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NASA Hubble Space Telescope image of a region of the galaxy M100 shows a
class of pulsating star called a Cepheid Variable. Though rare, these stars
are reliable distance indicators to galaxies. Based on the Hubble observation,
the distance to M100 has been measured accurately as 56 million light-years
(+/- 6 million light-years), making it the farthest object where intergalactic
distances have been determined precisely. Hubble's high resolution pinpoints
a Cepheid, which is located in a starbirth region in one of the galaxy's
spiral arms (bottom frame). The top three frames were taken on (from left
to right) May 9, May 4, May 31, and they reveal that the star (in center
of each box) changes brightness. Cepheids go through these changes rhythmically
over a few weeks. The interval it takes for the Cepheid to complete one
pulsation is a direct indication of the stars's intrinsic brightness. This
value can be used to make a precise measurement of the galaxy's distance.
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Hubble Space Telescope has the required sensitivity and resolution to detect
these "cosmic milepost" type stars out to great distances from Earth, according
to astronomers. Typically, Cepheids in a crowded region of a distant galaxy
are too faint and the resolution too poor, as seen from ground-based telescopes,
to be detected clearly. Hubble was used to make twelve one-hour exposures,
timed carefully in a two-month observing window, to discover 20 Cepheid
variable stars in the M100 galaxy. Though M100 is the most distant galaxy
in which Cepheid variables have been discovered, HST must find Cepheids
in even more distant galaxies before accurate distances can be used to calculate
a definitive size and age for the universe. |
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