PASADENA-The cluster of stars known as the Pleiades is one of the
most recognizable objects in the night sky, and for millennia has
been celebrated in literature and legend. Now, a group of
astronomers has obtained a highly accurate distance to one of the
stars of the Pleiades known since antiquity as Atlas. The new results
will be useful in the longstanding effort to improve the cosmic
distance scale, as well as to research the stellar life-cycle.

In the January 22 issue of the journal Nature, astronomers from the
California Institute of Technology and the Jet Propulsion Laboratory
report the best-ever distance to the double-star Atlas. The star,
along with “wife” Pleione and their daughters, the “seven sisters,”
are the principal stars of the Pleiades that are visual to the
unaided eye, although there are actually thousands of stars in the
cluster. Atlas, according to the team’s decade of careful
interferometric measurements, is somewhere between 434 and 446
light-years from Earth.

The range of distance to the Pleiades cluster may seem somewhat
imprecise, but in fact is accurate by astronomical standards. The
traditional method of measuring distance is by noting the precise
position of a star and then measuring its slight change in position
when Earth itself has moved to the other side of the sun. This
approach can also be used to find distance on Earth. If you
carefully record the position of a tree an unknown distance away,
move a specific distance to your side, and measure how far the tree
has apparently “moved,” it’s possible to calculate the actual
distance to the tree by using trigonometry.

However, this procedure gives only a rough estimate to the distance
of even the nearest stars, due to the gigantic distances involved and
the subtle changes in stellar position that must be measured.

Further, the team’s new measurement settles a controversy that arose
when the European satellite Hipparcos provided a distance measurement
to the Pleiades so much nearer the distance than assumed that the
findings contradicted theoretical models of the life cycles of stars.

This contradiction was due to the physical laws of luminosity and its
relationship to distance. A 100-watt light bulb one mile away looks
exactly as bright as a 25-watt light bulb half a mile away. So to
figure out the wattage of a distant light bulb, we have to know how
far away it is. Similarly, to figure out the “wattage” (luminosity)
of observed stars, we have to measure how far away they are.
Theoretical models of the internal structure and nuclear reactions of
stars of known mass also predict their luminosities. So the theory
and measurements can be compared.

However, the Hipparcos data provided a distance lower than that
assumed from the theoretical models, thereby suggesting either that
the Hipparcos distance measurements themselves were off, or else that
there was something wrong with the models of the life cycles of
stars. The new results show that the Hipparcos data was in error,
and that the models of stellar evolution are indeed sound.

The new results come from careful observation of the orbit of Atlas
and its companion–a binary relationship that wasn’t conclusively
demonstrated until 1974 and certainly was unknown to ancient watchers
of the sky. Using data from the Mt. Wilson stellar interferometer
(located next to the historic Mt. Wilson Observatory in the San
Gabriel range) and the Palomar Testbed Interferometer at Caltech’s
Palomar Observatory in San Diego County, the team determined a
precise orbit of the binary.

Interferometry is an advanced technique that allows, among other
things, for the “splitting” of two bodies that are so far away that
they normally appear as a single blur, even in the biggest
telescopes. Knowing the orbital period and combining it with orbital
mechanics allowed the team to infer the distance between the two
bodies, and with this information, to calculate the distance of the
binary to Earth.

“For many months I had a hard time believing our distance estimate
was 10 percent larger than that published by the Hipparcos team,”
said the lead author, Xiao Pei Pan of JPL. “Finally, after intensive
rechecking, I became confident of our result.”

Coauthor Shrinivas Kulkarni, MacArthur Professor of Astronomy and
Planetary Science at Caltech, said, “Our distance estimate shows that
all is well in the heavens. Stellar models used by astronomers are
vindicated by our value.”

“Interferometry is a young technique in astronomy and our result
paves the way for wonderful returns from the Keck Interferometer and
the anticipated Space Interferometry Mission that is expected to be
launched in 2009,” said coauthor Michael Shao of JPL. Shao is also
the principal scientist for the Keck Interferometer and the Space
Interferometry Mission.

The Palomar Testbed Interferometer was designed and built by a team
of researchers from JPL led by Shao and JPL engineer Mark Colavita.
Funded by NASA, the interferometer is located at the Palomar
Observatory near the historic 200-inch Hale Telescope.

The device served as an engineering testbed for the interferometer
that now links the 10-meter Keck Telescopes atop Mauna Kea in Hawaii.