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The Solar System is a tiny part of a much larger system known as the (1) _______________ galaxy. The first observational evidence of the shape of our galaxy were collected by (2) _______________ , but his conclusions were partially wrong because he could observe only a small portion of stars of the Galaxy. Distant stars are difficult to observe because (3) _______________ absorbs their light at visible wavelengths. Modern instruments sensitive to (4) _______________ and radio waves are capable to see through the obscuring material, and have helped astronomers to determine that spiral galaxies like our own consists of a central (5) _______________ , a flat (6) _______________, both embedded in a large spherical (7) _______________ . The disk can be decomposed into a (8) _______________ component of young stars, and a (9) _______________ component with older stars. The Sun is located near a short spiral arm of the galaxy, known as the (10) _______________ . The disk of the galaxy does not rotate as a solid object, different parts of it rotate at different rates, objects (11) _______________ from the galactic center take (12) _______________ to complete one full orbit. This is known as (13) _______________ rotation. Finally, there is clear evidence that at the center of the galaxy there is a (14) _______________ with a diameter not larger than the orbit of Mercury. This object is surrounded by a cloud of hot gas 1500 light-years across detected in radio waves, and known as (15) _______________ . The first to realize that there exist other galaxies apart from our own, was astronomer (16) _______________. The nearest large galaxy is the (17) _______________ galaxy, which was one of the first to be identified as external to the Milky Way. Galaxies are classified according to their observed shapes. (18) _______________ galaxies look like spheres or squashed spheres. On the other hand, (19) _______________ galaxies have a disk with arms and a bulge. Galaxies of this later type that have a boxy or peanut shaped structure through its center are classified as (20) _______________ galaxies. (21) _______________ galaxies often appear disorganized and without consistent structure. A good example of this type of galaxy is the (22) _______________ . Distances to galaxies are determined using a variety of “standard bulbs”, however, (23) _______________ are the most commonly used to for distant galaxies due to the accuracy that can be achieved. Among several other very important contributions to astronomy by Edwin Hubble, is the discovery of the expansion of the universe. He found that galaxies move away from us, and that the speed of recession is proportional to the (24) _______________ the galaxies. The constant of proportionality between those two quantities is known as the (25) _______________ .
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