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Dark Matter Mystery
Scientists are considering a number of possibilities. Candidates for dark matter
include MACHOS, WIMPS and GAS. A "Good News" and "Bad News" analysis is given
below for each of the candidates.
MACHOS (Massive Compact Halo Objects) Examples: brown dwarfs, white
dwarfs, neutron stars, black holes.
Brown Dwarf stars have a mass that is less than
eight percent of the mass of the Sun--too low to produce the nuclear reactions
that make stars shine.
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Recently, astronomers have found some objects that are
either brown dwarf stars or very large planets around other stars.
Observations of the brightening and then dimming of distant
stars--thought to be due to the gravitational lens effect of a
foreground star-- may also provide further evidence for a large
population of brown dwarfs in our galaxy.
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There is as yet no evidence that brown dwarfs are anywhere
near as abundant as they would have to be to account for the dark
matter in our galaxy.
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White Dwarfs are the final condensed states of
small to medium sized stars.
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White dwarfs are known to exist and to be plentiful. Maybe
they could be plentiful enough to explain the dark matter if young
galaxies produced white dwarfs that cool more rapidly than present
theory predicts.
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No good alternative to the present theory exists. Also the
production of large numbers of white dwarfs implies the production of
a large amount of helium, which is not observed.
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Neutron Stars or Black Holes are the final condensed states of large and very large
stars.
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They can be dark, especially black holes, which are totally dark, except for a negligible amount of so-called Hawking radiation. See http://chandra.harvard.edu/resources/faq/black_hole/bhole-31.html
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These objects are expected to be much scarcer than white
dwarfs. Also, the processes that produce these objects release a lot
of energy and heavy elements; there is no evidence of such a release.
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