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A Closer Look: Dark Matter
How can we determine the mass of far-away galaxies?
In this session’s
video, we mentioned that a new kind of matter has been proposed to explain
the motions of galaxies. In 1933, astronomer Fritz
Zwicky made careful observations of a gravitationally bound cluster of
galaxies called the Coma cluster, which is estimated at over 1000 galaxies,
located 350 million light years away in the northern constellation Coma
Berenices. Zwicky determined how fast the galaxies are moving relative
to each other and, using Newton's theory of gravity, calculated their mass:
the faster they are moving, the greater the mass of the cluster. Zwicky
found the mass of the cluster to be 400 times what one would expect by
inferring the mass from the brightness of the cluster. He termed this missing
matter "dark matter." Even though dark matter can be indirectly
observed in many places in the universe including our own galaxy (by measuring
the speed of rotation of stars around the galactic center), no one has
yet detected it directly using any kind of telescope. Current estimates
are that dark matter comprises 90% or more of all the mass in the universe.
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