Where the stars collide

Using an advanced adaptive optics, namely the GeMS of the Gemini South telescope in Chile a team of astronomers has captured the globular cluster Liller 1. The data show that the cluster could contain a mass of 1.5 million times greater than that of the Sun, making it an extreme atmosphere for the stars’ collisions.
A group of scientists has observed in detail a star cluster heavily obscured by dust in the Milky Way. It is a globular cluster, an environment where the stars are so densely distributed making extremely probable the collision of stars. “It’s a bit like a stellar pool table, in which the probability of collision depends on the size of the table and the number of balls that it contains,” said Francesco R. Ferraro from the University of Bologna, one of the team members that made the observations.


Credit: Gemini Observatory/AURA.

The cluster of stars, known by the name of Liller 1, is a goal difficult to study, partly because of its distance and then because it is located near the center of the Milky Way (at about 3,200 light years away from the center, to be precise), where the obscuration due to dust is very high. The very high resolution image of storage revealed a large cluster of stars, which scientists have estimated at least contain a mass of 1.5 million suns, values very similar to the most massive globular clusters in our galaxy, Omega Centauri and Terzan 5.
“Although our galaxy contains about 200 billion stars, there is so much empty space between them and because the stars are rarely one to another, they rarely can collide,” said Douglas Geisler, principal investigator of the proposal and observational researcher at the University of Concepcion, Chile. “The central regions of globular clusters and overcrowded show rather favorable conditions. The data we have collected confirm that Liller 1 is one of the best environments in our galaxy to observe stellar collisions. ”
The team led by Geisler specializes in the study of globular clusters closest to the center of the Milky Way, while Ferraro is led by an expert in the analysis of the infrared data of globular clusters. The two groups have worked together to make detailed observations of Liller 1.
Liller 1 is a compact ball of stars, or a globular cluster. Globular clusters orbit in a large halo around the center of our Galaxy, and many of the globular clusters closest objects are spectacular, also observed with small telescopes or binoculars. “This is not particularly spectacular, is so obscured by material in the central bulge of our galaxy to be almost completely invisible to our eyes, ‘commented Sara Saracino, University of Bologna, lead author of the article published in the journal The Astrophysical Journal. Liller 1 is nearly 30,000 light years from Earth, in one of the most inaccessible regions of our Galaxy, where thick dust clouds prevent visible light to emerge. “Only the infrared radiation can travel through these clouds and provide direct information about the stars that make up the clutter,” said Emanuele Dalessandro University of Bologna.
Observations of storage Liller 1 were made with the powerful adaptive optics system of the Gemini South telescope at the Gemini Observatory in Chile.
The jewel of the art GeMS (the “Gemini Multi-conjugate adaptive optics system”), in combination with the powerful infrared camera Gemini South Adaptive Optics Imager, was able to penetrate the dense fog surrounding Liller 1 and to provide astronomers an unprecedented view of its star system. This has been made possible thanks to the combination of two specific features of GeMS: first, the possibility to operate at wavelengths in the near infrared (especially in K-band); secondly, the use of a technique innovative and revolutionary able to remove the distortions of astronomical images due to atmospheric turbulence. To offset the effects of image degradation due to the terrestrial atmosphere, the system uses three GeMS guide stars, a constellation of five laser guide stars, and several deformable mirrors. The correction is so refined that this system allows astronomers to collect images of unprecedented clarity. In the best data collected in K-band, the images of Liller 1 have an angular resolution of only 75 thousandths of arc second, slightly higher than the theoretical limit of the mirror 8 meters of Gemini (known as the diffraction limit). This means that GeMS made corrections almost perfect to atmospheric distortions.
These images are comparable in sharpness, to those collected by the Hubble Space Telescope in infrared wavelengths. In comparison, however, they have a great advantage: a collection area much larger (the mirror of the telescope Gemini South has a diameter of 8 meters, in front of a mirror by 2.4 meters on board the Hubble Space Telescope).
The observations to be carried out with this project are aimed at many other globular clusters. The results obtained Liller 1 have encouraged the team to expand their collaboration and are working on other clusters, which promise to deliver results even more exciting.
The scientific context: stellar collisions
The stellar collisions are extremely important, as they can provide the key to understanding the origin of these exotic objects that cannot be explained by the evolution of individual stars. Almost clashes (almost) front, where the stars come together mixing their fuel and re-stoking the nuclear fusion, for example, seem to be at the origin of the so-called Blue Straggler, stars hotter than the average of storage to which they belong. But collisions can also occur in binary systems, with a narrowing of the system compared to its original size, which can promote the interaction between the two components and produce a variety of items such as low-mass X-ray binaries, millisecond pulsars, etc. In particular pulsars to millisecond they are – according to the most accredited theory at present – old neutron stars accelerated to rotation periods of millisecond through the increase of matter, and thus require the presence of a companion. It is suspected that Liller 1 guests a large population of these exotic objects. Although no millisecond pulsar has been directly observed to date, it is believed that there is a large population hidden within the cluster, because this would explain the emission of gamma rays detected, the most intense so far observed for a globular cluster.
“Our observations confirm that Liller 1 is one of the best” laboratories “in which to study the impact of the dynamics of a globular cluster of stellar evolution: it opens the opportunity for a kind of study of “stellar sociology”, designed to measure the mutual influence between the stars when they are forced to live in conditions of extreme overcrowding, “concludes Ferraro.

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