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In 1687, Sir Isaac Newton showed that the force of gravity between two celestial bodies increases as the product of their two masses and decreases as the square of the distance between them. Because of their gravitational attraction to the Sun, the earth and all the other planets in our solar system rotate around the Sun. But while the earth moves completely around the sun in just one year, Pluto, normally the outermost planet in the solar system, takes 249 years to do the same--even though its orbital path is only 40 times that of earth's.
Spiral galaxies, such the Milky Way and our nearest neighbor Andromeda, similarly rotate around a galactic center. Spiral arms twist around a central bulge; farther away from the center is the halo of the galaxy, seemingly sparsely populated with scattered clusters of stars.
Newton's law predicts that the movement of stars around the galactic center
should slow down with increasing distance from the center of the galaxy. But
scientists noticed a funny thing when studying the movement of star
clusters in Andromeda's halo.
JPEG Image (37.7 KB); Caption; Credits and Copyrights
Astrophysicists measure the velocities of distant objects by measuring the "Doppler shift" of an object. Objects whose characteristic wavelengths are shifted to the red end of the spectrum are moving away from the observer; those whose wavelengths are blueshifted are moving toward the observer. To measure the relative speed of two distant objects--say a star in one of Andromeda's spiral arms and a star in Andromeda's halo--scientists measure the difference between their redshifts or blueshifts.
Much to the surprise of the scientists who made the initial measurements, the rotational velocity of stars in Andromeda did not steadily drop off in the outer reaches of the galaxy. Instead, the speeds drop slightly and then level off at a constant value.
Scientists found similarly puzzling results when they put together rotation graphs for the Milky Way and other spiral galaxies. How could this be?
If Newton's law is to hold true--and scientists have no reason to believe it doesn't--there must be large quantities of mass that we can't see in the halos of spiral galaxies. This so-called dark matter must provide the gravitational pull that keeps the stars in the outermost reaches of the galaxies whirling around so quickly.
Dark Matter Halo
Moreover, theory implies that the thin, rotating disks that are spiral
galaxies are simply not stable enough to hold together on their own gravitational
force; disruptive vibrations would cause them to fly apart. If, however, spiral
galaxies are embedded in a halo of dark matter, they becomes stable.
JPEG Image (11.1 KB)
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