by S. Prasad Ganti

In the beginning, there was only visible light astronomy which meant viewing distant objects in space using our naked eye or with the help of an optical telescope. Gradually things changed and we started doing astronomy by capturing radiation from other parts of the electromagnetic spectrum, i.e. radio waves, infra red waves, x-rays. ultra violet waves and gamma rays. Astronomy has become richer as a result. We are able to map our universe in much greater detail since. 

Understandably earlier observations were done using naked eyes only. Our ancestors were able to view the dark skies as there was no light pollution. The only instruments they had were their eyes. With the invention of the telescope about four hundred years ago, our vision got extended. Telescopes became powerful. But still the observations were restricted to using our eyes. With the invention of photography, plates started replacing the eyes. Plates could be analyzed much after the images were captured. And by many more people than just the observer who captured the images. This was the state of art until about a hundred years ago.    

With that state of art, Edwin Hubble found out about other galaxies like Andromeda, as distinct from our Milky way and came up with an approximate distance. He analyzed the photographic plates captured using the 100 inch telescope at Mount Wilson. We have come a long way since then, by branching out to other parts of the electromagnetic spectrum. Other than visible light, radio waves from distant objects can penetrate the Earth’s atmosphere and be received on the ground. This was discovered accidentally by a radio engineer Karl Jansky at AT&T. Radio communication was in vogue at that time and Jansky’s job was to trace other sources of radio “noise” coming in the way of intended communication. Although initially ignored, decades later, Jansky’s discovery led to Radio astronomy. With radio antennas, instead of optical lenses, forming the basis of radio telescopes.    

A combination of radio and  optical telescopes provided us with a more vivid  picture of our universe. Still there were patches of sky which looked totally dark. As if the voids meant absence of any significant objects. Sounding rockets started launches into space beginning  in the 1950s. One such rocket contained sensors to detect X-rays. It found the first X-ray source titled CYG X-1 coming from the Cygnus constellation.   

In physics, any body which is hotter than absolute zero, emits radiation. Including our Earth and we ourselves. It is called black body radiation. Hotter bodies emit higher energy waves like Gamma rays, X-rays, visible light etc. Being much cooler, we emit infrared rays. Night vision glasses are built on capturing these infrared rays to see in the dark. Since we have all kinds of objects with different temperatures in our universe, we have different types of radiation to “see”.  Our sun emits most of its  radiation in the form of visible light. And our eyes evolved to be sensitive to visible light. Hence our bias towards visible light. 

In addition to different frequencies (and types) of radiation emitted by different objects in space, the frequency of the radiation can change enroute. As the source objects are moving away from us, the radiation gets red shifted (something like the siren of a moving ambulance changing its pitch as it approaches us or moves away from us), which means it gets stretched and longer wavelengths result. A moving object emitting  X-rays can get red shifted to visible light as it is perceived by us.     

There is a lot of interstellar dust between us and some of the source objects. The dust is composed of small grains made of carbon (soot) or silicon (sand), about the same size as visible light. It can absorb, reflect or scatter the visible light. It either blocks us from seeing the object totally or we can see it in a different frequency. 

All these phenomena result in us getting different wavelengths of radiation. Our atmosphere blocks most of those frequencies except for visible light and radio waves. The only way to observe those blocked wavelengths is by placing those telescopes in space. Chandra X-ray telescope and James Webb infrared telescopes are two such instruments which cannot be replaced by any earth based instrument.       

There are different kinds of messengers coming towards us in various forms. By combining the images from all these different types of telescopes, we get a complete mosaic of our universe. No more dark patches.