Free Astronomy Magazine September-October 2016

T he small smattering of bright blue stars upper left of centre in this huge 615 megapixel ESO image is the perfect cosmic laboratory in which to study the life and death of stars. Known as Messier 18 this open star cluster contains stars that formed together from the same massive cloud of gas and dust. This image was captured by the Omega- CAM camera attached to the VLT Sur- vey Telescope (VST) located at ESO’s Paranal Observatory in Chile. [ESO] constellation of Sagittarius, and con- sists of many sibling stars loosely bound together in what is known as an open cluster. There are over 1000 known open star clusters within the Milky Way, with a wide range of properties, such as size and age, that provide astronomers with clues to how stars form, evolve and die. The main appeal of these clusters is that all of their stars are born together out of the same material. In Messier 18 the blue and white colours of the stellar population indicate that the cluster’s stars are very young, proba- bly only around 30 million years old. Being siblings means that any dif- ferences between the stars will only be due to their masses, and not their distance from Earth or the composi- tion of the material they formed from. This makes clusters very useful in refining theories of star formation and evolution. Astronomers now know that most stars do form in groups, forged from the same cloud of gas that collapsed in on itself due to the attractive force of gravity. The cloud of left- over gas and dust — or molecular cloud — that envelops the new stars is often blown away by their strong stellar winds, weakening the gravi- tational shackles that bind them. Over time, loosely bound stellar sib- lings like those pictured here will often go their separate ways as in- teractions with other neighbouring stars or massive gas clouds nudge, or pull, the stars apart. Our own star, the Sun, was most likely once part of a cluster very much like Messier 18 until its companions were gradually distributed across the Milky Way. The dark lanes that snake through this image are murky filaments of cosmic dust, blocking out the light from distant stars. The contrasting faint reddish clouds that seem to weave between the stars are com- posed of ionised hydrogen gas. The gas glows because young, extremely