Exploring the Solar System Study Guide
From the dawn of time, humans have looked up at the stars. Planets were once thought to be stars, but planets were special because they wandered across the sky, against the other fixed stars. The word planet, in fact, comes from the ancient Greek for wanderer. We now know that planets look like stars because they reflect light from the sun, and that they and the earth orbit around the sun in a solar system. How do the planets stay in orbit? How does the solar system work? What makes our moon, the subject of so much poetry and romance, wax and wane in the sky?
About 5 billion years ago, a cosmic gas cloud began to condense into the star that is now our sun, which has been burning since that birth. Around the sun, the cosmic gas cloud also condensed into smaller bodies (picture small whirlpools of contraction around a large central one). What started as dust grains coalesced into rocks, then boulders, then objects the size of mountains. By collisions and gravitational attraction, which held the bodies together, the objects grew. Sometimes, the collisions created smaller bodies but, on the whole, growth in size ruled. The sun, the earth, and all the other planets and their moons were formed by about 4.5 billion years ago.
The sun is a star, just like the others we see as points of light in the night sky. Because it's so relatively close to Earth, the sun looms large. In fact, the nearest other star is about 250,000 times farther away than our sun. Wow!
Figure 2.1 shows the sun, its size, surface temperature, inner core, sunspots, prominences, and the outer corona. The earth is shown to the same scale of size, but not distance! To draw it to scale, the correct location of the earth's orbit would be about 110 times the diameter of the sun. See ifyou can figure out where the Earth (to the scale as the sun in the figure) would go in the room where you are reading this book.
The sun is 860,000 miles in diameter (1.4 million kilometers). Because the earth is a little less than 8,000 miles in diameter, that means that the sun's diameter is more than 100 times that of Earth's. In volume, more than one million Earths could fit inside the sun.
The secret of the sun is its very hot inner core of 10 million degrees Kelvin (essentially the same as °C for temperatures this high). In the core, nuclear fusion reactions take place. Hydrogen is fused into helium, with the release of energy. We care a lot about this release of energy, for without this energy, life as we know it would not and could not exist. Not only would the earth be cold, near absolute zero, but photosynthesis by plants and algae, which requires sunlight, would be nonexistent, meaning no food for animals such as people.
The sun's surface that we see (but don't look directly at it!) is about 4,000 degrees Kelvin (7,000° F, or about 4,000° C). But the sun extends even farther out as a corona of glowing gases visible only during a solar eclipse (discussed later in this lesson). Note that the sun spins on its axis, like the earth does, taking about a month to turn. We can see the turning by observing sunspots, which are dark regions of storms on the sun's surface. Sunspots are darker because they are a bit cooler, but still extremely hot. The number of sunspots rises and falls in a cycle that is approximately 11 Earth years in length, as do the number of solar storms, seen in the previous figure as a prominence erupting from the surface.
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