Free Astronomy Magazine March-April 2016

SPACE CHRONICLES bon and iron, as well as larger, grain- ier molecules. Not only does dust block out light, making dust-shroud- ed objects harder to see, it also pref- erentially scatters bluer light. As a result, cosmic dust makes objects ap- pear redder when seen through our telescopes than they are in reality. Astronomers can factor out this reddening when study- ing objects. Still, the less red- dening, the more precise an ob- servation is likely to be. The second reason we know the distance to IC 1613 so pre- cisely is that the galaxy hosts a number of notable stars of two types: Cepheid variables and RR Lyrae variables. Both types of star rhythmically pulsate, growing char- acteristically bigger and brighter at fixed intervals. Other than the two Magellanic Clouds, IC 1613 is the only irregular dwarf galaxy in the Local Group in which RR Lyrae type vari- able stars have been identified. As we know from our daily lives on Earth, shining objects such as light bulbs or candle flames appear dim- mer the further they are away from us. Astronomers can use this simple piece of logic to figure out exactly how far away things are in the Uni- verse— so long as they know how bright they really are, referred to as their intrinsic brightness. Cepheid and RR Lyrae variables have the special property that their period of brightening and dimming is linked directly to their intrinsic brightness. So, by measuring how quickly they fluctuate astronomers can work out their intrinsic bright- ness. They can then compare these values to their apparent measured brightness and work out how far away they must be to appear as dim as they do. Stars of known intrinsic bright- ness can act like standard can- dles, as astronomers say, much like how a candle with a spe- cific brightness would act as a good gauge of distance inter- vals based on the observed bright- ness of its flame’s flicker. Using standard candles — such as the variable stars within IC 1613 and the less-common Type Ia supernova ex- plosions, which can seen across far greater cosmic distances — astrono- mers have pieced together a cosmic distance ladder, reaching deeper and deeper into space. Decades ago, IC 1613 helped astronomers work out how to utilise variable stars to chart the Universe’s grand expanse. Not bad for a little, shapeless galaxy. T his wide-field view shows the sky around the dwarf galaxy IC 1613 in the constella- tion of Cetus (The Sea Monster). This picture was created from images forming part of the Di- gitized Sky Survey 2. The galaxy appears at the centre of the pic- ture as an irregularly shaped clump of faint stars. [ESO/Digi- tized Sky Survey 2] T his sequence starts with a broad view of the rather faint constellation of Cetus (The Sea Monster). As we zoom, we close in on a faint, but nearby galaxy, IC 1613. The final detailed image, captured with the OmegaCAM camera on ESO’s VLT Survey Tele- scope in Chile, shows an unusually clean small galaxy. [ESO/A. Fujii/ Digitized Sky Survey 2] n