The naked-eye Pleiades star cluster has long been known to professional and
amateur astronomers for the striking visible nebulosity that envelopes the
cluster’s brightest stars, scattering their light like fog around a

Radio and infrared observations in the 1980s established that this
results from a chance encounter by the young stars of the Pleiades with an
interstellar cloud, rather than being caused by debris from the cluster’s
formation. New data obtained at Kitt Peak National Observatory suggest that
the Pleiades are actually encountering two clouds, giving rise to an
and previously unknown occurrence: a three-body collision in the vast
emptiness of interstellar space.

This new perspective on the motion of interstellar gas near the cluster
comes from
high-resolution spectra obtained at an adjunct facility of Kitt Peak’s
telescope known as the CoudE Feed. The investigator was Richard White of
Smith College in Northampton, MA, who worked in collaboration with students
from Smith College and Amherst College.

“The idea of the Pleiades and one gas cloud in an interstellar train wreck
made this nearby cluster an especially interesting region for astronomers
to understand the details of physical and chemical processes in the
medium,” White says. “The presence of a second cloud interacting with the
cloud and with the cluster creates a situation more like a three-car crash
in a
demolition derby, which makes the Pleiades altogether unique as natural

The time scale for the unfolding of the interstellar collisions in the
is several hundred thousand years. “That is good news for those who enjoy
magnificent color images of the Pleiades images that grace textbooks and
coffee table books, which suffer no danger of obsolescence,” White says.
“It is bad news for those who would like to see celestial fireworks
unfolding from year to year.”

Known as the Seven Sisters for the seven stars said to be visible with the
naked eye, the Pleiades (M45) consists of more than 500 stars roughly 100
years old in a cluster located about 400 light-years from Earth.

Sodium atoms in gas found between Earth and the stars absorb two specific
wavelengths of yellow starlight (the same wavelengths of yellow light
by low-pressure sodium streetlamps). Because of the Doppler effect
to the shift in siren pitch produced when an ambulance is moving toward or
away from a listener), the motion of the gas along our line of sight
produces subtle shifts in the observed wavelengths.

In a paper published in the October 2003 Astrophysical Journal Supplement,
White interprets the new observations of sodium atoms in the Pleiades
region in
the context of other recent observations of the Pleiades. These
observations include significant new optical images of the Pleiades from the
Burrell Schmidt telescope on Kitt Peak, published earlier this year in
the Astrophysical Journal by Steven Gibson of the University of Calgary
and Kenneth Nordsieck of the University of Wisconsin, and radio maps of
neutral hydrogen that formed part of Gibson’s doctoral thesis.

The orientation of features in the optical and radio imagery provides clues
to gas and dust motions across the sky, which can be combined with the
spectroscopically measured velocities from Kitt Peak to allow astronomers to
reconstruct the three-dimensional configuration of the interstellar matter
near the Pleiades.

The sodium absorption lines reveal that there always is one feature between
Earth and the Pleiades stars, moving toward the cluster with a line of sight
velocity of about 10 kilometers per second. White associates this feature
the Taurus-Auriga interstellar cloud complex, the bulk of which lies about
40 light-years to the east.

Toward some stars, however, there are two or more absorption features.
White argues that a shock-wave from the collision between the Pleiades and
gas associated with the Taurus-Auriga complex can account for splitting of
one feature into three in some areas, primarily on the south and east sides
of the Pleiades. However, the presence of an additional feature in the
data, primarily on the west side and moving into the cluster at about 12
per second, defies understanding unless a second cloud also is converging
on the Pleiades, he concludes.

The only previously known three-body collisions in interstellar space are
close encounters by a star and a neighboring binary or triple star system
a globular cluster or in the cores of galaxies.

Previously released images of the Pleiades from Kitt Peak that amply
the surrounding nebulosity are available in the NOAO Image Gallery at and at

Located southwest of Tucson, AZ, Kitt Peak National Observatory is part of
the National Optical Astronomy Observatory, which is operated by the
Association of Universities for Research in Astronomy (AURA), Inc.,
under a cooperative agreement with the National Science Foundation.