Variable stars close to the galactic centre
RR Lyrae stars in the constellation of Sagittarius
Wide-field view of the Centre of the Milky Way
Videos
VISTA finds remains of archaic globular star cluster
Ancient stars, of a type known as RR
Lyrae, have been discovered in the centre of the Milky Way for the first
time, using ESO’s infrared VISTA telescope. RR Lyrae stars typically
reside in ancient stellar populations over 10 billion years old. Their
discovery suggests that the bulging centre of the Milky Way likely grew
through the merging of primordial star clusters. These stars may even be
the remains of the most massive and oldest surviving star cluster of
the entire Milky Way.
A team led by Dante Minniti (Universidad Andrés Bello,
Santiago, Chile) and Rodrigo Contreras Ramos (Instituto Milenio de
Astrofísica, Santiago, Chile) used observations from the VISTA infrared survey telescope, as part of the Variables in the Via Lactea (VVV)
ESO public survey, to carefully search the central part of the Milky
Way. By observing infrared light, which is less affected by cosmic dust
than visible light, and exploiting the excellent conditions at ESO’s Paranal Observatory, the team was able to get a clearer view of this region than ever before. They found a dozen ancient RR Lyrae stars at the heart of the Milky Way that were previously unknown.
Our Milky Way has a densely populated centre — a feature
common to many galaxies, but unique in that it is close enough to study
in depth. This discovery of RR Lyrae stars provides compelling evidence
that helps astronomers decide between two main competing theories for
how nuclear bulges form [1].
RR Lyrae stars are typically found in dense globular clusters. They are variable stars,
and the brightness of each RR Lyrae star fluctuates regularly. By
observing the length of each cycle of brightening and dimming in an RR
Lyrae, and also measuring the star’s brightness, astronomers can
calculate its distance [2].
Unfortunately, these excellent distance-indicator stars are
frequently outshone by younger, brighter stars and in some regions they
are hidden by dust. Therefore, locating RR Lyrae stars right in the
extremely crowded heart of the Milky Way was not possible until the
public VVV survey was carried out using infrared light. Even so, the
team described the task of locating the RR Lyrae stars in amongst the
crowded throng of brighter stars as “daunting”.
Their hard work was rewarded, however, with the
identification of a dozen RR Lyrae stars. Their discovery indicate that
remnants of ancient globular clusters are scattered within the centre of
the Milky Way’s bulge.
Rodrigo Contreras Ramos elaborates: “This discovery of
RR Lyrae Stars in the centre of the Milky Way has important implications
for the formation of galactic nuclei. The evidence supports the
scenario in which the nuclear bulge was originally made out of a few
globular clusters that merged.”
The theory that galactic nuclear bulges form through the merging of globular clusters is contested by the competing hypothesis that these bulges are actually due to the rapid accretion of gas. The unearthing of these RR Lyrae stars — almost always found in globular clusters — is very strong evidence that part of the Milky Way's nuclear bulge did in fact form through merging. By extension, all other similar galactic bulges may have formed the same way.
Not only are these stars powerful evidence for an important
theory of galactic evolution, they are also likely to be over 10
billion years old — the dim, but dogged survivors of perhaps the oldest
and most massive star cluster within the Milky Way.
[1] The nuclear stellar
bulge is the compact component in the innermost regions of the Milky
Way (and other galaxies) extending to a size of about 400 light-years.
[2] RR Lyrae stars, like some other
regular variables such as Cepheids, show a simple relationship between
how quickly they change in brightness and how luminous they are. Longer
periods mean brighter stars. This period-luminosity relationship can be
used to deduce the distance of a star from its period of variation and
its apparent brightness.
More Information
This research was presented in a paper to appear in The Astrophysical Journal Letters.
The team is composed of D. Minniti (Instituto Milenio de
Astrofísica, Santiago, Chile; Departamento de Física, Universidad Andrés
Bello, Santiago, Chile; Vatican Observatory, Vatican City State; Centro de Astrofisica y Tecnologias Afines - CATA),
R. Contreras Ramos (Instituto Milenio de Astrofísica, Santiago, Chile;
Pontificia Universidad Católica de Chile, Instituto de Astrofísica,
Santiago, Chile), M. Zoccali (Instituto Milenio de Astrofísica,
Santiago, Chile; Pontificia Universidad Católica de Chile, Instituto de
Astrofísica, Santiago, Chile), M. Rejkuba (European Southern
Observatory, Garching bei München, Germany; Excellence Cluster Universe,
Garching, Germany), O.A. Gonzalez (UK Astronomy Technology Centre,
Royal Observatory, Edinburgh, UK), E. Valenti (European Southern
Observatory, Garching bei München, Germany), F. Gran (Instituto Milenio
de Astrofísica, Santiago, Chile; Pontificia Universidad Católica de
Chile, Instituto de Astrofísica, Santiago, Chile)
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 16 countries:
Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland,
Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden,
Switzerland and the United Kingdom, along with the host state of Chile.
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 a major partner in
ALMA, the largest astronomical project in existence. And on Cerro
Armazones, close to Paranal, ESO is building the 39-metre European
Extremely Large Telescope, the E-ELT, which will become “the world’s
biggest eye on the sky”.
Links
- Research paper
- Variables in the Via Lactea (VVV) ESO public survey
Contacts
Dante Minniti
Universidad Andrés Bello
Santiago, Chile
Email: dante@astrofisica.cl
Universidad Andrés Bello
Santiago, Chile
Email: dante@astrofisica.cl
Rodrigo Contreras Ramos
Instituto Milenio de Astrofísica
Santiago, Chile
Email: rcontrer@astro.puc.cl
Instituto Milenio de Astrofísica
Santiago, Chile
Email: rcontrer@astro.puc.cl
Mathias Jäger
Public Information Officer
Garching bei München, Germany
Cell: +49 176 62397500
Email: mjaeger@partner.eso.org
Source: ESO
