Astronomers Andrei Berdyugin, Vilppu
Piirola and Pekka Teerikorpi at Tuorla
Observatory, have made the first complete maps of the
amount of polarisation of starlight above and below the
plane of the Milky Way galaxy.
For many decades, astronomers have known that in addition to the
stars and gas scattered through the near emptiness of space, there
are also large amounts of tiny particles which they call "dust".
Although the dust particles are very much smaller than the dust
we blithely suck up with domestic vacuum cleaners, the analogy is
not so far fetched, since for many purposes astronomers would
gladly do away with cosmic dust. Looking up into the Milky Way on a
good dark night, many "dark" patches can be readily seen delineated
by the brighter surrounding regions. One of the most famous of
these clouds of interstellar dust is the "Coal Sack" in the
southern hemishpere, near to the Southern Cross. Looking along
certain directions in the Milky Way, this dust builds up
effectively into a screen and blocks the light from stars further
away still.
The dust particles are known to be irregularly shaped. Because
they are also slightly magnetic, they tend to align themselves
along the directions folowed by magnetic fields in space. Aligned
like this, the screen of dust is very much like a pair of polaroid
sunglasses; they polarise slightly the light coming through layers
of dust from stars behind.
Andrei Berdyugin, Vilppu Piirola and Pekka Teerikorpi have been
working for several years to measure the polarisation of all the
stars at the North and South Galactic poles -- regions of the sky
perpendicular to the plane of the Milky Way. This way they can
build up a map of how the dust is layered above and below the Milky
Way.
The measurements not only reveal how much dust is scattered
amongst the stars --- long a thorny issue in astronomy --- but also
the direction of the magnetic field of the Milky Way.
The figure shows their measurements at the Southern Galactic
pole. For each star, the amount of dust along the line of sight
between us and the star is shown as a line. The length of the line
indicates how much dust (polarisation) is seen, and its direction
indicates the alignment of the Galactic magnetic field.
Apart from measuring the magnetic field of the galaxy, the maps
also allow the astronomers to measure how thick the dust layer is
in our Galaxy and its relation to the gas and stars in the Galaxy's
flattened disk. Although the amount of dust seen in the maps only
blocks a few percent of the light coming from behind, it is
important to know exactly how much because it plays an important
part in the puzzle of figuring out how far away distant galaxies
are from the Milky Way.
The paper on high latitude interstellar polarisation is
by Andrei Berdyugin, Vilppu Piirola and Pekka Teerikorpi
and has been published in the journal Astronomy
and Astrophysics.
Updated October 20th 2004.
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Polarisation measurements of starlight at the South
Galactic Pole. A line indicates for each measured star,
the amount of dust (polarisation) along the line of sight,
and the alignment of the polarisation. Maps like these
help build up a picture of how much dust we have to
contend with not only when looking around the Galaxy but
also out into the Universe at large. Click here for a more detailed view of the
polarisation map
The Royal Swedish Academy's 60 cm telescope at La Palma in the
Canary Islands, Spain, was used to measure the polarisations of the
stars. The telescope has recently been upgraded to allow fully remote
observing from Tuorla Observatory, in Turku, Finland.
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