Artist’s impression of eclipsing binary
Explanation of eclipsing binaries
Map of the Large Magellanic Cloud
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New results pin down the distance to the galaxy next door
After nearly a decade of careful
observations an international team of astronomers has measured the
distance to our neighbouring galaxy, the Large Magellanic Cloud, more
accurately than ever before. This new measurement also improves our
knowledge of the rate of expansion of the Universe — the Hubble Constant
— and is a crucial step towards understanding the nature of the
mysterious dark energy that is causing the expansion to accelerate. The
team used telescopes at ESO’s La Silla Observatory in Chile as well as
others around the globe. These results appear in the 7 March 2013 issue
of the journal Nature.
Astronomers survey the scale of the Universe by first measuring the
distances to close-by objects and then using them as standard candles [1]
to pin down distances further and further out into the cosmos. But this
chain is only as accurate as its weakest link. Up to now finding an
accurate distance to the Large Magellanic Cloud (LMC), one of the
nearest galaxies to the Milky Way, has proved elusive. As stars in this
galaxy are used to fix the distance scale for more remote galaxies, it
is crucially important.
But careful observations of a rare class of double star have now
allowed a team of astronomers to deduce a much more precise value for
the LMC distance: 163 000 light-years.
“I am very excited because astronomers have been trying for a
hundred years to accurately measure the distance to the Large Magellanic
Cloud, and it has proved to be extremely difficult,” says Wolfgang Gieren (Universidad de Concepción, Chile) and one of the leaders of the team. “Now we have solved this problem by demonstrably having a result accurate to 2%.”
The improvement in the measurement of the distance to the Large
Magellanic Cloud also gives better distances for many Cepheid variable
stars [2]. These
bright pulsating stars are used as standard candles to measure
distances out to more remote galaxies and to determine the expansion
rate of the Universe — the Hubble Constant. This in turn is the basis
for surveying the Universe out to the most distant galaxies that can be
seen with current telescopes. So the more accurate distance to the Large
Magellanic Cloud immediately reduces the inaccuracy in current
measurements of cosmological distances.
The astronomers worked out the distance to the Large Magellanic Cloud
by observing rare close pairs of stars, known as eclipsing binaries [3].
As these stars orbit each other they pass in front of each other. When
this happens, as seen from Earth, the total brightness drops, both when
one star passes in front of the other and, by a different amount, when
it passes behind [4].
By tracking these changes in brightness very carefully, and also
measuring the stars’ orbital speeds, it is possible to work out how big
the stars are, their masses and other information about their orbits.
When this is combined with careful measurements of the total brightness
and colours of the stars [5] remarkably accurate distances can be found.
This method has been used before, but with hot stars. However,
certain assumptions have to be made in this case and such distances are
not as accurate as is desirable. But now, for the first time, eight
extremely rare eclipsing binaries where both stars are cooler red giant
stars have been identified [6]. These stars have been studied very carefully and yield much more accurate distance values — accurate to about 2%.
“ESO provided the perfect suite of telescopes and instruments for
the observations needed for this project: HARPS for extremely accurate
radial velocities of relatively faint stars, and SOFI for precise
measurements of how bright the stars appeared in the infrared,”
adds Grzegorz Pietrzyński (Universidad de Concepción, Chile and Warsaw
University Observatory, Poland), lead author of the new paper in Nature.
“We are working to improve our method still further and hope to
have a 1% LMC distance in a very few years from now. This has
far-reaching consequences not only for cosmology, but for many fields of
astrophysics,” concludes Dariusz Graczyk, the second author on the new Nature paper.
Notes
[1] Standard candles are objects of
known brightness. By observing how bright such an object appears
astronomers can work out the distance — more distant objects appear
fainter. Examples of such standard candles are Cepheid variables [2]
and Type Ia supernovae. The big difficulty is calibrating the distance
scale by finding relatively close examples of such objects where the
distance can be determined by other means.
[2] Cepheid variables are bright unstable stars that
pulsate and vary in brightness. But there is a very clear relationship
between how quickly they change and how bright they are. Cepheids that
pulsate more quickly are fainter than those that pulsate more slowly.
This period-luminosity relation allows them to be used as standard
candles to measure the distances of nearby galaxies.
[3] This work is part of the long-term Araucaria Project to improve measurements of the distances to nearby galaxies.
[4] The exact light variations depend on the relative
sizes of the stars, their temperatures and colours and the details of
the orbit.
[5] The colours are measured by comparing the brightness of the stars at different near-infrared wavelengths.
[6] These stars were found by searching the 35 million LMC stars that were studied by the OGLE project.
More information
This research was presented in a paper “An
eclipsing binary distance to the Large Magellanic Cloud accurate to 2
per cent”, by G. Pietrzyński et al., to appear in the 7 March 2013 issue
of the journal Nature.
The team is composed of G. Pietrzyński (Universidad de Concepción,
Chile; Warsaw University Observatory, Poland), D. Graczyk (Universidad
de Concepción), W. Gieren (Universidad de Concepción), I. B. Thompson
(Carnegie Observatories, Pasadena, USA), B., Pilecki (Universidad de
Concepción; Warsaw University Observatory), A. Udalski (Warsaw
University Observatory), I. Soszyński (Warsaw University Observatory),
S. Kozłowski (Warsaw University Observatory), P. Konorski (Warsaw
University Observatory), K. Suchomska (Warsaw University Observatory),
G. Bono (Università di Roma Tor Vergata, Rome, Italy; INAF-Osservatorio
Astronomico di Roma, Italy), P. G. Prada Moroni (Università di Pisa,
Italy; INFN, Pisa, Italy), S. Villanova (Universidad de Concepción ), N.
Nardetto (Laboratoire Fizeau, UNS/OCA/CNRS, Nice, France), F. Bresolin
(Institute for Astronomy, Hawaii, USA), R. P. Kudritzki (Institute for
Astronomy, Hawaii, USA), J. Storm (Leibniz Institute for Astrophysics,
Potsdam, Germany), A. Gallenne (Universidad de Concepción), R. Smolec
(Nicolaus Copernicus Astronomical Centre, Warsaw, Poland), D. Minniti
(Pontificia Universidad Católica de Chile, Santiago, Chile; Vatican
Observatory, Italy), M. Kubiak (Warsaw University Observatory), M.
Szymański (Warsaw University Observatory), R. Poleski (Warsaw University
Observatory), Ł. Wyrzykowski (Warsaw University Observatory), K.
Ulaczyk (Warsaw University Observatory), P. Pietrukowicz (Warsaw
University Observatory), M. Górski (Warsaw University Observatory), P.
Karczmarek (Warsaw University Observatory).
ESO is the foremost intergovernmental astronomy organisation in
Europe and the world’s most productive ground-based astronomical
observatory by far. It is supported by 15 countries: Austria, Belgium,
Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy,
the Netherlands, Portugal, Spain, Sweden, Switzerland and the United
Kingdom. ESO carries out an ambitious programme focused on the design,
construction and operation of powerful ground-based observing facilities
enabling astronomers to make important scientific discoveries. ESO also
plays a leading role in promoting and organising cooperation in
astronomical research. ESO operates three unique world-class observing
sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO
operates the Very Large Telescope, the world’s most advanced
visible-light astronomical observatory and two survey telescopes. VISTA
works in the infrared and is the world’s largest survey telescope and
the VLT Survey Telescope is the largest telescope designed to
exclusively survey the skies in visible light. ESO is the European
partner of a revolutionary astronomical telescope ALMA, the largest
astronomical project in existence. ESO is currently planning the
39-metre European Extremely Large optical/near-infrared Telescope, the
E-ELT, which will become “the world’s biggest eye on the sky”.
Links
Contacts
Grzegorz PietrzyńskiUniversidad de Concepción
Chile
Tel: +56 41 220 7268
Cell: +56 9 6245 4545
Email: pietrzyn@astrouw.edu.pl
Wolfgang Gieren
Universidad de Concepción
Chile
Tel: +56 41 220 3103
Cell: +56 9 8242 8925
Email: wgieren@astro-udec.cl
Richard Hook
ESO, Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591
Email: rhook@eso.org
