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Galactic Hydrogen

6.1 Emissions outside the Solar System

Having examined the emissions from bodies within the solar system, we now look much further out into the Milky Way. The most abundant element in the Universe is Hydrogen and it exists in atomic and ionised forms in large amounts within our galaxy. As discussed in section 2.2.3, atomic hydrogen can emit a specific spectral line at 1420MHz. By measuring the strength of this 'line emission' it is quite possible for the amateur radio astronomer to map the hydrogen distribution in part of our galaxy. In order to do this, the observer needs good antenna and receiver equipment that is stable over hours and days of observing time. This usually requires the sensitive receiver elements to be temperature controlled to avoid drifts in noise level and receiver gain. Assuming this can be done, an amateur observer can produce a map such as that shown in Figure 6.1. by measuring the signal strength as a function of transit time at a number of declinations on different days and combining the data with computer software tools.

Figure 6.1 Map of atomic Hydrogen in the galactic plane

6.2 Galactic distribution of Hydrogen

The narrow spectral line emission from Hydrogen is Doppler shifted by the relative line-of-sight velocity of the source region to the observer. This provides a way for professional radio astronomers to separate out the signals from different spiral arms of the galaxy, that each have different relative velocities. By this means it is possible to plot the density of Hydrogen (related to signal strength) to the location of the source position (related to the Doppler shift) and generate a 'top down' view of the spiral nature of the galaxy from measurements made from the Earth, which of course lies within the galactic plane. This cannot be done optically as dust in the galactic plane largely obscures the spiral arms. In Figure 6.2 we see a 'top down' map of the galaxy produced in 1964. The sector marked in blue is that related to the amateur map in Figure 6.1 The dark sectors along the 0 ° to 180° line of galactic longitude is blank because there is insufficient Doppler shift for the calculations to be made.

Figure 6.2 Hydrogen distribution in the Milky Way

© Dr David Morgan 2011