Gonzalez and Richards Chapter Seven

Assumptions and implications are not the same thing.

Along the way, stars’ luminosity (absolute brightness, not apparent magnitude) and surface temperature were determined.  From that, a catalogue of stars was generated with our own Sun in the main sequence (G2).  At first we thought, our Sun was normal . . . but we had to redefine what was normal.  It turns out that our Sun is quite extraordinary – more later.  The spectrograph broke light down into the colors of the rainbow, as we have seen when light passes through a prism.  However, upon closer examination, we discovered black lines.  These lines were called “absorption lines” and have to do with chemical responses when photons and electrons cancel each other out.  “An absorption line results when electrons absorb photons in a particular energy level in atoms of a particular element” (p. 123). 

Our Sun has one signature – places where the lines appear; other stars have their own signatures.  Originally they thought that hydrogen would be the hottest and so everything was based upon the line caused by hydrogen absorption.  That didn’t work out because there was not a simple correlation between the star’s temperature and hydrogen absorption.  Now the system is rather complicated – but much more reliable. 

Because our Sun gives off so much visible light energy, it provides not only a good place to live – with full-spectrum lighting everywhere but in western Washington state apparently – but a good platform from which to observe experimentation. 

Although we can extract compositional information from the spectrum of a cool star, it is            inferior to that obtained from Sun-like stars, which are the golden mean of measurability in this respect.  Unlike those of hot stars, the optical spectrum of the Sun contains enough molecular lines for astronomers to derive useful data (such as isotope ratios) without dominating the spectrum.  This makes the Sun’s spectrum a nearly perfect compromise between the density of absorption lines and the integrity of the continuum (p. 125). 

And so we see that if there were to be complex or technological life out there somewhere, it would have to be on a planet revolving about a Sun-like star – and they might be looking right back at us! 

From that platform (Earth and Sun) we now jump to inferences we can make by examination of stars away out there.  As you can tell, they are merely points of light.  Even with the largest telescopes, all but the closest stars never resolve into disks but remain points of light.  This is good, it turns out.  When we map the heavens, we are literally moving from point to point with huge distances between.  At once it makes the trigonometry more simple and more difficult. 

Modern stellar astrometry has yet to reach a level of measurement precision limited by the angular sizes of stars (p. 126). 

However, look at all the information they pass our way:

First, we can study the Sun’s surface as a representative mid-range star.  Second, most stars have uniform surfaces with uniform properties, so knowing their surface details            would not add much important information.  Third, we don’t need to resolve stars to determine their basic properties, because they are spherically symmetric, simple, and thanks to their stellar specra, can keep few of their secrets to themselves (p. 126). 

So, we get the spectrographic results, calculate the angles and pretty much are able to ascertain what kind of star they are – are they the kind of stars that might have habitable planets around them or not? – up to this point: uniformly not. 

All of this stellar research helped us to verify some aspects of theory.  Because gravity from the Sun bent light from stars we were able to put legs on the General Law of Relativity.  In the opinion of the authors: “It turns out that the properties of stars are also delicately balanced for life.  And . . . are the key ingredients of habitable zones in the universe (p. 127). 

Another critical area of discussion has to do with the Circumstellar Habitable Zone.  This is “that region around a star where liquid water can exist continually on the surface of a terrestrial planet. . .” (p. 127).  Of course, the authors feel that this liquid water is required for “a few billion years.”  Be that as it may, it is intuitively obvious to life-forms such as ourselves that liquid water in huge quantities is required not only for simple and complex life, but for technological life.  Parameters:

They usually mark the inner boundary of the Circumstellar Habitable Zone as the point where a planet loses its oceans to space through a runaway greenhouse effect [you want to be thinking about Venus at this point], and define its outer boundary as the point where oceans freeze or carbon dioxide clouds form, both of which increase a planet’s albedo [the fraction of incident light or electromagnetic radiation that is reflected by a surface or body (as the moon or a cloud) Webster’s Ninth Collegiate Dictionary.] and trigger a vicious cycle of increasing coldness until the oceans freeze over completely [e.g., Mars]. 

Actually, it is much more precise than the Venus/Mars notes above: In the case of our Earth, if we moved it 5% closer to the Sun, we would evaporate the oceans and the solar wind would blast away the vapor.  If we moved it 20% further away, we would freeze them to the point where their internal circulation was of no benefit in warming them or us. 

Another of the details the authors remind us of has to do with the asteroids.  Mars, as it turns out, takes an incredible number of hits for us.  Because meteors accelerate as they move toward the Sun, Mars does not get hit as hard as Venus or Mercury, but it gets hit far more often than we do.  Its thin atmosphere insures that many more of them hit the ground than would otherwise hit the earth – you might imagine that a micro-meteorite could ruin your entire day. . . . 

What then of the Earth’s atmosphere?  The further toward the outer extreme in the habitable zone, the more heat is required to maintain liquid water.  Therefore, more carbon dioxide (a greenhouse gas) is required to heat the atmosphere and thus heat the surface to keep the water circulating.  But that is not entirely beneficial either:

A thick carbon dioxide atmosphere isn’t a problem for some types of life forms, but large mobile creatures require an oxygen-rich atmosphere with a relatively low concentration of carbon dioxide.  Moreover, a planet near the inner edge of the habitable zone, like Earth, will be more biologically fecund and diverse than one further out.  The sunlight’s energy maintains a large number of photosynthetic organisms, which, in turn, support a lush and diverse biosphere.  . . . . A planet that produces biomass more slowly than Earth may not be able to ready itself for the arrival of complex life before its host star leaves the main sequence.  It would also be more threatened by sudden external shocks to its ecosystem.  So it may be no accident that Earth resides very close to the inner boundary of the Sun’s habitable zone.  Habitability probably varies dramatically within the zone. Or to put it less charitably, it may very well be much narrower than currently assumed (p. 130). 

The authors then go on to describe the necessary ingredients for the habitable planet’s core.  It must be an iron/nickel core so as to “generate a life-protecting magnetic field” (p. 131).  It needs sulfur to reduce the core’s melting point: “Too little sulfur, and a liquid core will not only require a higher temperature to remain liquid but will be likely to freeze solid when the temperature drops.  Too much sulfur, however, and a pure iron solid core may not form at all” (p. 131).  There is also the need for potassium because P40 is a long-lived radioactive isotope that “helps to keep the mantle convecting and the crustal plates moving” (p. 131). 

In the aggregate:

When multiplied out, these additional factors greatly reduce the best estimates for the width of the Circumstellar Habitable Zone for complex life and probably also for simple life.  In short, a planet’s true habitable zone depends on much more than just the intensity of light from its host star (p. 131). 

Given the authors’ presuppositions, they require that the Sun be a main-sequence star for 4.5 billion years.  The earth then appears to have been in the habitable zone for most of its history and liquid water seems to have been present throughout.  From these assumptions, observations and deductions, it would appear that “Stars more than about 1.5 times the Sun’s mass are probably not viable habitats for complex life, because they spend relatively little time on the main sequence before they become red giants and while in the main sequence, their luminosity changes relatively quickly” (p. 132).  Whereas, I am not certain about the time dynamics of astrophysics and geophysics, it stands to reason that the band of habitability around a star is narrow and the parameters required for complex life are many. 

The authors then explore the possibility of life around M class dwarfs.  It turns out that they are too cold, the distance required too near, the radiation too great, the gravitation [stripping planets of any moon they might have had] too great thus synchronizing the planet’s rotation exposing one side to searing radiation and one side to freezing (p. 134).  Also any other planets orbiting would probably be closer to the star and thus perturb each other’s orbits.  One thing we have here in our solar system is rather smooth alignment of the planets.  They are heavy enough and far enough apart so as to avoid too much gravitational interaction (p. 135). 

Earlier in this review (with respect to the “Introduction”), I noted matters pertaining to the authors’ perspective on the “Anthropic Principle.”  Now (pp. 136-8), the authors flesh out their version of the principle “. . . which states that we should expect to observe conditions, however unusual, compatible with or even necessary for our existence as observers” (p. 136).  The implications of this are stark: in contrast with the customary view expressed in textbooks, that we infest an unremarkable solar system and hence ought to find many versions of ourselves “out there” somewhere, in reality, the parameters for complex/technological life are so many and fine that we might just as well rather expect to find ourselves alone in the (center of the) universe!  For instance, “. . . the Sun is among the 9 percent most massive stars in the Milky Way galaxy; most stars are M dwarfs.”  In addition, “The Sun is a highly stable star.  Its light output varies by only 0.1 percent over a full sunspot cycle (about eleven years), perhaps a bit more on century timescales” (p. 137).  Then the authors explore where we are not:

Another way to apply “Anthropic” reasoning is to consider what our home world is not like.  For example we’re not living on a large moon orbiting a gas giant planet, a planet with a significantly eccentric orbit, a terrestrial planet lacking a large moon, or a planet orbiting an M dwarf star.  These are all probably more common planetary configurations than ours.  If our local environment were simply a random slice of space, we’d be much more likely to find ourselves in one of those other settings or, even more probably, in some vast and empty track of space between stars or even between galaxies.  Quite reasonably the Weak Anthropic Principle says that we don’t “because” none of those places is compatible with the existence of technological life.  According to the Weak Anthropic Principle, and common sense for that matter, we can expect to find ourselves only in a place compatible with our existence.  Apparently only improbable places qualify (pp. 137-8).  

The next section in the book “A Latent Ruler” has to do with using the sun and trigonometry to measure other places.  When measuring things, it is helpful to know a “side.”  This, in turn, helps us to know angles and other sides.  Because mundane life on earth includes a free trip around the Sun each year, we know that one “side” is two AUs or Astronomical Units – adding up to about 186 million miles, give or take a trip to the corner grocery.  So, every six months we measure angles to stuff out there in space and from that calculate distances to nearer objects like Alpha Centauri (the Parallax method).  If we lived on Jupiter (thus having continuously dry-cleaned lungs) it would take us over six years to get our measurements – worse if we lived on Saturn.  On the other hand, if we lived around an M dwarf star, we would only have about 10% of that distance for the known “side.”  That would mean that the angles would be shaved even finer than they are with our “side” at 186 million miles.  “Put another way, it would take stellar parallax observers of one thousand M dwarf star/planet systems to observe the volume of space we can survey from our single planet” (p. 138).  Remember: a light year is 5.878 trillion miles . . . it takes four of those to get to the nearest star system.  So, two of the sides of the triangle are something like 23.512 trillion miles.  One side is only 186 million miles and so you can guess how small the angle is off 90 degrees.  The further out the star, the finer the angle. . . .   

So how does this help us?  Well if we are ever going to find ET (or another place to live when we ruin this one – so the reasoning goes), we really should be looking around the systemic neighborhoods of stars more or less like our sun.  Great, so how is this the place where they told us that Trig would save our lives? 

This is critically important if we are to understand the physics of stars, since the parallax of a star combined with its apparent brightness of a star [sic] gives us its luminosity. 

Luminosity tells us the total energy that a star generates.  This in turn, tells us about nuclear fusion and it dependence on temperature and pressure at the center of the star (p. 139). 

All this in turn tells us how much radiation a star is producing and whether or not we might expect to find a “habitable zone” around it and thus should expend the energy to find out if there is, in fact, a terrestrial planet in that habitable zone. 

Let’s summarize:

. . . the observer’s home planet must be far enough from the host star, the planet’s         atmosphere must not be too thick or murky, and there must be enough stars in the host star’s vicinity.  . . . Had Earth been closer to the Sun, it would probably have been a hothouse with a thick atmosphere like that of Venus.  If it had been much farther from the Sun, Earth would have needed a much thicker – and more obstructive – atmosphere with a great deal more carbon dioxide, to keep water flowing on its surface. But such an atmosphere, even with surface water, would still be hostile to animal life.  So in both cases, observations from the ground would be inferior to those we enjoy.  Once again, the most habitable location also provides an excellent overall setting for scientific discovery (p. 140). 

The final section, “A Handy Astrophysics Lab,” tells us that not only does our Sun help us with the math, it also helps us with the Physics.  This is not counter intuitive: the Sun is the closest star and so we can develop our understanding of all those other inferior loci throughout the galaxy by contrasting them with it.  Take spectrography for instance: “. . . the Sun’s atmosphere has qualities of both hot stars (a well-defined continuum in its optical spectrum and absorption lines arising from transitions in ionized atoms) and cool stars (molecular absorption lines in its spectrum and a convection zone near its surface).  If astronomers could only exist near a very rare class of star, such as a blue supergiant, then they would have had a hard time extending the knowledge of their home star to the more common main-sequence stars” (p. 141). 

Finally, our Sun produces what are called neutrinos.  What on earth are they?  It was speculated that “The particles would be so tiny, subtle, and noninteracting that most could pass through the entire Earth without hitting a single atom” (p. 141).  Great, so how do you catch one to look at it?  Scientists have developed these devices that catch a couple each day and show it so by getting them to react with stuff that the Sun’s other radiation cannot touch – under ground!  This helps us with our models of the very large and the very small in the universe.  The Sun is just barely luminous enough for neutrino astronomy and helioseismology to be practical scientific enterprises.  Observers with our level of technology living on one of the far more common K or M dwarf stars would not have these important observational tests available to them” (p. 142).  And so, when Supernova 1987A blew up away in the Milky Way’s satellite galaxy, the Large Magellanic Cloud, they would not have had any idea why they were getting bombed by unusual radiation.  We do: we got hit by neutrino radiation – not to worry. . . we won’t glow in the dark or sprout additional appendages. 

Login to add comments