of these features, figure 1 shows a detailed map of the MWG that was constructed from observations of different parts of the galaxy where each was observed at electromagnetic wavelengths spanning from the radio end of the spectrum to the X-ray end.
Figure 1: Structure of the Milky Way Galaxy. This map of the MWG was constructed from observations across the electromagnetic spectrum of different portions of the galaxy. Image credit: NASA/JPL-Caltech/R. Hurt
Figure 2 shows images of ten spiral galaxies that come the closest to matching the central bulge and spiral structure of the MWG. In every case, the spiral arms of these ten galaxies fail to manifest the symmetry of the MWG’s spiral arms. Also, they possess more spurs and “feathers” (bridges) between the spiral arms than does the MWG and/or a central bulge different in size and shape than the MWG’s. As I explain in my book, The Creator and the Cosmos,1 and in the fine-tuning compendium on our website,2 each of these three structural differences with the MWG makes the other galaxies noncandidates for the possibility of hosting advanced life.
Figure 2: Spiral Galaxies That Most Closely Match the Structure of the MWG. Top left to right: Andromeda Galaxy, NGC 4526, M100, Pinwheel Galaxy; middle left to right: NGC 908, M83, NGC 4921, NGC 1232; bottom left to right: NGC 6384, NGC 3344. Image credits: Adam Evans, NASA/ESA/Hubble Space Telescope/High-Z Supernova Search Team, Schulman Foundation, ESA/NASA, ESO, ESO, NASA/ESA/Hubble Heritage Archive, ESO, NASA/ESA/Hubble, NASA/ESA/Hubble
Now, astronomers have found another feature of spiral galaxies that must be fine-tuned for advanced life to be possible. They have discovered that spiral galaxies are a little like human beings in that it is not good for them to be too tightly or too loosely wound up.
When a galaxy’s spiral arms are tightly wound, the arms will be too close together for a planet in between any two of the spiral arms to safely host advanced life. Such a planet will be too close to the powerful ultraviolet radiation emitted by the young giant stars that reside in the spiral arms and too close to gravitational disturbances from giant molecular clouds that also reside in the spiral arms. Furthermore, when the spiral arms are tightly wound, they generate an inordinate number of spurs and feathers between the arms. These spurs and feathers expose planets in between the spiral arms to additional deadly radiation and gravitational disturbances.
When a galaxy’s spiral arms are loosely wound, the symmetry of their structure is easily disturbed. Such structure is prone to disruption by nearby galaxies, even nearby dwarf galaxies, and by internal inhomogeneities inside the galaxy.
What about galaxies with no spiral arms? Such galaxies are noncandidates for advanced life since the density of stars in galaxies without spiral arms is far greater than in galaxies with spiral arms. The star density is so great that planets with features that make the existence of life possible will experience disturbed orbits and high bombardment frequency by comets, asteroids, molecular clouds, and planetesimals.
MWG’s Spiral Arms Are Just-Right
A study by astronomers Si-Yue Yu and Luis Ho shows that only certain kinds of spiral galaxies will possess spiral arms that are not too tightly or too loosely wound.3 Yu and Ho analyzed 79 spiral galaxies where high-resolution images of the galaxies’ spiral structure were available. They found that:
- the larger a galaxy’s central bulge, the more tightly wound are its spiral arms;
- the higher the stellar concentration, the more tightly wound are the spiral arms; and
- the larger the total galaxy stellar mass, the more tightly wound are the spiral arms.
For galaxies where the central stellar velocity dispersion was greater than 100 kilometers per second, the two astronomers noted that the lower the central stellar velocity dispersion, the more tightly wound are the spiral arms.
All these correlations that the pair discovered are consistent with the predictions of the density wave theory for the origin and maintenance of spiral arms. This additional evidence for density wave theory means that, contrary to the assertions of young-earth creationists,4 astronomers really do possess a good and observationally verified understanding of galaxy spiral arm structure in the context of an approximately 14-billion-year-old universe.5
Though not addressed by Yu and Ho, the correlations they found provide additional evidence for the fine-tuning of the MWG for the benefit of advanced life. Relative to other large spiral galaxies, the MWG’s mass consists of few stars. This low ratio of stellar mass to total mass is exactly what is required for the MWG to have its spiral arms wound up to the degree required for the existence of advanced life on a planet located halfway between two of its spiral arms at the just-right distance from the center of the galaxy (see figure 3).
Figure 3: Location of the Solar System in the Milky Way Galaxy. The yellow dot shows the solar system’s location halfway between two of the MWG’s smaller arms. Image credit: NASA/JPL-Caltech/R. Hurt
The degree of fine-tuning required for a spiral galaxy’s arms to be ideally wound adds to the more than one hundred characteristics of a galaxy6 that must be fine-tuned for advanced life to be possible. Such extensive fine-tuning shouts to all of us that there must exist a super-intelligent, super-powerful Being who was intent on creating a home where human beings could live, thrive, and discover him.