The star cluster NGC 3766 in the constellation of Centaurus
The Swiss 1.2-metre Leonhard Euler Telescope in its dome at La Silla
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Minute variations in brightness reveal whole new class of stars
Astronomers using the Swiss 1.2-metre
Euler telescope at ESO’s La Silla Observatory in Chile have found a new
type of variable star. The discovery was based on the detection of very
tiny changes in brightness of stars in a cluster. The observations
revealed previously unknown properties of these stars that defy current
theories and raise questions about the origin of the variations.
The Swiss are justly famed for their craftsmanship when creating
extremely precise pieces of technology. Now a Swiss team from the Geneva
Observatory has achieved extraordinary precision using a comparatively
small 1.2-metre telescope for an observing programme stretching over
many years. They have discovered a new class of variable stars by
measuring minute variations in stellar brightness.
The new results are based on regular measurements of the brightness
of more than three thousand stars in the open star cluster NGC 3766 [1]
over a period of seven years. They reveal how 36 of the cluster’s stars
followed an unexpected pattern — they had tiny regular variations in
their brightness at the level of 0.1% of the stars’ normal brightness.
These variations had periods between about two and 20 hours. The stars
are somewhat hotter and brighter than the Sun, but otherwise apparently
unremarkable. The new class of variable stars is yet to be given a name.
This level of precision in the measurements is twice as good as that
achieved by comparable studies from other telescopes — and sufficient to
reveal these tiny variations for the first time.
“We have reached this level of sensitivity thanks to the high
quality of the observations, combined with a very careful analysis of
the data,” says Nami Mowlavi, leader of the research team, “but
also because we have carried out an extensive observation programme
that lasted for seven years. It probably wouldn’t have been possible to
get so much observing time on a bigger telescope.”
Many stars are known as variable or pulsating stars, because their
apparent brightness changes over time. How the brightness of these stars
changes depends in complex ways on the properties of their interiors.
This phenomenon has allowed the development of a whole branch of
astrophysics called asteroseismology, where astronomers can “listen” to
these stellar vibrations, in order to probe the physical properties of
the stars and get to know more about their inner workings.
“The very existence of this new class of variable stars is a challenge to astrophysicists,” says Sophie Saesen, another team member. “Current
theoretical models predict that their light is not supposed to vary
periodically at all, so our current efforts are focused on finding out
more about the behaviour of this strange new type of star.”
Although the cause of the variability remains unknown, there is a
tantalising clue: some of the stars seem to be fast rotators. They spin
at speeds that are more than half of their critical velocity, which is
the threshold where stars become unstable and throw off material into
space.
“In those conditions, the fast spin will have an important impact
on their internal properties, but we are not able yet to adequately
model their light variations,” explains Mowlavi. “We hope our
discovery will encourage specialists to address the issue in the hope of
understanding the origin of these mysterious variations.”
Notes
[1] This star cluster is one of several
included in this major monitoring programme. NGC 3766 lies about 7000
light-years from Earth in the southern constellation of Centaurus (The
Centaur) and is estimated to be about 20 million years old.
More information
This research was presented in a paper
“Stellar variability in open clusters I. A new class of variable stars
in NGC 3766”, by N. Mowlavi et al., published in the journal Astronomy & Astrophysics on 12 June 2013.
The team is composed of N. Mowlavi, F. Barblan, S. Saesen and L.
Eyer. All four authors are from the Geneva Observatory in Switzerland.
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
Nami MowlaviGeneva Observatory, University of Geneva/ISDC
Switzerland
Tel: +41 22 37 92 194
Email: Nami.Mowlavi@unige.ch
Sophie Saesen
Geneva Observatory, University of Geneva
Switzerland
Tel: +41 22 379 24 46
Email: Sophie.Saesen@unige.ch
Richard Hook
ESO Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591
Email: rhook@eso.org
