Free Astronomy Magazine November-December 2015

SPACE CHRONICLES from the build-up of smaller galaxies during the early days of the Uni- verse. If some of these small galaxies still remain today, they should now contain many extremely old stars. The Sculptor Dwarf Galaxy fits the bill as a primordial galaxy, thanks to a large number of ancient stars, visi- ble in this image (below left). Astronomers can determine the age of stars in the gal- axy because their light carries the signatures of only a small quantity of heavy chemical el- ements. These heavy ele- ments accumulate in galaxies with successive gener- ations of stars. A low level of heavy elements thus in- dicates that the average age of the stars in the Sculp- tor Dwarf Galaxy is high. This quantity of old stars makes the Sculptor Dwarf Galaxy a prime tar- get for studying the earliest peri- ods of star forma- tion. In a recent study, astronomers combined all the data available for the galaxy to cre- ate the most accu- rate star forma- tion history ever determined for a dwarf spheroidal galaxy. This anal- ysis revealed two distinct groups of stars in the galaxy. The first, predom- inant group is the older population, which is lacking in heavier elements. The second, smal- ler population, in contrast, is rich with heavy elements. Like young people crowding into city centres, this youthful stellar population is concentrated toward the galaxy’s core. The stars within dwarf galaxies like the Sculptor Dwarf Galaxy can exhibit com- plex star formation histo- ries. But as most of these dwarf galaxies’ stars have been isolated from each other and have not inte- racted for billions of years, each collection of stars has charted its own evolution- ary course. Studying the similarities in dwarf galax- ies’ histories, and explain- ing the occasional outliers, will help to explain the de- velopment of all galaxies, from the most unassuming dwarf to the grandest spi- rals. There is indeed much for astronomers to learn from the Milky Way’s shy neighbours. T his image of the sky around the Sculptor Dwarf Galaxy was created from pictures from the Digitized Sky Survey 2. The galaxy appears as a small faint cloud close to the centre of the picture. [ESO/Digitized Sky Survey 2] T his video zoom takes a closer look at the Sculptor Dwarf Galaxy, pictured in new image from the Wide Field Imager camera, in- stalled on the 2.2-metre MPG/ESO telescope at ESO’s La Silla Observatory. This elusive galaxy is a close neighbour of our galaxy, the Milky Way. Despite their proximity, both galaxies have very distinct histories and char- acters. This galaxy is much smaller, fainter and older than the Milky Way. [DSS, ESO] n