Free Astronomy Magazine November-December 2015

SPACE CHRONICLES able discovery — a previously un- known component of our home gal- axy, the Milky Way. “The central bulge of the Milky Way is thought to consist of vast numbers of old stars. But the VISTA data has revealed something new — and very young by astronomical standards!” says Dékány, lead au- thor of the new study. Analysing data from the survey, the astronomers found 655 candidate variable stars of a type called Ce- pheids. These stars expand and con- tract periodically, taking anything from a few days to months to com- plete a cycle and changing signifi- cantly in brightness as they do so. The time taken for a Cepheid to brighten and fade again is longer for those that are brighter and shorter for the dimmer ones. This re- markably precise relation- ship, which was discov- ered in 1908 by Ameri- can astronomer Henrietta Swan Leavitt, makes the study of Cepheids one of the most effec- tive ways to measure the dis- tances to, and map the posi- tions of, distant objects in the Milky Way and beyond. But there is a catch — Ce- pheids are not all the same — they come in two main classes, one much younger than the other. Out of their sam- ple of 655 the team identified 35 stars as belonging to a sub-group called classical Cepheids — young bright stars, very different from the usual, much more elderly, residents of the central bulge of the Milky Way. The team gathered information on the brightness, pulsation period, and deduced the distances of these 35 classical Cepheids. Their pulsation periods, which are closely linked to their age, revealed their surprising youth. “All of the 35 classical Cepheids dis- covered are less than 100 million years old. The youngest Cepheid may even be only around 25 million years old, although we cannot ex- clude the possible presence of even younger and brighter Cepheids,” ex- plains the study’s second author Dante Minniti, of the Universidad Andres Bello, Santiago, Chile. The ages of these classical Cepheids provide solid evidence that there has been a previously unconfirmed, continuous supply of newly formed stars into the central region of the Milky Way over the last 100 million years. But, this wasn’t to be the on- ly remarkable discovery from the survey’s dataset. Mapping the Ce- pheids that they discovered, the A stronomers using the VISTA telescope at ESO’s Paranal Obser- vatory have discovered a previously unknown component of the Milky Way. By mapping out the locations of a class of stars that vary in brightness called Ce- pheids, a disc of young stars buried behind thick dust clouds in the central bulge has been found. This diagram shows the locations of the newly discovered Cepheids in an artist’s rendering of the Milky Way. The yellow star in- dicates the position of the Sun. [ESO/Microsoft WorldWide Telescope] T his video shows the locations of the newly discover- ed Cepheids (red dots) in an artist’s rendering of the Milky Way. [ESO/Microsoft Worldwide Telescope] team traced an entirely new feature in the Milky Way — a thin disc of young stars across the galactic bulge. This new component to our home galaxy had remained un- known and invisible to previous sur- veys as it was buried behind thick clouds of dust. Its discovery demon- strates the unique power of VISTA, which was designed to study the Milky Way’s deep structures by wide-field, high-resolution imaging at infrared wavelengths. “This study is a powerful demon- stration of the unmatched capabil- ities of the VISTA telescope for probing extremely obscured galactic regions that cannot be reached by any other current or planned sur- veys,” remarks Dékány. “This part of the galaxy was com- pletely unknown until our VVV sur- vey found it!” adds Minniti. Further investigations are now need- ed to assess whether these Cephe- ids were born close to where they are now, or whether they originate from further out. Understanding their fundamental properties, interactions, and evolu- tion is key in the quest to under- stand the evolution of the Milky Way, and the process of galaxy evo- lution as a whole. n