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Gonzalez and Richards Chapter Eight

Assumptions and implications are not the same thing.

Posted Monday, August 15, 2005 by Gerald Vreeland

Chapter 8 of The Privileged Planet is entitled "Our Galactic Habitat."  As you might guess this is at once one of the most fascinating chapters (well, it is to me, anyway) and at the same time one of the most challenging in terms of the science, theory and mathematics.  It seemed necessary for me to read each section four times just to make certain I knew what the authors were trying to tell me.  Part of it, again, had to do with the incredible number of and depth of end notes.  

 

Several of these will be brought forward for comment in the paragraphs to follow.  Be warned, there are two things you will feel from reading this chapter (or its review): you will begin to feel infinitesimally small but at the same time, you may begin to feel incredibly fortunate.

The authors begin the chapter with a section on the various mythological understandings of the broad white strip of nebulous light across the night sky.  We land upon a name pproximating that given it through the Greek myths: The Milky Way galaxy.  Even in more recent times we considered that this was the only place that was in the Universe.  Now, of course, we know that it is but one in billions of "island universes."  However, is it only one in billions or is it the only one in billions?  This is the question the authors will seek to answer. However, before we extrapolate the results to the entire observable universe, we probably ought to indicate what is known in our present galaxy.  Because of our technology - and a little inference! - we have been able to identify three types of galaxies: spiral, elliptical and irregular "- the latter admittedly a bit of catchall" (p. 144).  We live in a spiral galaxy.  With our telescopes we can observe others like ours and we observe that they move faster in the center and the movement is most likely more chaotic, as it is in the center of our own galaxy.  We live out in a smooth patch of space between the Sagittarius and Perseus arms of the Milky Way Galaxy. Probably along the line of the "Golden Ratio," the Galaxy resembles a nautilus shell.

 

In our Galaxy, there are nearly countless stars of the various types indicated in the previous chapter.  However, there are a lot of other things of interest:

 

Astronomers can also see a motley assortment of matter between the stars.  These include ghostly giant molecular clouds, up to millions of times more massive than the Sun; diffuse interstellar clouds; supernova remnants; and the winds from dying red giant stars and their descendants, the planetary nebulae.  They sometimes can see a glowing interstellar cloud when bright stars heat its inside.  Such a fluorescing cloud is call is called an H II region, because its hydrogen gas is ionized.  More indirectly, astronomers often see interstellar clouds called reflection nebulae illuminated from the outside, when the light from a nearby radiant star reflects off its dust (pp. 145-6).

 

I should probably add that some of these regions are millions of degrees Fahrenheit and/or would instantly microwave us into a cinder were we in range.  Spectrography helps us to identify distance and contents.  All the tools help us to figure out what the Galaxy looks like.  We have ours divided into four regions: the "halo, bulge, thick disk, and thin disk. Each region is

characterized by the ages, compositions, and movements of the objects within it (p. 146).

 

The halo contains only old metal-poor stars in highly elliptical orbits. . . .  The bulge, as fat as the disk is thin, contains stars spanning a large range in metal content, from about one-tenth to three times the Sun's.  It is unlike the halo in that some stars still form there today.  The orbits of its stars are also elliptical, but less so than those in the halo.

The flattened thick and thin disks overlap.  The thin disk contains the greater diversity of objects, including most of the stars in the Milky Way, while the thick disk is more puffed up with older, more metal-poor stars. In [our] neighborhood, only a few percent of the stars are members of the thick disk; Arcturus, the brightest star in the constellation Boötes, is probably the most prominent thick-disk star in the solar neighborhood. Even within the thin disk, older objects are more spread out on both sides of the mid-plane (p. 146).

 

Where are we?  We are about 7 kiloparsecs from the galactic center or about 22,400 light-years away - give or take a few bazillion miles [admittedly, using a ruler on a chart like this in the

book (p. 145) is like using a sledge hammer to perform brain surgery - you can go through the motions but accuracy will be a challenge. . .].  Now, imagine looking at an old LP record - edge on!  Distances get a bit finer from top to bottom: We are about 250 light-years from the galactic mid-plane.  But the really astonishing thing is that we've been able to study just about every conceivable kind of star and interstellar phenomenon from our little backwater of space.

 

As we've said, the gas and dust in our eddy is rather diffuse compared with other regions nearby (the two arms of the galaxy) and further off (the galactic core - A.K.A. the zone of avoidance because you cannot see any inter-galactic objects looking that way).  But we wouldn't want to move out of our little place close to the plain: because although we might get rid of some dust, we would get a lot more visual noise from bright stars and such in the center of the galaxy thus increasing the size of "the zone of avoidance."  Too much glare. . . .

 

Although stuff gets downright. . . well. . . “milky” when we look to the center or away to the Perseus arm, we have a wonderful vantage point from which to look at other close spiral galaxies.  When we do, we see that the reason the arms are so fuzzy is not because of increased star concentration; rather

 

. . . stars are only about 5 percent more concentrated in the arms.  The arms appear so much brighter because they contain star nurseries, where the brightest stars are born and die.  Their intense radiation illuminates the surrounding gas and dust, both strongly concentrated in the arms.  Therefore, while spiral arm dwellers would get a closer view of the rarer very massive 0 stars, overall, this bright, thick dust would obscure their view of the local and distant universe (p. 147).

 

It would also most likely microwave them into oblivion. . . .  Alright, so there's how you know; now how do you know you know?  We have, in our galactic neighborhood, some superimposed galaxies - those occulting each other or colliding with one another.  We can tell by how much we can see of what's behind through what's in front the density and light transparency of the gaps versus the disks and arms.  We're off in celestial inferential Never-NeverLand; but the guesses are educated and seem to reflect what we can see of our home galaxy.

 

The authors then explore some other regions of the galaxy and hypothesize what it might be like to observe from there.  First the orbital galactic globular clusters would be too bright and chaotic. They would not offer a good view of the universe beyond - but oh, the view of the Milky Way!  Away from them, yet still in the halo, most of the stars are of the same type.  Although you might be able to look away and see other island universes, you would have a hard time inferring anything about them:

 

It's only because we have a good grasp of the physics of stars that we can properly interpret the distant galaxies - after all, they're made of the same stuff we get to see up close in our immediate galactic neighborhood. In short, halo dwellers would have a hard time fathoming distant galaxies, and the cosmos as a whole (p. 150).  

 

At the risk of being tedious, I'm going to quote another paragraph - because it talks about a region of space that I'm most enamored of - ask me if you want to see an 8 X 10 glossy of the region that my cousin shot through the Mt.Wilson observatory.

 

Even though we don't have a bird's-eye view of the Milky Way, our perch is quite felicitous.  First, we live among a small collection of galaxies called the Local Group.  The Milky Way and Andromeda (Messier 31) Galaxies are its two largest members, followed by the galaxy in the constellation Triangulum (Messier 33).  Both M31 and M33 are spiral galaxies, with the latter presenting us with a nearly face-on view.  These teach us much about the overall structure of our home galaxy (p. 150).

 

Actually, the process is not as haphazard as it sounds: we can actually map the galaxy from the inside.  We understand "differential rotation"  - the fact that the inside goes faster than the

outside in a spiral galaxy - like a vortex.  And from motion and varying densities of stars and dust, we can pretty well determine the size and shape of things.  So it is good that we do not live in an irregular galaxy or an elliptical galaxy because the motions are much more chaotic.

 

It is also advantageous for us that we do not live in one of the giant galactic clusters - where there are literally hundreds of galaxies.  We might have a better idea about the dynamics of our own galaxy; but we would lose in terms of being able to see other areas of what to us is the visible universe - it would be completely occulted by our near fuzzy, nebulous neighbors.

Conclusion:

 

In short, settings in the halo, a globular cluster, the bulge, a spiral arm, an isolated galaxy, a denser cluster of galaxies, an irregular galaxy, or an elliptical galaxy would be less revealing than ours.  We occupy the best overall place for observation in the Milky Way galaxy, which is itself the best type of galaxy to learn about stars, galactic structure, and the distant universe simultaneously; these are the three major branches of astrophysics (p. 151).

 

The authors conclude this section by recalling the romanticized past wherein people used to hypothesize civilizations on the Moon, Mars, Venus, Jupiter, and even the Sun.  Thanks to our

deep-space probes we know that the possibility of simple life, much less complex or technological life is beyond those spheres.  As our knowledge of the universe expands we find that the parameters for complex/technological life get ever more narrow.

 

We like a good far-flung space western as much as the next fellow, but when a self-declared expert starts talking about civilizations all over the galaxy not as whimsical fiction but as cosmic inevitability [e.g., Sagan], we get skeptical.  For just as most of the Solar System doesn't meet the strict requirements for complex or technological life, the same seems to be true for most of our galaxy (p. 152).

 

And so, just as we have a Circumstellar Habitable Zone in our own solar system, there is likewise and Galactic Habitable Zone.  "And its first requirement is to maintain liquid water on

the surface of an Earth-like planet.  But it's also about forming Earth-like planets and the long- term survival of animal-like aerobic life" (p. 152).  And so the authors move on to how terra-

planets are formed.

 

The authors then tell us of they think our planet was formed, based upon the observations of the formations of star systems still going on today - or however long ago their light left them. Hang on:

 

The Big Bang produced hydrogen and helium and little else.  Over the next 13 billion years, this mix was cooked within many generations of stars and recycled.  Beginning with the fusion of hydrogen atoms, massive stars make ever-heavier nuclei deep in their hot interiors, building on the ashes of the previous stage and forming an onion-like structure.  Exploding as supernovae, the massive stars eventually return atoms to the galaxy . . . producing heavy elements that didn't exist before.  As a result, our galaxy's metal content . . . has gradually increased to its present value, which is close to the Sun's. Today, metals make up nearly 2 percent of the mass of the Milky Way's gas and dust in the disk (pp. 152-3).

 

Their conclusion?  Star dust literally courses through our veins (p. 153).  Well One way or another, I suppose, they're right. . . .  We must remember that to these authors, anything above

the weight of hydrogen and helium are considered metals.  We must cook and blow elements until our important standbys are present: carbon, oxygen, nitrogen - and enough iron and

potassium-40 to spin a magnetic planetary core; If there is too little metal, you get a preponderance of gas giant planets; if there is too much metal, you get a preponderance of planetesimals that pock-mark your home-world like the moon and Mars!  The authors then attempt to demonstrate that location is everything.  You cannot be too far out or there will be an absence of material to work with; you cannot be too far toward the vortex or there will be too much chaotic movement.  "To find enough heavy elements billions of years before our system began to form, you should look at our galaxy's inner region. . . . But our inner galaxy is one tough, crowded neighborhood, with little patience for some planet trying to pretty itself up with even a little primitive life" (p. 156).

 

We have learned much by the observation of supernovae.  From Type la and Type II supernovae we can determine the synthesis of elements in the "ensuing maelstrom" (p. 157).  We also know that the rate of these explosions has been decreasing since the beginning of galactic formation. We are also told that the production of Iron narrows the time frame for forming Terra-planets. This, of course, relates to the need for tectonic plate activity.  The authors conclude this section by noting that there were not enough of these heavier building blocks to form Earth-like planets until the galaxy was several billion years old - and then these elements were primarily in the inner regions.  "By themselves, these factors limit the time and place in our galaxy wherein Earth-like planets can form" (p. 159).  However, this makes me wonder if there was some great migration that gradually brought us out from a star-forming region to the relatively safer backwater in space that we now enjoy.  

 

As we find out, troubles come in pairs: we can be impacted into oblivion or we can by radiated into incineration.  It is thought by some that some of the mass extinctions were the result of

impacts (or ice-ages, or solar flares, or predation by voracious mammals or aliens, or. . .).  However, there is nothing to indicate conclusively that anything of the "planet-killer" class has

struck us for a long time.  Radiation has also been low and we have not passed through and dust or gas clouds that would be superheated by the friction of our motion.  So, at least as long as

there has been life, we have had relatively smooth sailing.  As we look off in the distance, this should be the case for only the next billion years or so. . . . Well, I won't be around in this form, anyway. . . . 

 

At this point, I must go into something of a digression: it seems that the authors really put the stake into the evolutionary vampire's heart.  "High-energy radiation may work its destruction on unprotected life less dramatically than large impacts, but it is no less effective" (p. 161).  From there, I would like to look at end note 48:

 

[Referring to the study by Vulic, Lenski and Radman] While this study has come closer than any other in claiming to produce a new species of bacteria, it has not actually done

so.  The researchers produced a genetic barrier between two identical lines, which they admit is "much smaller than the barrier between such clearly distinct species as E. coli and Salmonella enterica."  Thus, even with the highly artificial and extreme selective pressures applied by the scientists to rapidly reproducing bacteria in a laboratory setting, there is still no evidence that random genetic mutations yield evolutionary innovations above the species level.  There is only evidence for negative mutations, which produce cripples eliminated from a population by natural selection.  Add to this the fact that the Earth experienced much higher radiation levels than it has now, via nearby supernovae, solar flares, and potassium-40 in the oceans, yet life hardly did anything interesting for about two billion years. [!]

 

Even if we granted Scalo and Wheeler's premises about biological evolution, we disagree with one aspect of the application of their theory to astrophysical settings: They argue

that a higher rate of nonlethal intermittent radiation events wil accelerate evolution.  In every astrophysical setting we can think of, however, an increase in the rate of the low  intensity events is also accompanied by an increase in the rate of the high-intensity events (this is true for supernovae, gamma ray bursts, stellar flares, etc.).  Thus, within their  model, an accelerated rate of evolution will be accompanied by a higher probability of sterilization.  Complete sterilization will be less likely for the simplest life and most likely for the most complex life. Ironically, even within this model, there is still a preferred place for life in the Galaxy, since low supernova rates will slow the evolution of life and high supernova rates will kill off complex life (p. 386)

 

All that to say that we need to avoid solid impacts and radiation impact as well.  It is a good idea that we are where we are because: "'energetic transient radiation events,' . . . include active

galactic nucleus (AGN) [super-massive black hole] outbursts, supernovae, and gamma ray bursts.  These and other radiation sources are more threatening in the inner regions of the Milky

Way galaxy, simply because stars are more concentrated there" (p. 161).  With respect to the first of these: "Black holes are fearsome objects, distorting space, time, and common sense, so

densely packed that not even light can escape their horizons" (p. 162).  And so "The safest place to be during an AGN outburst is in the outer disk, far from the nucleus, and probably close to the mid-plane, where Earth happens to be.  The worst place to be is in the bulge, with scorching radiation and stars with highly inclined and elliptic orbits, which can come close to the energetic nucleus or pass through its jet [from the accretion disk of the black hole]" (p. 162).

 

The Type II supernovae belch out radiation as well.  The authors illustrate this with a picture of M81, "an elegant spiral galaxy in Ursa Major."  It is many millions of light years away and

although it is regularly impossible to resolve any of its stars from the ambient glow with even our largest telescopes, supernova 1993J was visible for several months.  This Supernova appears to be in one of the outer spiral arms on our side of the galaxy.  The galaxy appears oblique to us comparable to the angle of a fried egg in a pan on the stove at arm's length.  Do a thought experiment: imagine you lived only a few light-years from it - you would be either blown out of the galaxy or incinerated by the radiation.  The authors also discuss gamma ray bursts that strike us from deep space.  But since these events are weak and rare, we needn't worry about them - unless one happens in the neighborhood because they are "among the most energetic transient events since the Big Bang" (p. 164).

 

We can also be thankful that we do not live in a gas or dust cloud.  Some stars move at speed of up to 250 kilometers per second.  At such speeds friction would produce X-rays and dust would peel the bridge right off the paint.  "According to John Lewis a dust grain traveling that fast 'would have a kinetic energy equal to the explosive power of over one hundred thousand times its weight of TNT'" (p. 387).

 

As with the position in the habitable zone in the solar system, what can be said about the application of the Weak Anthropic Principle to the habitable zone of the galaxy?  Because of timing and positioning, it would appear that our Solar System in something of a golden mean for building Earth-like planets.  Also, "The Sun's orbit in the disk is also more nearly circular than  most other stars of its age, and its motion perpendicular to the disk is less pronounced" (p. 165).  

 

We are close to the corotation circle (matching the angular velocity of the nucleus).  We should not be too close to it or we will pass through one of the arms of the galaxy with all the trouble mentioned above.  "But stars actually at the circle will resonate with the spiral arm pattern, eventually getting sent on large excursions and invariably visiting spiral arms.  So for

maximizing the time away from spiral arms, a star should be close to, but not actually at, the corotation circle" (p. 165).  "The Sun's nearly circular orbit and proximity to the corotation

circle make it less likely that it has recently crossed or will soon cross a spiral arm" (p. 165).  

 

Of course, it is not impossible that we should escape any possible threat of supernovae radiation.  The spiral arms that we are between are ragged and it is possible that something close to us could explode and shower us with unwanted radiation.  At the point that it passes the planet, the side facing the radiation flow would be at risk – at whatever level! – the side facing away would be virtually unscathed – a 50/50 proposition.  However, as we look around the neighborhood, the only ones that look about ready to blow (by size and coloration) are at a safe distance.  Be that as it may, we appear to be in the best location between the arms and relative to the mid-plain for habitability (p. 166).  Because of the advance in precision of our measuring of the galaxy it could be that we will find that the Galactic Habitable Zone shrinks down to very small and isolated regions of space.  Certain classes of planets, in certain types of solar systems, in certain areas of the galaxy (between spiral arms and adjoining the corotation circle) could potentially force us into a group of one in the galaxy, if not the universe (cf. p. 167). 

 

The final section of this chapter is “Other Galaxies.”  Some of the parameters for complex and/or technological life elsewhere are difficult to meet.  For instance: “About 98 percent of galaxies in the local universe are less luminous – and thus, in general, more metal-poor – than the Milky Way” (p. 167).  This means that other island universes might be devoid completely of terrestrial planet of about our size.  We should probably exclude elliptical galaxies because their stars and hence any possible planetary systems are on much more chaotic orbits and would be more likely to take a trip to their respective galactic cores.  They would be more likely to pass through dust and gas clouds at high rates of speed and thus burn up any possible atmosphere. 

 

Some galaxies have interactions with other galaxies – with potential calamity.  We see those that have, are and will soon pass through each other.  The titan gravitational forces will tend to strip off stars from galaxies – much less planets and planetesimals.  It is thought that many galactic nuclei have super massive black-holes at their centers.  These become active – blasting out radiation – when bombarded with new material, planetary or stellar. 

 

Our Local Group is rather small by comparison with some groups (e.g., the Virgo and Coma galactic clusters).  This is a good thing because when a cluster has thousands of galaxies, certain ones tend to become dominant and strip off stars with their powerful gravitational forces.  In turn, these close configurations of galaxies and stars have more of a tendency to attract each other and cause more reactions with super-super-massive black holes. 

 

Massive cD elliptical galaxies are found in the centers of may rich clusters, presumably having grown at the expense of many hapless smaller galaxies.  With their intense

nuclear activity, such super galaxies would be anything but super places to live.  Overall, then, rich clusters are probably less habitable than sparse groups (p. 168). 

 

And so the authors conclude:

 

Clearly, other regions of the Milky Way galaxy and other regions of the nearby universe are, by and large, quite different from our present location.  Precious few plots of galactic real estate are as amenable to complex life as ours, to say nothing of its value for observation.  Our home is a fairly comfortable porch from which we can gaze out to the ends of space and the beginning of cosmic time.  And as we’ll see, not all times in the history of the universe are like our present (p. 168).

Guillermo Gonzalez and Jay W. Richards, The Privileged Planet: How our Place in the Cosmos is Designed for Discovery (Washington, DC: Regnery, 2004).

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