Astronomers Have Found the Existence of Spiral Arms in Galaxies

question 95

Multiple Choice

Astronomers have found the existence of spiral arms in galaxies difficult to account for because the

• A) inner part of a galaxy rotates in a shorter time than the outer parts,so the arms should have wound up so tightly that they would have disappeared over the lifetime of the galaxy.
• B) arms should have been destroyed by collisions with other galaxies over the galaxy's lifetime.
• C) outer regions of a galaxy including the spiral arms have no significant rotation,so the arms should have fallen into the center of the galaxy.
• D) outer parts of a galaxy rotate faster than the inner parts,so the arms should have straightened out into spokes like those of a bicycle wheel.

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