4. Galaxy Rotation Curves and Mass Accounting
4. Galaxy Rotation Curves and Mass Accounting
Mass Accounting is a general term that refers to the way in which we know dark matter exists. When we add up all the matter in the universe that we can see, we fall short of the amount predicted by important calculations and theories. Dark matter is the mysterious substance that makes up this difference.
One of the ways we can detect the presence fof dark matter is by studying galaxies. A galaxy is a a grouping of stars held together by the force of gravity. The stars within the galaxies rotate, and the galaxies themsleves rotate.
The term galaxy rotation curve refers to the way the rotation velocity (speed) of stars and glass clouds is spread out from the center of a galaxy. The shape of this rotation curve is determined by the amount of matter in the galaxy.
Galaxies typically have bright stars within them, that we call luminous matter, surrounded by a darker ring, which we call a halo. The rotation velocity of the luminous area of the galaxy is the same as the rotation velocity of the halo. This lets us know that dark matter is present inside the halo - matter we cannot see is rotating within it.
There are many possible sources for dark matter, one of them being the dark halos of galaxies. To find dark matter in the halos, we measure the velocity at which the galaxies within are rotating.
Posted by kelechi on March 26, 2005 at 05:56 PM