New images from the Very Large Telescope in Chile show the interior of the Milky Way. This is Terzan 5 – a cluster of stars 20,000 light years away from Earth. Astronomers have discovered that this star cluster is in fact composed of star populations created during at least two different eras; the earliest some 12 billion years ago and the second 6 billion years ago.
This discovery could confirm that the central region of a galaxy originates from the merging of pre-formed – and internally evolved – star systems, that large galaxies are formed by smaller objects merging.
Francesco Ferraro is an astronomer at the University of Bologna in Italy. Along with his colleagues at the university, he has studied Terzan 5 in depth. But the region where it is located has been hard to study because of its high concentration of interstellar dust.
He says: “This discovery has been astonishing because we were expecting Terzan 5 to be just a cluster like many others. The discovery of two different populations suggests that this object is a cosmic relic, from the same period of the formation of the galaxy and therefore a building block that formed the structure of the galaxy as we know it today.”
The astronomers made this discovery because they were able to identify two distinct stellar populations within Terzan 5: a bright one whose stars are centrally concentrated and a second one whose stars are fainter.
Says Alessio Mucciarelli, another astronomer at the University of Bologna: “This hypothesis was verified by the study of high resolution spectrum images which allowed us to distinguish a different chemical composition between the two populations. This represents a strong endorsement of the two different ages.”
A technical jewel lies behind the scenes of this discovery, called the Multi-Conjugate Adaptive Optics Demonstrator (MAD), a cutting-edge instrument that allows the Very Large Telescope to achieve superbly detailed images in the infrared.
It overcomes the blurring of ground-based telescope images which normally results from the Earth’s turbulent atmosphere. It is a prototype of even more powerful, next-generation adaptive optics instruments.
Says Emanuele d’Alessandro, another astronomer at the University of Bologna: “Observations made in the past with space telescopes and ground telescopes were not able to detect this peculiarity of Terzan 5. MAD was mounted on the Very Large Telescope for a limited period of time as a test for this kind of instrument, but also as a test for the new generation of 40 meter diameter telescopes like the European ELT (Extremely Large Telescope).”
Its success means that MAD could be used alongside the next generation of Large Telescopes -the Extremely Large Telescope – the largest optical-infrared telescope in the world, which is currently being designed.
According to scientists, this could be only the first of a series of discoveries about the origin of galaxies. Several similar systems could be hidden behind the cosmic dust: these objects in the Milky Way – as old as the galaxy – contain the history of how our galaxy was formed.