Windows on the Universe. Relative Transparency of the Earth’s Atmosphere to Radiation of Different Wavelengths

As the previous section has shown, different types of emission mechanism are produced in response to different physical conditions. Within the boundaries of each emission mechanism, a wide range of radiation wavelengths can also be produced.

Observing the Universe at different wavelengths and scrutinising spectra can therefore give us an insight into the different phenomena and physical environments present throughout the cosmos. In some ways, we have become so used to seeing the night sky at optical wavelengths, that spacecraft images of it in anything other than visible light often take us by surprise. It is important to remember at all times, however, that visible light is such a tiny part of the electromagnetic spectrum that to ignore the rest would be folly.

If we start at the lowest energies of electromagnetic radiation, the Universe becomes the realm of the radio galaxies. The entire sky is dotted with their tremendously powerful radio emitting lobes. Also visible at radio wavelengths are objects from our own Galaxy known as supernova remnants. These are the glowing remains of exploded high mass stars. In the direction of Sagittarius will be the centre of the Milky Way. At certain radio wavelengths, vast clouds of molecular gas can be seen. These also exist within our own Galaxy and are the clouds out of which stars eventually form.

The different wavelengths of radio trace out different molecular species. By mapping the strength of each molecule’s radio emission, contour maps of the clouds can be obtained, which show the number of each type of molecule at each point throughout the cloud. The nearest molecular clouds would appear to be enormous to us. For instance, the molecular cloud with which the Orion nebula is associated encompasses the whole of the constellation at radio wavelengths.

At the next man-made division - microwave wavelengths - the view is totally different, since the cosmic microwave background dominates. Instead of a dark sky punctuated with both point and extended sources, the entire sky is bright and glowing with energy. Superficially, the brightness will look the same in all directions.

There are variations in the radiation however, and the first, most obvious is known as the dipolar anisotropy. It is produced by the motion of the Earth relative to the cosmic microwave background radiation. When all the components of the Earth’s motions are summed, such as the component due to the Solar System’s motion around the centre of our Galaxy and the Galaxy’s motion within the Local Group, the resultant velocity increases the temperature of the CMBR in the direction of the Earth’s motion. It also decreases its temperature by a corresponding amount in the antithetical direction.

Underlying this is the microwave contribution from the material and objects in the Milky Way. If all of these are removed and the remaining microwaves subjected to very careful analysis, fluctuations, which coincide with the emergence of Galactic structure, would become visible.

Changing our observations to the next, more energetic bands of radiation, we arrive at the infrared. The sky is once again dark and stars have reappeared on the scene. In general these are stars cooler than can be easily seen at optical wavelengths. They are the red dwarfs and red giant stars. Also present are the nascent stars in stellar nurseries such as the Orion nebula.

These protostars and other young stellar objects are obscured from view at optical wavelengths by their dusty birth clouds but, at infrared wavelengths, shine through. The Milky Way continues to stretch across the sky again, illuminated not so much by stars but by the infrared glow of warm dust clouds. A second band also stretches across the sky. This one intersects the Milky Way at an angle of just under 80° and marks the plane of our Solar System. It is glowing because there is warm dust in interplanetary space.

Beyond infrared, the familiar optical spectrum occurs. Our view of the Universe at these wavelengths was described in chapter 1. At higher energies, the ultraviolet, the very hottest stars in the Galaxy dominate the view. They are the О and В-type supergiant stars which have surface temperatures of 20. 000 K or more. Using Wien’s law λ_max can be calculated to be 70 nm; a wavelength in the extreme ultraviolet region of the spectrum. Thus, whereas most stars would fade to obscurity, the brightest stars in the Galaxy would become even brighter if we could see the ultraviolet region of the spectrum.

As well as the individual stars, there is also a glow from all around. This is known as the local ‘bubble’ and is a roughly spherical ‘cavity’ within which the Solar System and surrounding stars sit. It is thought to have been produced by a supernova explosion long ago. The shock wave from that explosion has swept interstellar atoms into a bubble shape which now glows at ultraviolet wavelengths.

Passing on to even more energetic wavelengths, the sky continues to glow faintly at X-ray energies. Unlike the locally produced ultraviolet, the X-ray background comes from vast distances and is thought to be the combined emission from distant galaxies. Clusters of galaxies glow faintly in X-rays thanks to thermal bremsstrahlung, as do other regions of hot, tenuous plasmas present in our own Galaxy.

The general background glow is also punctuated by point sources of intense X-rays. Some of these are the nuclei of powerful active galaxies. Others mark the positions in our Galaxy where stars are being ripped to pieces by incredibly strong gravitational fields. Both phenomena are explained theoretically as being due to the action of black holes.

If we were to continue to the highest energy range of the electromagnetic spectrum, the night sky would look perfectly black again except for the occasional flash of an errant gamma ray. As yet, these bursts, which occur once every few days from totally random directions, are unexplained and make up one of the most fascinating aspects of modem high energy astronomy.

The preceding paragraphs were designed to illustrate just how biased our view of the cosmos would be if we were to persist in myopically peering at it with optical wavelengths alone. In a perfect world, an astronomer would be able to tune his telescope to pick up whatever wavelength of radiation he wanted to detect. Unfortunately, certain factors mitigate against this.

One is that different styles of focusing devices and detectors are needed in order to receive different types of radiation. Another far more serious problem is that, even if you were to build a detector for each kind of radiation and mount them in a field, only some of them would receive signals. This is because the Earth’s atmosphere stops certain types of radiation from reaching the surface of our world (see Fig. 2. 7).

Fig. 2. 7. Relative transparency of the Earth’s atmosphere to radiation of different wavelengths. Not all electromagnetic radiation is capable of penetrating the atmosphere. The famous ozone layer absorbs much of the high energy radiation

Above wavelengths of 20 metres, radio waves are reflected by the Earth’s ionosphere. Most, but not all of the infrared radiation is absorbed by molecules in the atmosphere. Optical wavelengths, obviously, pass through without hindrance.

Apart from the ultraviolet radiation which causes suntans, all the high energy photons are blocked by various interactions with atoms in the atmosphere. These take place between altitudes of a few tens to a few hundreds of kilometres. The type of radiation we wish to observe determines the style of detector which has to be built and whether or not it has to be made into a satellite and sent into space.