Aspects of the Sun Since the beginning of time, human beings have been fascinated by the dramas unfolding in the firmament above--the shifting arch of the sun, the changing faces of the moon, the regular patterns of stars turning as the seasons change. The study of these subtle patterns and relationships in the cosmos evolved into Astronomy (or 'law of the stars'), one of the oldest of the sciences. While different cultures, religions, and scientific theories all offer contrasting explanations for cosmic phenomena, there is one point on which they all agree: there would be no life on Earth if not for the sun. Modern science has now proven that the sun, whose reliable path across the sky gives us the contours of our days and the duration of our nights, is the focal point of our solar system. The sun is, in fact, a star that is nearly 110 times the size of Earth and comprises more than 99% of the solar systems mass, which creates enough gravitational pull to extend to the outer reaches of our solar system and keep all manner of matter (planets, asteroids, meteors and even dust) in orbit. Like other stars, the sun is made up of various gases, the most prevalent being hydrogen at around 74%, and helium at around 25%. Because the suns gravitational force inward is equal to that of its thermal pressure outward, it is in a state of hydrostatic equilibrium, which causes the sun to compress into a sphere. While the sun is a near perfect sphere and appears to the naked eye as a precise and sharply defined yellow circle against the sky (hence its ancient astronomical symbol of a circle with a point at its center), it does not actually have a definite edge or boundary. Instead, the density of its gases decreases exponentially with distance from the core. Like most stars, the sun generates heat and energy via nuclear fusion, which takes place in the core at the center of the sun. This energy must travel through a number of different layers before it reaches the photosphere, the first layer of the suns atmosphere, where it escapes into space as sunlight. It is the photosphere that we see when we look at the sun. The layer is about 100 km thick, which is relatively thin compared to other solar layers, and is much cooler than the suns outermost atmospheric layer called the corona. One of the suns greatest mysteries is the incredible heat of the corona. Usually heat decreases with distance from the core; this is true for the sun until you reach the corona, which becomes suddenly hotter by a factor of nearly 200 times that of the photosphere. The exact process of heating and maintaining the corona is still one of the unsolved solar mysteries that continue to fascinate and perplex scientists today. Another solar mystery is the curious nature of solar flares. Equivalent to millions of 100-megaton hydrogen bombs detonated simultaneously, solar flares are the most intense and energetic explosions that occur in our solar system. (A) These explosions occur on the suns photosphere and are difficult to view through the layers bright emissions, even with specialized equipment. Solar flares are directly linked to another more easily observable solar phenomenon--sunspots. With the invention of the telescope in 1608, astronomers were finally able to look into the face of the sun and see that it is not a perfect and unchanging yellow disc as they had expected, but that it is often marred by discernibleble mishes or dark spots. (B) These dark areas are the coolest regions on the suns photosphere and are characterized by intense magnetic activity. The frequency of sunspot occurrences follows an eleven-year solar or sunspot cycle. (C) At the minimum end of the cycle, there is very little sunspot activity and at the maximum end, there might be hundreds of visible sunspots. An increased number of sunspots indicates a correlated increase in solar flare activity. (D) This period of activity can pose a serious danger to satellites and astronauts. One solar flare can emit enough magnetic energy to cause serious damage to a satellite, or change the satellites orbit. It can also shake the Earths magnetic field and cause dangerous surges in power lines, resulting in blackouts over large areas. Because of these dangers and our increasing reliance on satellites, it has become even more important for scientists to understand the nature of solar weather and to determine more accurate methods of predicting solar activity. The word 'they' in the passage refers to