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The Milky Way’s ancient heart

Astronomers found RR Lyrae stars at our Milky Way galaxy’s core. They now believe ancient globular star clusters may have merged to form the core of the Milky Way.

This image, captured with the VISTA ...Image via ESO/VVV Survey/D. Minniti

The VISTA telescope in Chile captured this image. It shows the central part of the Milky Way, a region normally hidden behind obscuring dust. VISTA can see into this region because it can see in the infrared. Thus it can study the stars close to center of our galaxy. Image via ESO/VVV Survey/D. Minniti

The European Southern Observatory’s infrared VISTA telescope has discovered ancient stars – known as RR Lyrae stars – in the center of our Milky Way. This is the first time anyone has seen this sort of star at the galaxy’s center. RR Lyrae stars are commonly found in globular clusters, those vast symmetrical clusters that lie outside the disk of our galaxy and contain some of its oldest stars. The discovery of RR Lyrae stars at the heart of our Milky Way suggests to these astronomers that the bulging center of our galaxy likely grew through the merging of primordial globular star clusters. These stars may even be the remains of our galaxy’s most massive and oldest surviving star cluster.

This work is published October 12, 2016 in the peer-reviewed Astrophysical Journal Letters.

Dante Minniti of Universidad Andrés Bello in Chile and Rodrigo Contreras Ramos of the Instituto Milenio de Astrofísica in Chile used observations from the VISTA infrared survey telescope to find a dozen ancient RR Lyrae stars at the heart of the Milky Way that were previously unknown. Their statement explained:

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.

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 [a 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 center of the Milky Way’s bulge.

This discovery – RR Lyrae stars in the heart of our Milky Way – may help astronomers decide between two competing theories for how these bulges form at the center of galaxies like ours. One theory suggests that galactic bulges form through the merging of globular star clusters. The competing hypothesis is that these bulges form due to the rapid accretion [accumulation via gravity] of gas. Rodrigo Contreras Ramos said:

The evidence supports the scenario in which the bulge was originally made out of a few globular clusters that merged.

M13, the largest and brightest globular star cluster visible in Northern Hemisphere skies.  In 1974 astronomer Frank Drake used the Arecibo radio telescope to broadcast the first deliberate message from Earth to outer space. It was directed at this globular cluster.

This is M13, the largest and brightest globular star cluster visible in Northern Hemisphere skies. Globular clusters like these, which surround the center of our Milky Way (see below), are the classic place to find RR Lyrae variable stars.

About 150 globular star clusters surround our galaxy.  They orbit our galaxy's center.

About 150 globular star clusters are known to surround our Milky Way galaxy. They orbit our galaxy’s center and contain its most ancient stars.

Bottom line: Astronomers have used an infrared telescope in Chile – called VISTA – to discover RR Lyrae stars in the center of our Milky Way galaxy. As RR Lyrae stars typically reside in ancient globular clusters over 10 billion years old, this discovery suggests that the bulging center of the Milky Way likely grew through the merging of primordial star clusters.

Deborah Byrd

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