The latest report from the European Southern Observatory (ESO):
The Milky Way’s Ancient Heart
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.
The video, based on observations made in the infrared with the VISTA infrared survey telescope shows the central region of the Milky Way galaxy. Several variable stars within the field of views are marked with circles. They can also be identified as they brighten and fade in regular intervals. At the end the video zooms on one of the newly discovered RR Lyrae stars (marked with a red circle), which are too faint to be seen clearly in the large field of view. Credit: ESO/VVV Survey/D. Minniti
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].
This video sequence starts from a wide field view of the Milky Way and closes near the galactic centre. Here astronomers using the VISTA infrared survey telescope discovered several ancient stars, of a type known as RR Lyrae. 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. Credit: ESO, Digitized Sky Survey 2, N. Risinger (skysurvey.org. Acknowledgment: Davide De Martin and S. Guisard (www.eso.org/~sguisard), Music: Johan B. Monell (www.johanmonell.com)
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”.
This video zooms across a part of the sky close to the centre of our Milky Way, which was observed with the VISTA infrared survey telescope. Within this part of the sky astronomers discovered a dozen new variable stars. Their discovery suggests that the bulging centre of the Milky Way likely grew through the merging of primordial star clusters. Credit: ESO/VVV Survey/D. Minniti. Music: 5th Dimension
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.
Notes
[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.
