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  Title:Origin of the Solar System

  The orderly nature of our solar system leads most astronomers to conclude that the planets formed at essentially the same time and from the same primordial (original) material as the Sun. This material formed a vast cloud of dust and gases called a nebula. The nebular hypothesis suggests that all bodes of the solar system formed from an enormous nebular cloud consisting mostly of hydrogen and helium as well as a small percent of all the other heavier elements known to exist

  The heavier substances in this frigid cloud of dust and gases consisted mostly of such elements as silicon, aluminum, iron, and calcium—the substances of today’s common rocky materials. Also prevalent were other familiar elements, including oxygen, carbon, and nitrogen.

  Nearly five billion years ago, some external influence, such as a shock wave traveling from a catastrophic explosion (supernova), may have triggered the collapse of this huge cloud of gases and minute grains of heavier elements, causing the cloud to begin to slowly contract due to the gravitational interactions among its particles. As this slowly spiraling nebula contracted, it rotated faster and faster for the same reason ice-skaters do when they draw their arms toward their bodies. Eventually, the inward pull of gravity came into balance with the outward force caused by the rotational motion of the nebula. By this time the once vast cloud had assumed a flat disk shape with a large concentration of material at its center, called the protosun (pre-Sun). Astronomers are fairly confident that the nebular cloud formed a disk because similar structures have been detected around other stars.

  During the collapse, gravitational energy was converted to thermal energy (heat), causing the temperature of the inner portion of the nebula to dramatically rise. At such high temperatures, the dust grains broke up into molecules and energized atomic particles. However, at distances beyond the orbit of Mars, the temperatures probably remained quite low. At -200℃, the tiny particles in the outer portion of the nebula were likely covered with a thick layer of ices made of frozen water, carbon dioxide, ammonia, and methane. Some of this material still resides in the outermost reaches of the solar system in a region called the Oort cloud.

  The formation of the Sun marked the end of the period of contraction and thus the end of gravitational heating. Temperatures in the region where the inner planets now reside began to decline. The decrease in temperature caused those substances with high melting points to condense into tiny particles that began to coalesce (join together). Such materials as iron and nickel and the elements of which the rock-forming minerals are composed—silicon, calcium, sodium, and so forth—formed metallic and rocky clumps that orbited the Sun. Repeated collisions caused these masses to coalesce into larger asteroid-size bodies, called protoplanets, which in a few tens of millions of years accumulated into the four inner planets we call Mercury, Venus, Earth, and Mars. Not all of these clumps of matter were incorporated into the protoplanets. Rocky and metallic pieces that still remain in orbit are called meteoroids.

  As more and more material was swept up by the inner planets, the high-velocity impact of nebular debris caused the temperatures of these bodies to rise. Because of their relatively high temperatures and weak gravitational fields, the inner planets were unable to accumulate much of the lighter components of the nebular cloud. The lightest of these, hydrogen and helium, were eventually whisked from the inner solar system by the solar winds.

  At the same time that the inner planets were forming, the larger, outer planets (Jupiter, Saturn, Uranus, and Neptune), along with their extensive satellite systems, were also developing. Because of low temperatures far from the Sun, the material from which these planets formed contained a high percentage of ices—water, carbon dioxide, ammonia, and methane—as well as rocky and metallic debris. The accumulation of ices partly accounts for the large sizes and low densities of the outer planets. The two most massive planets, Jupiter and Saturn, had surface gravities sufficient to attract and hold large quantities of even the lightest elements—hydrogen and helium.

  1. According to paragraph 1, which of the following best describes the “nebular hypothesis”?  Our solar system formed from a large cloud consisting mostly of hydrogen and helium and of small amounts of other elements.  Our solar system formed from gases and heavier elements thrown off by the Sun as it rotated in the center of the nebular cloud.  The primordial matter that evolved into our solar system consisted mostly of familiar elements such as oxygen, carbon, and nitrogen.  A cloud of dust and gases gathered into a rotating nebula composed mostly of the rocky materials seen on Earth today.

  2. The word “frigid” in the passage is closet in meaning to

   moving  giant  original  cold

  Paragraph 2

  Nearly five billion years ago, some external influence, such as a shock wave traveling from a catastrophic explosion (supernova), may have triggered the collapse of this huge cloud of gases and minute grains of heavier elements, causing the cloud to begin to slowly contract due to the gravitational interactions among its particles. As this slowly spiraling nebula contracted, it rotated faster and faster for the same reason ice-skaters do when they draw their arms toward their bodies. Eventually, the inward pull of gravity came into balance with the outward force caused by the rotational motion of the nebula. By this time the once vast cloud had assumed a flat disk shape with a large concentration of material at its center, called the protosun (pre-Sun). Astronomers are fairly confident that the nebular cloud formed a disk because similar structures have been detected around other stars.

  3. Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.

   Possibly due to some explosion about five billion years ago, this nebular cloud began to collapse, causing gravitational contractions as its particles interacted.

   About five billion years ago, a supernova may have exploded, causing a huge cloud of gases and heavier elements to form

   Gravitational attraction among particles of gases and heavier elements caused some explosive event nearly five billion years ago.

   About five billion years ago, a shock wave from an external event caused this huge cloud of gases to collapse into small grains of heavier elements.

  4. In paragraph 2, why does the author describe how ice-skaters use their arms to increase their speed of rotation?

   To help describe the armlike structures on a spiraling nebula

   To help explain why a nebula rotates faster when it contracts 

  To show why spinning ice-skaters are not pulled down by gravity

   To show how the motion of a nebula differs from that of an ice-skater

  5. The word “detected” in the passage is closet in meaning to

   formed  predicted  discovered  recorded

  6. According to paragraph 2, why do astronomers believe that the nebular cloud formed a disk around the protosun? 

  They can still see some debris from the disk.

   They have observed that disks have formed around other stars.

   They know that any rotating cloud of gas tends to contract into a disk shape. 

  They have conducted experiments with gravity that have confirmed their belief.

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