by Dr. Guillermo Gonzalez
How often have you heard that “the Sun is just an average star?” If you’ve watched many TV documentaries or read introductory astronomy books, chances are you’ve heard it more than once. In fact, even most astronomers still believe the Sun is just an average star. As you may have already guessed, I’m not one of them. But, holding a minority position does not necessarily make me (or any scientist) wrong. Showing that the Sun is not average is easy.
A review of what astronomers can know about stars from observation helps frame the issue. Astronomers learn about stars by studying the light they emit. Measuring extrinsic quantities such as apparent brightness, position in the sky, angular motion, radial velocity, trigonometric parallax, and high-resolution spectra allows them to calculate intrinsic properties, such as luminosity, surface temperature, surface gravity, surface composition, and sometimes mass.
After a century of detailed observations and theoretical development, astronomers have found a number of fundamental relations among these quantities. And, they’ve employed the “boundary conditions” of stars to infer something of their hidden interiors. As a result, some quantities are more “fundamental” than others. The less fundamental parameters (surface temperature, surface gravity, density) can be derived from the more fundamental ones. Mass and initial composition are among the most fundamental stellar properties, with age usually regarded as the independent parameter.
Stars range in mass from about one-twelfth to 100 times the Sun’s mass. Given only this information, one might conclude that the Sun is on the low end of the observed mass range. However, it is not. In fact, the Sun is among the 4 to 8% most massive stars in the galaxy. The frequency with which stars occur in each mass bin (also called the stellar mass function) varies widely. In other words, how does the number of low mass stars compare to high mass stars? Well, like almost everything else in nature, low mass members of the group are more numerous than the bigger ones.
While the Sun’s mass is below midrange, it is far above the average value. Let’s compare this to a more everyday example. Humans range in weight from a few to about 1800 pounds. Thus, the midrange is near 900 pounds. However, the average value is probably near 150 pounds. The midrange value is very different from the average, because very few people are near the upper end of the scale. Similarly, the average star has a mass near 20% of the Sun’s mass; these stars are called M dwarfs.
Astronomers cannot give a simple straightforward answer about how atypical the Sun’s mass is. One ambiguity results from mass loss. Some stars have lost most of their initial mass. For example, white dwarfs range in mass from less than half to 1.4 times the Sun’s mass. They are the cooling cinders of once more-massive stars. An astronomer wanting to compare the Sun’s mass to all stars within a given volume of galactic space would rank the Sun near the 4% most massive stars. However, if he or she wants to compare the Sun’s mass to the initial mass of the stars in the same volume, the ranking would come closer to 8%. In either case, the Sun’s mass proves atypical. This ranking will also change as the galaxy ages. Low mass stars can burn their hydrogen fuel for hundreds of billions of years, while stars more massive than the Sun that formed early on are now white dwarf stars, neutron stars, or black holes.
Using high-resolution spectroscopy, astronomers can determine the relative abundances of over 30 chemical elements in the atmosphere of a solar-type star. They've known since the middle of the twentieth century that the compositions of stars are not all the same. Older stars are more “metal-poor” (with less of the elements more massive than hydrogen and helium) than younger ones.
As detector technology and stellar atmosphere modeling software have improved, astronomers have been able to study more subtle stellar composition variations. Beginning about 10 years ago such data have shown the Sun’s metallicity to be moderately above average when compared to nearby stars. But, again, there is more than one way to compare stellar metallicity. The nearby star sample contains a mix of stars born over the entire history of the Milky Way galaxy. Restricting the comparison to stars similar in age to the Sun (4 to 5 billion years) makes the Sun appear even more rare in metal richness. Only now are nearby stars forming with the metallicity as high as the Sun’s.
The Sun’s metallicity can also be compared to nearby stars selected according to other criteria. For example, the Sun can be compared to other stars with giant planets. Doing so places the Sun on the metal-poor tail of the distribution. Interestingly, the star in this sample most similar to the Sun, 47 Ursa Majoris, also has the most similar system of planets (two giant planets in relatively large circular orbits). This finding implies that the final state of a planetary system is very sensitive to metallicity. So, not only is the Sun’s metallicity atypical compared to the general field population (most of which lack giant planets), but also atypical compared to nearby stars with giant planets.
A highly stable star, the Sun's light output varies by only 0.1% over a full sunspot cycle (approximately 11 years), though it may vary a bit more on longer timescales. Most (or even all) of this variation likely results from the formation and disappearance of sunspots and faculae (brighter areas) on the Sun’s photosphere. Lower mass stars tend to vary more, both via spots and relatively stronger flares. Among Sun-like stars of comparable age and sunspot activity, the Sun exhibits smaller light variations.
Some scientists argue that viewing the Sun near its equator from the ecliptic plane biases the measurement of its light variations. By contrast, astronomers observe other stars with randomly oriented rotation axes. This dependence on observer perspective results from the fact that sunspots tend to occur near the equator and the faculae have a higher contrast near the Sun’s limb. If the Sun were viewed over one of its poles, the light variation would be greater.
However, numerical simulations show that observer viewpoint cannot explain the low variations in the Sun’s brightness. Other anomalies exist in the sunspot cycle, but more research is needed to confirm them.
Uncommon Location and Orbit
A couple of solar anomalies concern the Sun's placement in the galaxy. First, it is located relatively close to the disk’s midplane. Given that the Sun oscillates vertically relative to the disk, and, like a ball on a spring, should spend most of its time near the extrema of its motion, this location is surprising. Second, the Sun is located very close to what is called the corotation circle—that place in the disk where the orbital period of the stars equals the orbital period of the spiral arm pattern. Stars both outside and inside the corotation circle cross spiral arms more often.
Some stellar parameters are both extrinsic and intrinsic. How can this be? Some parameters that are extrinsic to a single star can be intrinsic to a grouping of stars, like a star cluster or the galaxy. After all, the galaxy is made of stars (along with other things). Astronomers have determined the galactic orbits of several thousand nearby stars, and a variety of trends emerge. For example, old disk stars have less circular orbits than young disk stars. Compared to nearby stars of similar age, the Sun’s orbit in the plane of the disk is more nearly circular, and its vertical motion is smaller. With only the Sun’s galactic orbit to go by, one might conclude that it formed very recently, not 4.6 billion years ago (as known from the radiometric dating of meteorites and stellar evolution models).
What Does All This Mean?
These solar anomalies don’t have to be attributed to anything in particular. You could shrug your shoulders and say something like, “what a coincidence,” or “it’s just chance.” But, these answers are not very satisfying. Because the Sun plays an essential role in allowing and sustaining life on Earth, to conclude that the values of some of the Sun’s parameters required fine-tuning to permit such life seems reasonable.
Indeed, this “observer bias,” the Weak Anthropic Principle (WAP), simply states that observable environmental parameters must be consistent with life's existence. The WAP does not offer a complete “explanation” for the particular parameters observed for life's environment, as it cannot account for their origins. But the WAP does offer a way of recognizing new habitability requirements.
Uncommon Habitability of Earth
Many of the anomalous solar parameter values discussed in preceding paragraphs can be directly linked to the habitability of Earth. Some are obvious. For example, the low amplitude of variations in the Sun’s energy output keeps Earth’s climate from experiencing excessively wild climate swings. Other links may be less obvious.
The Sun’s mass affects the fine-tuning necessary for the existence of life on Earth in a couple of ways. First, the lifetime of a star on the main sequence (its stable burning period) strongly depends on its mass. While on the main sequence, a star fuses its abundant hydrogen fuel relatively slowly, leading to a gradual increase of luminosity. The most massive stars last only a few million years on the main sequence. Stars twice the Sun’s mass last just under two billion years, while stars half the Sun’s mass last nearly 60 billion years!
Thus, the more massive stars don’t last long enough for Earth-like complex life to come on the scene. This makes the fact that the Sun is not among the roughly 3% most massive stars understandable. If it were, we wouldn't be here. But what about lower mass stars?
Although low mass stars remain on the main sequence much longer than the Sun, this advantage does not factor into a discussion of the fine-tuning required for life because the age of the galaxy is only a little greater than the expected main sequence lifetime of a solar-mass star. Because low mass stars are less luminous than the Sun, a planet must orbit it closely to maintain liquid water on its surface. But, low mass stars tend to have flares similar in strength to those on the Sun. This means that the relative effects of flares will be more severe for life on a planet orbiting a low mass star.
What’s more, low mass stars are cooler and emit less ultraviolet radiation. This feature might seem like an advantage, but it’s not. Ultraviolet radiation hitting a planet’s atmosphere regenerates its ozone layer. The stronger its ozone shield, the better a planet protects itself from sudden increases in the ultraviolet radiation either from strong flares or nearby supernovae. Thus, the Sun offers better protection to strong transient radiation events than do lower mass stars.
How does the Sun’s metallicity (high relative to stars without planets and low relative to stars with planets) affect life on Earth? Assuming the Sun and its planets formed together from the same molecular cloud about 4.6 billion years ago, then the metallicity of that birth cloud supplied a very important initial condition to the formation of the planet-for Earth is made almost entirely of metals. If a lesser metal abundance were available early on, smaller terrestrial planets would have resulted.
The formation of gas giant planets, such as Jupiter, is a bit more complicated. Astronomers believe a large rocky core (about 10 to 15 times Earth’s mass) would first be necessary. Once in place, the core’s gravity would gravitationally accrete and retain abundant hydrogen gas from the protoplanetary disk, and runaway growth would result. This must be accomplished within about 10 million years, after which most of the gas is lost. If the gas is lost before the growth process is completed, then a gas giant will not form.
Because their dependence on metallicity is different, terrestrial and gas giant planets cannot be expected to form together from an arbitrary initial metallicity. Some systems may contain a terrestrial planet, but no gas giants. The presence of a gas giant in a circular orbit, both via the deflection of comets from the inner solar system and the delivery of water from asteroids early on enhances the habitability of a system. On the other extreme, a system may end up with too many gas giants, thus destabilizing other planetary orbits.
The high mean metallicity of the Sun relative to stars without giant planets probably correlates with the minimum metallicity required to build Earth-size terrestrial planets along with gas giant planets in large circular orbits. On the other hand, the very low metallicity of the Sun relative to stars with giant planets is probably related to the increased instability of planetary orbits in the circumstellar habitable zones of systems forming from metal-rich molecular clouds.
Finally, what does the Sun’s galactic location and orbit have to do with life on Earth? For starters, not all places in the Milky Way are equally habitable. So, to a certain degree, the place in which we find ourselves between spiral arms, in the thin disk, far from the galactic center-fits what we know about the distribution of building blocks and threats to life in the Milky Way. Even Earth's proximity to the corotation circle may be of considerable importance, given that this configuration maximizes time intervals between spiral arm crossings. Simultaneously having a nearly circular orbit also helps Earth avoid spiral arm crossings. We certainly don’t want those arms crossing too often, given the high frequency of supernovae that occur there.
The reason for Earth's proximity to the midplane is less obvious. Interstellar dust provides more radiation protection and that dust is most dense in the midplane. But whether or not the midplane position provides a decisive degree of protection from supernovae radiation remains unclear. Perhaps another deeper explanation is warranted.
As astronomers are learning more about the solar system’s surroundings and are able to better place the Sun in its proper context, they are showing the Sun to be rare indeed. They are also discovering that the conditions required for life are far more numerous and narrow than commonly believed. These findings, along with the many examples of fine-tuning in chemistry, biochemistry, physics, and cosmology, argue against chance explanations. A nonchance explanation called intentional design implies both mind and will. And I call that intentionality a God thing.
Sidebar: Implications for Design
The fact that the Sun is anomalous in several respects violates the popular Copernican Principle or “Principle of Mediocrity.” The Copernican Principle derives its name from Nicolaus Copernicus, who in the sixteenth century proposed a heliocentric model for the solar system, removing Earth from a place of centrality. Historical revisionists have reinterpreted this “dethronement” of Earth as demonstrating its “mediocrity.” Thus, they claim Earth and the solar system to be unexceptional in every way.
A more balanced account of history lends no support to this metaphysical hypothesis. Copernican advocates, assuming there is nothing exceptional about Earth’s capacity for life, have, over the past four centuries, speculated about life on just about every body in the solar system, including the Sun! The start of the twentieth century found Copernicans discussing advanced civilizations on Mars and in lush forests on Venus. However, they have been in constant retreat since the invention of the telescope.
By the end of the century, these revisionists retreated to the point of speculating about the possible presence of microbial life below the surfaces of Mars and Europa. Yet, Earth remains as the only oasis in the solar system.
Strong adherence to the Copernican Principle also misled astronomers’ expectations concerning other planetary systems. If Earth is mediocre, then there should be many other planetary systems that look pretty much like this one. Given these expectations, the first extrasolar planet, discovered in 1995, came as a shock—51 Pegasi b orbits its host star every 4.2 days. Since then, additional discoveries have shown that giant planets in very tight circular or large eccentric orbits are the rule, not the exception.
Progress in “astrobiology” would have proceeded more rapidly had scientists allowed for the possibility that Earth, the Sun, and the solar system are atypical. Instead, the anomalous characteristics of the Sun received scant attention in the astronomical literature. These anomalies supply important clues to habitability constraints for Earth life. They provide much needed guidance in a field of study that still suffers from lack of direction.
About author: Guillermo Gonzalez, Ph.D., is an assistant professor of astronomy at Iowa State University. His specialties are high-resolution quantitative stellar spectroscopy and astrobiology. He has published his research in many astronomical journals, including the Astrophysical Journal, Astronomical Journal, Astronomy & Astrophysics, Solar Physics, and Monthly Notices of the Royal Astronomical Society.
To assert that some of the Sun’s parameters must be fine-tuned to within a narrow range to permit life on Earth is reasonable. This assertion is called the Weak Anthropic Principle. Earth's Sun is unlike other stars in these characteristics:
Mass - The Sun's mass helps determine its lifespan. The Sun delivers fewer and less intense transient radiation events than do lower mass stars.
Initial composition - Not only is the Sun’s metallicity atypical compared to the general field population (most of which lack giant planets), but it is also atypical compared to nearby stars with giant planets.
Stability - As a star, the Sun is highly stable, which prevents wild climate changes. Observer viewpoint cannot explain the low brightness variations of the Sun.
Location - The Sun holds a surprising position in the galaxy. It is between spiral arms in a circular orbit. A location very close to the so-called corotation circle prevents it from crossing spiral arms too frequently.
Because these parameters must fall within a certain narrow range for Earth life to be possible, their convergence within that range argues for fine-tuning. Adding these examples to many additional ones from chemistry, cosmology, and physics greatly increases the overall required degree of fine-tuning for life. And fine-tuning argues for a Fine-Tuner, more frequently referred to as God, the Creator.
- Main sequence: The longest-lived and most stable part of a star's life cycle during which it fuses hydrogen in its core
- Interstellar extinction: The absorption of light by dust between stars in the Milky Way galaxy
- Mass bin: Refers to stellar mass function—the interval in mass on a plot of number frequency of stars with mass
- Ecliptic plane: The imaginary plane defined by Earth's orbit around the Sun
- Faculae: Brighter regions on the Sun's disk, usually associated with active regions
- Sun's limb: The visible edge of the Sun's disk
- Disk star: A star that orbits within the Milky Way's flattened disk
- Protoplanetary disk: The disk of gas, dust, and larger bodies that eventually forms a planetary system around a star
- Circumstellar: Surrounding a star
Here, the astronomer’s definition of a metal is employed (i.e., any chemical element heavier than helium). The term “metallicity” is applied to a scaled solar mixture of elements. A star can have a lower or higher metallicity than the Sun.
For more examples of possible anomalies related to the sunspot cycle, see D. F. Gray, “Stars and Sun: Treasures and Threats,” in The Tenth Cambridge Workshop on Cool Stars, Stellar Systems and the Sun, ed. R. A. Donahue and J. A. Bookbinder (1998), 193-209.
Increasing evidence indicates strong links between the Earth’s climate and the Sun’s activity. Such evidence includes the correlation between global temperature and sunspot cycle length during the past century, and the correlation between carbon-14 variations in the atmosphere and auroral activity, and the “Medieval Maximum” and the “Little Ice Age.” For further information on this topic, see T. I. Pulkkinen et al., “The Sun-Earth Connection in Time Scales from Years to Decades and Centuries,” Space Science Reviews 95 (2001), 625-37.
For example, it appears that the universe is “designed for discovery.” In other words, Earth dwellers seem to get a better overall view of the universe than most other places and times. A place in the midplane of the Milky Way and between spiral arms maximizes scientists' view of the distant universe. See G. Gonzalez, “The Measurability of the Universe: A Record of the Creator’s Design,” Facts for Faith 4 (2000), 42-48.
See D. R. Danielson, “The Great Copernican Cliche’,” American Journal of Physics 69, no. 10 (October 2001): 1029-35; G. Gonzalez and H. Ross, “Home Alone in the Universe,” First Things no. 103 (May 2000), 10-12.