Multi-wavelength observational astronomy
Gaseous halos of galaxies, as presented on this site, are only one example of how powerful the combination of data obtained in different wavebands can be. The reason for this fact is that one sees in different wavebands radiation emitted by various components of the objects under study. Below, I will try to present a basic overview of which type of emission one can see in the various wavebands. The section on astronomical observatories also explains to some extent how telescopes and instruments are optimised to best observe radiation with different wavelengths.
Click here to skip past the historical background and go directly to the information about radiation processes....
The electromagnetic spectrum - or: Some historical background
Mankind has been fascinated by the sky for as long as memory goes back. Even ruins from the most ancient civilizations, thousands of years ago, show signs of astronomical activities.
Without current-day technology, the only window to the Universe to ancient civilizations was of course the optical. Until 1609, when Johannes Kepler built the first telescope, all observations were carried out with the unaided eye. The invention of the telescope truly revolutionised astronomy. It made much fainter objects visible than can ever be seen with the eye.
Yet, for centuries to come, the optical window remained to be the only one. No other observations could be conducted. In the beginning because there was not even an understanding of electromagnetic processes (i.e. the processes leading to the emission of light, where light is to be understood in a more generic sense than just photons that our eyes can "see"). Later, despite the development of a more advanced knowledge about electromagnetism, no technology existed to register photons (i.e. the particles that we can consider light to be made of) other than optical. This changed only in 1931(!), when Karl Jansky, a radio engineer working for Bell Laboratories at the time, noticed an unknown source of interference in radio transmissions that he was conducting as part of his work in the field of telecommunications.
Karl Jansky's big achievement was to realise, although not being an astronomer, that the interference occurred about 4 minutes earlier every day - and to understand that this is the typical behaviour of a celestial object located outside our Solar system! Based on this realisation he figured out, conducting more, dedicated measurements to establish the nature of the disturbance, that the interfering radio source had to be located in the area of the centre of our Galaxy. He published this in an article in 1933 - and of course he was right: The first celestial radio source had been detected. And thus the first ever extraterrestrial emitter of "light" (or better, more generally, "radiation") other than optical was finally found and radio astronomy born.
Now we know why the radio regime - and no other - had to be the second window opened to the Universe (after the optical), for a very simple reason: These are the only two regimes in which our atmosphere is transparent, as shown in the figure above!
Sketch of the electromagnetic spectrum and the transparency of the Earth's atmosphere, from the highest energies/frequencies on the left (Γ-rays) to the lowest energies/frequencies (radio emission) on the right.
And since mother nature had not equipped us with radio ears, it tooks us quite a while to figure out how to detect this type of radiation. There is, by the way, only one possibility how we could have opened a different window first: We'd have had to figure out how to observe astronomical objects from outside the Earth's atmosphere before developing radio communications technology (which is a pretty unlikely scenario...).
Yet later, first on balloons, then on rockets, we learned how to transport detectors out of the densest part of the atmosphere and out there to detect other types of electromagnetic radiation from space: in the infrared, ultraviolet, X-ray and Gamma-ray part of the electromagnetic spectrum - all of which had been inaccessible to us from the ground. More about this topic can be found in the section on astronomical observatories.
Radiation processes in different parts of the electromagnetic spectrum
In the following text a selection of diagnostically important radiation processes are listed for each waveband. As in other sections of this server, no claim is made for completeness of the information given. Which radiation processes are particularly "important" will always depend on the viewpoint of the observer and his/her scientific objectives. But BEFORE going into the different wavebands, I will briefly explain four general types of emission processes.
Varieties of these general emission mechanisms are observed in different wavebands. Therefore, listing them in an introduction of any single waveband would not make sense. Now a look at some of the most important diagnostic emission processes in various wavebands.
- Radio astronomy
- Infrared astronomy
- Optical astronomy
- Ultraviolet astronomy
- X-ray astronomy
- Gamma-ray astronomy
Radio astronomy is not by chance listed first here. In the radio regime one observes the photons with the lowest energies (a few millionths of an electron Volt [eV], compared to about 1 eV of an optical photon). The table lists the wavebands in a sequence of increasing photon energy.
Piecing together the puzzle: Multi-wavelength observations
As an example for the use of multi-wavelength observations in determining a whole range of important properties of a particular objects I present data of the starburst galaxy NGC 4666.