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Artist’s impression of the yellow hypergiant star HR 5171
Zooming in on the yellow hypergiant star HR 5171
Mix of new and old observations reveals exotic binary system
ESO’s Very Large Telescope Interferometer
has revealed the largest yellow star — and one of the ten largest stars
found so far. This hypergiant has been found to measure more than 1300
times the diameter of the Sun, and to be part of a double star system,
with the second component so close that it is in contact with the main
star. Observations spanning over sixty years, some from amateur
observers, also indicate that this rare and remarkable object is
changing very rapidly and has been caught during a very brief phase of
its life.
Using ESO’s Very Large Telescope Interferometer (VLTI), Olivier
Chesneau (Observatoire de la Côte d’Azur, Nice, France) and an
international team of collaborators have found that the yellow
hypergiant star
HR 5171 A [1] is absolutely huge — 1300 times the diameter of the Sun and much bigger than was expected
[2]. This makes it the largest yellow star known. It is also in the
top ten of the largest stars known — 50% larger than the famous red supergiant Betelgeuse — and about one million times brighter than the Sun.
“The new observations also showed that this star has a very close binary partner, which was a real surprise,” says Chesneau. “The two stars are so close that they touch and the whole system resembles a gigantic peanut.”
The astronomers made use of a technique called
interferometry
to combine the light collected from multiple individual telescopes,
effectively creating a giant telescope up to 140 metres in size. The new
results prompted the team to thoroughly investigate older observations
of the star spanning more than sixty years, to see how it had behaved in
the past
[3].
Yellow hypergiants are very rare, with only a dozen or so known in our galaxy — the best-known example being
Rho Cassiopeiae.
They are among the biggest and brightest stars known and are at a stage
of their lives when they are unstable and changing rapidly. Due to this
instability, yellow hypergiants also expel material outwards, forming a
large, extended atmosphere around the star.
Despite its great distance of nearly 12 000 light-years from Earth, the object can just about be seen with the naked eye [4]
by the keen-sighted. HR 5171 A has been found to be getting bigger over
the last 40 years, cooling as it grows, and its evolution has now been
caught in action. Only a few stars are caught in this very brief phase,
where they undergo a dramatic change in temperature as they rapidly
evolve.
By analysing data on the star’s varying brightness, using
observations from other observatories, the astronomers confirmed the
object to be an eclipsing binary system where the smaller component
passes in front and behind the larger one as it orbits. In this case HR
5171 A is orbited by its companion star every 1300 days. The smaller
companion is only slightly hotter than HR 5171 A’s surface temperature
of 5000 degrees Celsius.
Chesneau concludes “The companion we have found is very
significant as it can have an influence on the fate of HR 5171 A, for
example, stripping off its outer layers and modifying its evolution.”
This new discovery highlights the importance of studying these huge
and short-lived yellow hypergiants, and could provide a means of
understanding the evolutionary processes of massive stars in general.
Notes
[1] The star is also known as V766 Cen, HD 119796 and HIP 67261.
[2] Comparable objects seem to all be red supergiants
which reach 1000–1500 times the radius of the Sun and have initial
masses not exceeding 20–25 Solar masses. The radius of a yellow
supergiant was expected to be 400–700 times that of the Sun.
[3] Spectral data were obtained using the
Anglo–Australian Telescope with the University College London Echelle
Spectrograph (UCLES), at the South African Astronomical Observatory
(SAAO), with PUCHEROS, from the Pontificia Universidad de Chile (PUC)
and through coronagraphic observations with the Near-Infrared
Coronagraphic Imager (NICI) on the Gemini South telescope. Archival
photometric datasets examined include infrared photometry from the South
African Astronomical Observatory spanning the time frame from 1975 to
2013 and other datasets from 1983 to 2002, including some amateur
observations. The agreement of professional results with those from
amateur astronomer Sebastian Otero (2000–2013) is considered by the
authors to be “excellent”, and “illustrates the quality of these amateur
observations”.
[4] The visual magnitude of HR 5171 A is seen to vary between 6.10 and 7.30. It and can be viewed in the constellation of Centaurus (The Centaur).
More information
This research was presented in a paper “The
yellow hypergiant HR 5171 A: Resolving a massive interacting binary in
the common envelope phase”, by Chesneau et al., to appear in the journal
Astronomy & Astrophysics.
The team is composed of O. Chesneau (Laboratoire Lagrange, Univ. Nice
Sophia-Antipolis, CNRS, Observatoire de la Côte d’Azur, Nice, France
[Lagrange]), A. Meilland (Lagrange), E. Chapellier (Lagrange), F.
Millour (Lagrange), A.M. Van Genderen (Leiden Observatory, Leiden, The
Netherlands), Y. Nazé (Le Fonds de la Recherche Scientifique, Liège,
Belgium), N. Smith (Steward Observatory, Tucson, USA), A. Spang
(Lagrange), J.V. Smoker (ESO, Santiago, Chile), L. Dessart (Aix
Marseille Université, CNRS, Laboratoire d’Astrophysique de Marseille,
Marseille, France), S. Kanaan (Instituto de Física y Astronomía,
Universidad de Valparaíso, Chile [IFA]), Ph. Bendjoya (Lagrange), M.W.
Feast (South African Astronomical Observatory, South Africa [SAAO]),
J.H. Groh (Geneva Observatory, Geneva, Switzerland), A. Lobel (Royal
Observatory of Belgium, Brussels, Belgium), N. Nardetto (Lagrange), S.
Otero (American Association of Variable Star Observers, Cambridge, MA,
USA), R.D. Oudmaijer (School of Physics & Astronomy, University of
Leeds, UK), A.G. Tekola (SAAO and Las Cumbres Observatory Global
Telescope Network, Goleta, CA, USA), P.A. Whitelock (SAAO), C. Arcos
(IFA), M. Curé (IFA) and L. Vanzi (Department of Electrical Engineering
and Center of Astro Engineering, Pontificia Universidad Catolica de
Chile, Santiago, Chile).
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Links
Contacts
Olivier Chesneau
Laboratoire Lagrange / Univ. Nice Sophia-Antipolis, CNRS - Observatoire de la Côte d’Azur
Nice, France
Tel: +33 (0)4 92 00 19 79
Email: olivier.chesneau@oca.eu
Richard Hook
ESO, Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591
Email: rhook@eso.org
