The Sun plays an essential role in allowing and sustaining life on Earth, the values of some of the Sun’s parameters are fine-tuned to permit life on earth, which points to design. The Sun has a Just-Right Mass to give enough heat to support life, which is uncommon. The Sun is a single star, which is the only kind of star system that can support life ( there are binary stars etc. ). It gives off the right amount of energy. The visible light is made of many colors. We can see three of them (our brain constructs all sorts of color hues from that information). its fusion reaction is finely tuned. The outward pressure from the fusion reactions keeps the stars from collapsing. The sun is the most perfectly round natural object known in the universe. It has the right amount of life-requiring metals. It seems to contain just the right amount of life-requiring metals, even compared with otherwise similar nearby stars. It contains enough metals for building a habitable planet, but it doesn’t have too much, which might have produced an unstable planetary system with too many massive planets. It is a highly stable star. 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. Uncommon Location and Orbit: 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.
The Sun, a typical middle-aged star, is the most important astronomical body for life on Earth 4
The Sun has a Just-Right Mass
A star more massive than the sun would burn too quickly and too erratically to support life. If the sun were less massive, Earth would have to be closer to the sun in order to get enough heat to support life. But, if Earth was closer to the sun, the sun would exert so much gravitational force on the earth it would cause Earth’s rotation period to slow down to a point that one side of the planet would get too cold and the other side would get too hot to support life. In comparison to Earth, the sun is a very massive object. As illustrated below, if the sun was the size of a basketball, the earth would be less than the size of a BB used in a BB gun. The sun has the just-right mass to support life on Earth.
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. 8
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
The Sun is a Single Star
Life is not possible on a planet that does not orbit any star because the planet would be too cold to support life. Life is also not possible on a planet that orbits a binary star system or a multi-star system because the gravitational forces exerted on the planet by the additional stars would frequently pull the planet out of a temperature zone where life could exist. Earth orbits a single star system which is the only kind of star system that can support life. [See also, Hugh Ross, “Search for Planets Draws a Blank” (reasons.org)]
Right amount of energy given off
The amount and type of energy given off by the Sun correspond closely to what is expected from its surface temperature (6000 degrees Kelvin). Stars hotter than the Sun are more bluish (and emit a relatively greater amount of UV radiation) and cooler stars are more reddish (with greater amounts of IR radiation). The visible light is made of many colors. We can see three of them (our brain constructs all sorts of color hues from that information). Other organisms with eyes do not necessarily see the world the way we do; some cannot see color, but some can see ultraviolet in addition to color (many insects). Some have organs to sense infrared, for hunting warm prey (snakes).
Fusion reaction finely tuned
The outward pressure from the fusion reactions keeps the stars from collapsing. The inward pressure from gravitation keeps the stars from exploding. If the fusion reactions in the core become too weak, a star can and does collapse. Such collapse can provide new conditions in a core that result in new types of fusion reactions so that expansion follows. If fusion reactions in the core become too strong, a star can and does explode. Such events can be observed. When a star explodes it shines with extreme brightness for a while; it turns from an unnoticed to a "new" star, a "Nova". Stars, like our Sun, where inward pressure and outward pressure is nicely balanced, fluctuate but little in brightness and give off a steady stream of energy. The balance is achieved by self-regulation: a slight decrease in fusion energy would result in contraction that would heat up the core and increase fusion rate, and vice versa. Other stars, where the balance is not so well tuned, pulsate noticeably. Living on a planet circling a pulsating star presumably would be difficult or impossible. Thus, the reason that the Sun neither expands (from the ongoing explosion within) nor collapses (from its own weight) is that the two forces keep the balance. In the distant future, when this balance is disturbed because most of the hydrogen is used up, the Sun will expand. This will be the end of the solar system as we know it. 3
The sun is the most perfectly round natural object known in the universe 5
Now a team led by the University of Hawaii's Dr Jeffrey Kuhn have made the first precise measurement of the sun's equatorial bulge, or its "oblateness". The results were a big surprise. "We were shocked," says Kuhn. The sun doesn't bulge much at all. It is 1.4m kilometers across, but the difference between its diameter at the equator and between the poles is only 10 kilometers. Scaled to the size of a beachball, that difference is less than the width of a human hair. Only an artificial sphere of silicon that was created as a standard for weights is known to be more perfectly spherical. From a long-running, space-based experiment we show that, when analyzed with sufficiently high spatial resolution, the Sun’s oblate shape is remarkably constant, and almost completely unaffected by the solar cycle variability seen on its surface. 6
The right amount of life-requiring metals
Our star seems to contain just the right amount of life-requiring metals, even compared with otherwise similar nearby stars. It contains enough metals for building a habitable planet, but it doesn’t have too much, which might have produced an unstable planetary system with too many massive planets. As we discussed in Chapter Eight, overly massive planets would lumber obtrusively through a planetary system, making the presence of terrestrial planets in stable circular orbits less likely. Additionally, at least 50 percent of main-sequence stars are born in binary systems, most of which are inhospitable for similar reasons. 7 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. 8
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. The low amplitude of variations in the Sun’s energy output keeps Earth’s climate from experiencing excessively wild climate swings.
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 extreme 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. 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.
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.
Is the Sun an oddball star?
Meanwhile, astronomer Jorge Meléndez at Portugal’s University of Porto leads an international collaboration looking for solar twins. “My group has already studied about 75 percent of stars similar to the Sun in the whole Hipparcos million-star catalog,” says Meléndez. “Surprisingly, we see that the Sun is actually chemically different from most solar twins.” 9
7. THE PRIVILEGED PLANET, Guillermo Gonzalez and Jay W. Richards, page 282
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