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NASA's Hubble Space Telescope has caught the eerie, wispy tendrils of
a dark interstellar cloud being destroyed by the passage of one of the
brightest stars in the Pleiades star cluster. Like a flashlight beam
shining off the wall of a cave, the star is reflecting light off the
surface of pitch black clouds of cold gas laced with dust. These are
called reflection nebulae.
The famous cluster is easily visible in the evening sky during the
winter months as a small grouping of bright blue stars, named after
the "Seven Sisters" of Greek mythology. Resembling a small dipper,
this star cluster lies in the constellation Taurus at a distance of
about 380 light-years from Earth. The unaided eye can discern about
half a dozen bright stars in the cluster, but a small telescope will
reveal that the Pleiades contains many hundreds of fainter stars.
In many cases, the nebulae surrounding star clusters represent
material from which the stars have formed recently. However the
Pleiades nebulosity is actually an independent cloud, drifting
through the cluster at a relative speed of about 6.8 miles/second
In 1890, American astronomer E. E. Barnard, observing visually with
the Lick Observatory 36-inch telescope in California, discovered an
exceptionally bright nebulosity adjacent to the bright Pleiades star
Merope. It is now cataloged as IC 349, or "Barnard's Merope Nebula."
IC 349 is so bright because it lies extremely close to Merope--only
about 3,500 times the separation of the Earth from the Sun, or about
0.06 light-year--and thus is strongly illuminated by the star's light.
In the new Hubble image, Merope itself is just outside the frame on
the upper right. The colorful rays of light at the upper right,
pointing back to the star, are an optical phenomenon produced within
the telescope, and are not real. However, the remarkable parallel
wisps extending from lower left to upper right are real features,
revealed for the first time through Hubble's high-resolution imaging
capability. Astronomers George Herbig and Theodore Simon of the
University of Hawaii obtained these broadband observations with
Hubble’s Wide Field and Planetary Camera 2 on September 19, 1999.
Herbig and Simon propose that, as the Merope Nebula approaches
Merope, the strong starlight shining on the dust decelerates the dust
particles. Physicists call this phenomenon "radiation pressure."
Smaller dust particles are slowed down more by the radiation pressure
than the larger particles. Thus, as the cloud approaches the star,
there is a sifting of particles by size, much like grain thrown in the
air to separate wheat from chaff. The nearly straight lines pointing
toward Merope are thus streams of larger particles, continuing on
toward the star while the smaller decelerated particles are left
behind at the lower left of the picture.
Over the next few thousand years, the nebula--if it survives the
close passage without being completely destroyed--will move on past
Merope, somewhat like a comet swinging past our Sun. This chance
collision allows astronomers to study interstellar material under
very rare conditions, and thus learn more about the structure of the
dust lying between the stars.
December 6, 2000
Image Credit: NASA and The Hubble Heritage Team (STScI/AURA)