Galactic Green

Our Milky Way Is Amazingly Designed for Life

“It was the best of times,it was the worst of times." The opening line of Charles Dickens's A Tale of Two Cities applies just as well to two groups of researchers today as it did to Paris and London in the time of Dickens's novel. For those who accept the anthropic principle (the observation that the universe is fine-tuned to make possible the existence of advanced life) and its theological implications, this is the best of times. For those who don't, it may seem the worst.

Three Australian astronomers, Simon J. Mutch, Darren J. Croton, and Gregory B. Poole, have discovered that the description "ideally timed for life" applies not only to Earth's sun and moon but also to our Milky Way Galaxy (hereafter Milky Way or MWG). In a recently published paper in the Astrophysical Journal,¹ the three astronomers declare that the Milky Way is currently experiencing a "midlife crisis" that holds significance for our existence.

To describe this "crisis," we must first explain how galaxies are categorized by color. Though it may seem counter to the colors we usually associate with hot and cold, young stars, which tend to be hot, are blue-colored, while old stars, which tend to be cooler, are red-colored. So galaxies in which star formation proceeds aggressively shine with a blue color, while galaxies in which star formation has ceased appear red. As Mutch, Croton, and Poole point out, astronomers have typically categorized galaxies as belonging to either the red population or the blue population.

Going Green Is Best

The Milky Way, however, fits into neither the red nor the blue category. It has taken on a green hue. This is because, while star formation in the MWG has subsided some, it has not yet ceased. Thus, our galaxy contains a combination of blue stars and stars that aren't yet old enough to be red but have aged enough to be yellow. Blended together, these stars give the galaxy a green appearance.

Green galaxies are rare, but they are exactly what advanced life requires. A galaxy dominated by blue stars will bathe its planets with many flares—flares too abundant and intense, and with too much ultraviolet and x-ray radiation, to permit life to exist on any of the planets. A galaxy dominated by red stars will also bathe its planets with many flares—again, flares of deadly intensity. A red galaxy also exposes its planets to more supernova and nova events (stellar explosions) than advanced life can possibly handle.

Another problem for galaxies dominated by red stars is that they lack the necessary level of ongoing star formation to sustain their spiral structure. But galaxies dominated by blue stars, where star formation is advancing aggressively, experience major disturbances (warps, bends, spurs, and feathers) in their spiral structure, so they cannot maintain a stable spiral form, either.

But the green Milky Way, in addition to being of appropriate size and mass to contain the elements that life requires, has another characteristic that allows for the existence of advanced life within it: its spiral arms are stable, well separated, highly symmetrical, free of any significant warps or bends, and relatively free of spurs and feathers. In part, these spiral-arm features are possible because the galaxy is dominated by yellow stars which are complemented by a significant population of blue stars.

The Australian astronomers label the Milky Way's present condition as a "midlife crisis" (tongue in cheek) because our galaxy is indeed experiencing a "midlife" event: it is transitioning from a star-forming site to a no-longer-star-forming site. And this midlife period appears to be the "best of times" for the sustainment of living things. Just as a human being's midlife tends to mark a transition from intense focus on building material resources to a wider focus on pursuing the meaning of life, so, too, the Milky Way has transitioned from its role in building the required ingredients for advanced life (carbon, nitrogen, oxygen, phosphorus, calcium, iron, etc.) to one in which it can now, for a relatively brief time period, sustain advanced life.

However, that's just part of the "best of times" story. Mutch, Croton, and Poole go on to show that the Milky Way manifests several other unique features that favor its ability to support advanced life. For instance, for a galaxy of its size and mass, the MWG is "under-luminous," that is, not as bright as it should be.²This might not sound good at first, but the unexpected lack of luminosity actually means that Earth is shielded from exposure to life-threatening levels of radiation—levels that would otherwise make this the "worst of times" for our planet's living things.

As a bonus, this under-luminosity, combined with Earth's particular location in a dark region of the galaxy between spiral arms, makes for exceptionally dark nighttime skies. Such dark skies enable astronomers to explore the universe in much greater detail and depth than they would otherwise be able to do.

The Best Neighbors

The Milky Way is also extraordinarily fortunate in its neighbors. Its nearest companion galaxies, the Large and Small Magellanic Clouds, are located 160,000 and 200,000 light years away, stretch 15,000 and 7,000 light years across, and contain mass equivalent to sixteen billion and seven billion solar masses, respectively. They rank as the largest "satellite" (companion) galaxies in the Local Group (the galaxy cluster in which the Milky Way resides).

The configuration of the Milky Way with its two nearby companions, the Magellanic Clouds, is exceedingly rare—and significant for life. Even after extensive searching, aided by the Sloan Digital Sky Survey, astronomers have found no other example of this configuration anywhere in the universe.³ They did find 22,581 isolated galaxies otherwise similar to the Milky Way and without another large galaxy in their vicinity. A few of them (3.5 percent) have a neighbor as large as or larger than the Small Magellanic Cloud within 500,000 light years' distance. But astronomers have found no close analogues—that is, no real twins—to the Milky Way–Magellanic Cloud galactic system.

Here's why that system makes such a difference for the possibility of life, especially advanced life. The Large and Small Magellanic Clouds are near enough and massive enough to send a steady supply of gas into the Milky Way. They are also big enough and close enough that their gravity efficiently funnels a steady supply of small and gas-rich dwarf galaxies into the MWG. This steady, gradual supply is so well-balanced as to sustain the Milky Way's spiral structure without disturbing its overall symmetry, structure, and location.

By contrast, the Andromeda Galaxy, also a member of the Local Group, with a total mass similar to that of the Milky Way, has experienced relatively infrequent merger events with good-sized dwarf galaxies.⁴ As a result, its spiral structure is warped and disturbed to such a degree as to make it an unsuitable site—the "worst of times"—for the existence and sustenance of advanced life. Astronomers have noted that Andromeda's warping and distortion represent the norm for large spiral galaxies; in fact, Mutch, Croton, and Poole comment that the Andromeda Galaxy could easily serve as a "template" for the typical spiral galaxy.⁵

The Milky Way has maintained its spiral structure thanks to its just-right age, its just-right star-formation history, and the just-right size and location of companion galaxies within its just-right galaxy cluster. I find it hard to imagine that all these "just-rights" could be the result of mere happenstance. To me, they represent one more part of God's signature, a reminder of the careful craftsmanship that shows up throughout the universe—on all size scales—giving humanity good reason to wonder and to look for a future that will be the "best of times" yet to come. •

Endnotes
1. Simon J. Mutch, Darren J. Croton, and Gregory B. Poole, "The Mid-Life Crisis of the Milky Way and M31," The Astrophysical Journal (Aug. 1, 2011): http://iopscience.iop.org/0004-637X/736/2/84.
2. Chris Flynn et al., "On the mass-to-light ratio of the local Galactic disc and the optical luminosity of the Galaxy," Monthly Notices of the Royal Astronomical Society (Nov. 2006): http://tinyurl.com/74wqajo; F. Hammer et al., "The Milky Way, an Exceptionally Quiet Galaxy: Implications for the Formation of Spiral Galaxies," The Astrophysical Journal (June 10, 2007): http://iopscience.iop.org/0004-637X/662/1/322/.
3. Lulu Liu et al., "How Common Are the Magellanic Clouds?" The Astrophysical Journal (May 20, 2011): http://iopscience.iop.org/0004-637X/733/1/62.
4. David L. Block et al., "An almost head-on collision as the origin of two off-centre rings in the Andromeda galaxy," Nature (Oct. 19, 2006): nature.com/nature/journal/v443/n7113/full/nature05184.html.
5. Mutch, Croton, and Poole, op. cit.

From Salvo 21 (Summer 2012)
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