DELVING INTO THE DEEP SKY
Located far beyond our Solar System, thousands of deep-sky objects are within easy range of astronomers armed with a telescope and a passion to hunt them down
Deep-sky objects include double and multiple star systems, open star clusters containing hundreds of individual stars and vast, concentrated assemblages of stars known as globular clusters. Many delightful nebulae can be discerned – glowing clouds of gas and dust sprinkled throughout our galaxy, with evidence of starbirth, stellar evolution and even the death of stars. Far beyond the Milky Way lie countless other galaxies, many of which can be seen through a telescope eyepiece.
Most star atlases, maps and computer programs display the positions and names of a huge variety of deep-sky objects. A great guide to refer to is the Messier list of deep-sky objects. Compiled in the late18th century by French astronomer Charles Messier, the list comprises an eclectic mix of 110 deep-sky objects visible from the Northern Hemisphere. 40 Messier objects are galaxies lying far beyond our own. 30 are open clusters, gravitationally bound collections of stars, many of them relatively young in cosmic age. 28 are globular clusters – vast conglomerations of stars that form a halo around our galaxy. Seven are nebulae – clouds of dust and gas within the Milky Way. Four are planetary nebulae – gas and dust puffed out from senile stars. And one is a supernova remnant – the debris left by a star that exploded.
Messier’s list is a great introduction to deep-sky observing, but it’s by no means exhaustive. It doesn’t incorporate multiple stars and many star clusters, nebulae and galaxies. Many more deep-sky objects are listed in the New General Catalogue of Nebulae and Clusters of Stars, compiled by John Dreyer in 1888. Hundreds of these objects are within the reach of someone armed with a good telescope, a number of them brighter and more visible than Messier objects. Hunting for deep-sky objects armed with just a star map and telescope requires a reasonably good star chart and a modicum of patience. The hunt itself is the draw to observing. It can be frustrating at times, but when success is attained the rewards are tremendously satisfying.
When searching for faint deep-sky objects, it’s important to be dark-adapted so that your pupils are dilated to their maximum, thus enabling the most light to be seen. The darker the observation site, the more fainter objects can be seen. In the dark, pupils dilate to their maximum size, allowing the maximum amount of light into the eye. Stargazers can take advantage of dark adaptation, but it takes time. Step outside from a bright room into a dark backyard and it may be difficult to see any stars at first. After a while, stars will become clearer, and after about half an hour in darkness you will be able to see stars to the limit of your vision. But be warned… any bright light – be it car headlights or a neighbour’s security light – will instantly ruin your dark adaptation.
In bright light, colour can be distinguished because the retina’s colour-sensitive cone cells are triggered. But in dim light, only the rod cells, concentrated around the outer edges of the retina, are activated.
The rods can’t distinguish colour, and they have less resolving power than the cones, so they deliver less detailed images. A dim object, such as a diffuse nebula, may be difficult or impossible to see when it is looked at directly. Through the eyepiece, a faint object appears brighter and shows more detail if you direct your view slightly to one side of the object so that light falls on the rods – known as ‘averted vision’.
Each eye has a ‘sweet spot’ of maximum rod sensitivity to dim light. If you’re looking through your right eye, look slightly to the right of the dim object, and if using your left eye look slightly to the object’s left. It’s a pretty effective technique.