Electromagnetic Radiation in the Universe
One important point to remember in discussing electromagnetic radiation is that the spread in its frequency is characteristic of the temperature of the emitting source. This was in particular, it was noted that for a body at absolute temperature T there is a spread in the frequency of the emitted radiation which peaks at a frequency around 10^llT oscillations per second. The visible optical radiation lies in the frequency range 4 × 10¹⁴ (red) to 7 × 10¹⁴ (blue) implying that the surface temperature of a star emitting such radiation must be in a range somewhat greater than 4000K to 7000K: roughly 3000 K-10,000 K. Because of the spread in frequencies emitted, the colour of a star is not precisely that given by the peak frequency above. For example, a star at 3000K will indeed be reddish but one, like the sun, at around 6000K will emit all the colours of the rainbow to some degree, which then combine together to give white light as observed. In discussing the formation and nature we shall come to understand how these large surface temperatures arise. Turning now to lower frequencies we come to radiation in the infrared and microwave region. The former arises largely from heated dust lying within galaxies, the heating being due to hot radiation from stars in the process of forming or dying. These processes. As far as microwave radiation is concerned, a most important observation was made in 1965 when Penzias and Wilson (Nobel Laureates, 1978) using a very sensitive detector discovered such radiation clearly arriving at the earth from outside our galaxy. They knew the radiation was extra-galactic since its intensity remained the same day and night throughout the year; if it had been coming from the sun or our galaxy the intensity would have changed as the direction of the detector changed with the earth’s motion. Subsequent investigation culminating in a detailed study in 1989 using the Cosmic Background Explorer (COBE) satellite established that the whole of space is pervaded with microwave radiation (cosmic microwave background radiation) having exactly a distribution of frequencies corresponding to a temperature of 2.736K. The presence of this background microwave radiation has profound implications for understanding the way in which the universe came into being which will be discussed in the next section. At even lower frequencies we come to the part of the electromagnetic spectrum in the radio wave region. From the 1940s onwards many thousands of discrete radio sources have been identified. Many have been found to coincide in position with known galaxies, and radio waves from within our own galaxy have also been detected. All such radiation can be understood as due to the acceleration of charged particles, particularly electrons, in the very large magnetic fields which exist within galaxies. Finally, turning to frequencies higher than those for optical radiation, we come to extreme ultraviolet and x-ray astronomy. This has developed rapidly with the use of satellites carrying appropriate detectors and, for example, many x-ray sources have been pin-pointed, mostly outside our own galaxy. Given that x-ray frequencies are around 10¹⁷ oscillations per second upwards  it follows from the above relation between peak frequency and temperature that the objects emitting x-rays must be at extremely high temperatures-around 10⁶K-10⁷ K. They are obviously not ordinary stars, which have much lower temperatures, the possible origins of this radiation will be discussed. Even higher frequency radiation has also been detected by satellite observations emanating from the central disc of our galaxy, implying temperatures around 10⁹K. This is attributed simply to collisions between very high energy cosmic ray particles and the dust mentioned earlier which lies between stars. From this brief survey of the electromagnetic radiation present throughout the universe it is clear that many questions about its origins have to be answered. Much depends on the way in which stars are formed and develop. But before considering this it is important to discuss how the universe as a whole has developed into its present state.