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It is believe that most comets are members of the

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It is believe that most comets are members of the solar system and that they travel on elliptical _________, most of which are extremely long and have high eccentricity near unity. Explanation: Orbits. The orbits of comets are oriented at random in space. A comet can be observed from the Earth only when it traverses that end of its orbit where it passes close to the sun. The head of a comet is made up of two parts--the nucleus and the ______. Explanation: Coma. The nucleus is primarily made of ice and dust. For this reason, it is often compared to a "dirty snowball." The coma is the diffuse gaseous component of the head of a comet. As a comet approaches the sun, the coma usually grows in brightness, but sometimes it becomes so diffuse that it disappears. As they approach the sun, many comets develop tails of gas and dust that extend for millions of miles away from the head. An _____________ is different from a comet in that it doesn't have a coma or a tail. Asteroid. This difference is caused by the composition of the two objects--a comet is made up of ice and dust, which have the potential to heat up and form a coma and tail, while an asteroid is solid rock. Nearly all measurements of astronomical distances depend directly or indirectly on the principle of ________________. Explanation: Triangulation. Six quantities describe the dimensions of a triangle: The lengths of the three sides and the values of the three angles. Any three of these quantities in succession around the perimeter of the triangle determine the triangle uniquely. If you have two sides and an included angle or two angles and an included side, you can determine the triangle. Distances can be determined without having to span them directly. The ______________ motion of a star is the rate at which its direction in the sky changes, and is usually expressed in seconds of arc per year. Explanation: Proper. This motion is almost always an angle that is too small to measure with much precision in a single year. In an interval of 20 to 50 years, many stars change their directions by easily detectable amounts. The star of largest known proper motion is Barnard's Star, which changes its direction by 10.25 arcseconds per year. Just to get an idea of how small the motion is, there are 360 degrees in a circle. An arcminute is 1/60 of a degree, and an arcsecond is 1/60 of an arcminute.
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Our Sun, Some Basics About Stars and Clusters Measures of the amounts of light energy, or _____________ ________, received from stars are among the most important and fundamental observational data of astronomy. Explanation: Luminous Flux. They are used in estimating both the distances and the actual output of energy of stars. The measure of the amount of light flux received from a star or other luminous object is called ______________. tailed Explanation: Magnitude. Six levels of classifications were set up from first through sixth magnitude, with the first denoting the brightest-appearing stars. This system of stellar magnitudes began in ancient Greece, but is still used today
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