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#9.2 考古 by chidaben2003
Essay #17. 639 (23835-!-item-!-188;#058&00639-00)
345
In the 1980's, astronomer Bohdan Paczynski proposed a way of determining whether the enormous
dark halo constituting the outermost part of the Milky Way galaxy is composed of MACHO's (massive
compact halo objects), which are astronomical objects too dim to be visible. Paczynski reasoned that if
MACHO's make up this halo, a MACHO would occasionally drift in front of a star in the Large Magellanic
Cloud, a bright galaxy near the Milky Way. The gravity of a MACHO that had so drifted, astronomers
agree, would cause the star's light rays, which would otherwise diverge, to bend together so that, as
observed from Earth, the star would temporarily appear to brighten, a process known as microlensing.
Because many individual stars are of intrinsically variable brightness, some astronomers have contended
that the brightening of intrinsically variable stars can be mistaken for microlensing. However, whereas
the different colors of light emitted by an intrinsically variable star are affected differently when the star
brightens, all of a star's colors are equally affected by microlensing. Thus, if a MACHO magnifies a
star's red light tenfold, it will do the same to the star's blue light and yellow light. Moreover, it is highly
unlikely that a star in the Large Magellanic Cloud will undergo microlensing more than once, because the
chance that a second MACHO would pass in front of exactly the same star is minuscule. |
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