Article by Michael Burton, Director of the Armagh Observatory and Planetarium
Our view of the cosmos is biased by the vista that is apparent to our eyes. This is what the view in what we call the optically visible portion of the spectrum. To the unaided eye it is a view of a universe full of stars, together with five planets, one Moon and of course the Sun. When augmented with a telescope, our eyes can then see a universe full of galaxies – giant cities of stars.
Yet this is not a representative view of the universe. It misses many types of astronomical objects. The electromagnetic spectrum stretches from low energy radio waves to extremely high energy gamma rays. Optical light takes up just a tiny portion of this spectrum, from blue light with a wavelength of 0.4 microns, to red light with a wavelength of 0.7 microns. A micron is a millionth of a metre, and to give that some perspective, the typical human hair is about 50 microns thick. So very small!
Evolution has given us eyes responsive to optical light as this is the dominant portion of the electromagnetic spectrum emitted by our local star, the Sun. Our atmosphere, by good fortune, also happens to be transparent to optical radiation. So not only do the Sun’s rays reach us direct, so too does the light from the stars in the sky.
If our eyes had evolved to be sensitive to infrared light, for instance, radiation of slightly longer wavelength to optical light, they would have been sensitive to the heat emitted by objects. We would have been able to see in the “dark”. However, we would also have been largely unaware of the spectacle of the starry sky. For the Earth’s atmosphere also emits strongly in the infrared, so drowning out the weaker infrared light that comes from the stars. It is interesting to speculate how civilisation might have evolved in such conditions, without the vista of the night sky that ultimately stirred the development of the scientific methodology that underpins our modern, technologically-based society.
There is another region of the spectrum where radiation can reach us directly from distant objects in the cosmos. That is in the radio wavebands. Radiation with wavelengths from about 1cm to 10m can pass largely unobstructed through the atmosphere and so be detected by telescopes on the ground. Radio astronomy is concerned with the measurement of such radiation and then using it to better understand the nature of celestial objects.
We are used to receiving radio signals broadcast by TV and radio stations. However it was a great surprise when, in 1933, Karl Jansky detected radio emission from space. Using a radio antenna that he built that is not unlike, in design, that now used for TV aerials, he was investigating the static interference in radio transmissions. In doing so he unexpectedly discovered a radio source of cosmic origin coming from the direction of the centre of our Galaxy.
While some radio telescopes of today still do look a bit like Jansky’s original antenna – arrays of dipoles sensitive to the longest wavelength radiation – most radio telescopes now look much closer to optical telescopes in form. Except that they are (generally) far, far bigger! Size is essential for a radio telescope if image clarity is desired. For the image resolution that any telescope can achieve is directly proportional to the wavelength of the radiation being measured, divided by the diameter of the telescope. Since radio waves are over a million times longer than optical waves, this means a radio telescope would have to be a million times larger to achieve the same image quality!
Actually it is more complicated than this because the atmosphere blurs the quality of optical images. Radio telescopes can also be combined together to achieve the resolution of a single telescope whose diameter is the size of their distance apart, a technique known as interferometry. Though the sensitivity is only the equivalent of the collecting area of the individual dishes, not the area they are spread over. Nevertheless, radio astronomers have been able to achieve remarkable fidelity in their best images, far better than that of the best optical images obtained of astronomical sources.
While optical astronomy is largely concerned with the study of stars, which emit much of their radiation in these bands, radio astronomy is mostly concerned with studying the gas of interstellar and intergalactic space. Very few stars emit significant amounts of radio emission. However, clouds of gas in space are prolific emitters of radio waves.
Clouds of atomic gas – largely hydrogen atoms in space – emit radiation with a wavelength of 21 cm. Molecules emit radiation of higher frequency (and shorter wavelength). For instance, the carbon monoxide molecule emits at a wavelength of 3 mm. Its measurement allows us to study the regions where stars form in our Galaxy, the cores of giant molecular clouds found mostly in the central plane of our Galaxy. [Note: carbon monoxide is the second most common molecule in space. However the vastly more abundant hydrogen molecule does not, in general, emit radiation, so it cannot be studied directly in space except in special circumstances]. Finally the hot, ionised gas around luminous stars emits radiation from isolated electrons in the gas, as they swing by the protons. This allows astronomers to study the intense activity and mass loss from these stars, a central part of the process that is recycling material from the stars into the gas that occurs as part of the galactic ecosystem.