With a subject as old as astronomy, you might expect a very strong structure to be underlying the field. And in a certain way, there is – the heavens have been observed for millennia, providing a bedrock upon which astronomy continues to build. Yet it is exactly due to this age and history that things are not necessarily well organized.

Every respectable astronomer from days past would compile their own, new, catalogue of astronomical objects, filled with various conventions, terminology and classification schemes. Perhaps it is best compared with an old castle: the foundations run deep and are strong, but the corridors lie higgledy-piggledy in the most confusing fashion. Take almost any visible object on the night sky, and you will find it has over twelve alternative names, all originating from different catalogues – and those are just the main names. So which name do you use as an astronomer?

Simple: you find the name most commonly used in your subfield and stick with that. Unfortunately, unless you are extremely good in memorizing the many names of objects, you will then still find yourself sitting through a riveting lecture, without realizing that they are actually talking about the object you are working on. And not to mention some of the strange conventions still used in astronomy! Despite working with scales of parsecs, objects with masses millions of times that of the sun, and time scales reaching all the way back to the Big Bang, there is still is a convention in astronomy to express everything in units of centimetres, grams, and seconds. Thankfully this convention is ever so slowly on its way out, being replaced by more conventional units. But the catalogue problem persists.

How come no definitive catalogue has been yet established, allowing one to refer to an object by just a single name? The history of this problem lies in the way in which astronomy has evolved. The earliest astronomy was conducted with the naked eye. To help guide the eye in finding the right stars in the night sky, the stars were linked together into constellations. Using the brightness of stars as a second measure, a system was built to help define not just the rough position of an astronomical object, but also a way of classifying them.

Empires rose and fell, but astronomy continued to track the heavens, discovering ever more objects. It could be argued that with the exception of the telescope – which extended the distance to which the human eye could see – nothing truly revolutionary happened all the way up to the start of the 19th century. Then suddenly new parts of the electromagnetic spectrum were discovered, from infrared to ultraviolet. Each time a new part of the spectrum was discovered, scientists would turn their instruments to the skies, each time revealing the night skies to be blazing in a completely different fashion. Take for instance radio waves: these allowed exotic ‘lighthouse’-like objects called pulsars to be found, showing regular pulses of radio emission. These had never been seen at other wavelengths, but lit up the sky in radio. Pulsars result from some the most extreme physics known to mankind, and their discovery allowed theories such as general relativity to be tested, rightly resulting in a Nobel Prize.

The first instruments capable of detecting new classes of electromagnetic radiation could not localize the astrophysical sources very well. As a consequence, new catalogues were started, using the brightness of an object to classify them. But the brightest object in the optical part of the spectrum need not be the strongest radio, or indeed X-ray emitter, as the emission can result from different processes. It was only after instrumentation continued to improve and was able to provide ever-tighter boundaries on the location of emission sources that the position of objects could be pinpointed, allowing objects from various wavelengths to be linked together.

Slowly, astronomers began to recognize that using classification systems based on brightness would result in an ever-growing list of names for each object. Therefore, while old satellite missions would classify objects based on their brightness, new missions nowadays use coordinates in the names of new sources. This method helps to quickly link together names from different missions, and to realize whether they are referring to the same object. However, given that ‘Aquila X-1’ sounds far nicer than ‘H1908+005’, it should not be expected that the coordinate classification scheme will truly replace the old system, and they will thus continue to live alongside each other.

This is not necessarily a bad thing, as the older names reveal the whole history of an object, showing how each time a new part of the spectrum was discovered, a new aspect of astronomical sources was revealed. But even with the whole electromagnetic spectrum we continually find that we are missing parts of the puzzle. We have for instance been missing whole populations of black holes, expected to be out there, but never seen. It is due to perhaps the most groundbreaking discovery in astronomy, since branching out across the electromagnetic spectrum, that the signature of these objects was finally discovered without using the electromagnetic spectrum.

It was only last year that gravitational waves were first detected, but their discovery caused a flurry of excitement amongst astronomers. Originating from the merger of very massive objects such as black holes, gravitational waves – ripples in space time – manifest themselves in distances changing less than an attometer (10^-18). Aside from the mind bogglingly engineering challenges going into detecting changes in such phenomenological small distances, gravitational waves reveal a completely new way of viewing the universe. With gravitational waves, we can follow the final moments of massive binary systems and probe the extreme physics behind merging black holes and other compact objects.

For centuries we have observed the universe across the electromagnetic spectrum, but now, for the first time we can look to the heavens in a new manner. Yes, it adds yet a new catalogue of names to the alarmingly vast number used in astronomy. But is it a fair trade for getting a fresh look at the universe? Without a doubt.