NASA scientists recently published a stunning reproduction of the Milky Way Galaxy using data taken with the Spitzer telescope. As seen in the image below, two large spiral arms emanate from a central bar and encompass a number of smaller arms and substructure.
First, stars in elliptical galaxies generally have more radial orbits that frequently take the stars in close to the center of the galaxy. The density of stars in galactic centers causes large gravitational disturbances to any putative planetary system around such stars. In contrast, spiral galaxy stars exhibit more circular orbits that significantly decrease the possibility of gravitational disturbances from other stars.
Second, the spiral structures reflect regions of higher density stars and the region between the spiral structures exhibits a lower density. The Sun’s orbit in the Milky Way minimizes the number of passages through the spiral arms which, in turn, minimizes the chance for gravitational disruption of the planetary orbits.
Third, and perhaps most importantly, the existence of the spiral arm structure goes hand-in-hand with ongoing star formation. However, for a galaxy to continue forming new stars for more than ten billion years, it must continually receive new supplies of gas to replenish the gas used up by previous stars.
A paper published in the Astrophysical Journal found evidence that shows where the Milky Way received its supply of fresh gas. Using a detailed study of the stars in the halo around the Milky Way, a team of astronomers discovered that the halo exhibits a high degree of “clumpiness.” This clumpiness provides strong evidence that the Milky Way absorbed a large number of smaller dwarf galaxies during its history. The dynamics of these collisions would transfer the gas from the dwarf galaxies into the disk of the Milky Way, providing fuel for more star formation. In fact, it appears that such a process is occurring now in a location called the Virgo Stellar Stream.
Evidence consistent with the idea that Earth was designed to support life continues to mount. Not only does Earth orbit the right distance from a just-right star, it also resides in a just-right galaxy that collides with enough dwarf galaxies to continually form new stars and maintain its spiral structure. But, the collisions are not so frequent that they disrupt the planets orbiting around the sun.