Measurement of the radiation from thermal and nonthermal radio sources

Wow that’s a heading and a half. If only I knew what it meant. As I stated when I first started this blog, many moons ago, I am an amateur astronomer. My first set covered the basics of what radio astronomy actually is. I then moved on to a series about the different types of waves found in the electromagnetic spectrum. All of these blogs can be found on my website

One of the problems I have found as an amateur is knowing what is actually out there to blog about, I mean if I don’t know it exists, how can I know to include it? So it was with this in mind that I asked some astronomer friends for idea’s. Hence the subject matter referred to above, Measurement of the radiation from thermal and non thermal radio sources.

My first port of call was to google thermal and non thermal radio sources. I would like to tell you that it told me everything I needed to know about the subject matter. What it actually did was it bought up a long list of university papers from around the world. So rather than diving straight in and writing a thesis of my own I decided to break it down into small easily digested portions. So here is my idiots guide to thermal and non thermal radio sources.

As previously covered, radio is radiation. We can measure this radiation in its various forms.

“While thermal emission depends on the temperature of the emitting source, non-thermal emission depends on other things, such as the relative proportions of excited states of atoms and magnetic field strength

Examples of non-thermal radiation include synchrotron radiation, maser emission, and Compton scattering. Both synchrotron and maser emission are important in radio astronomy. ” –  Swinburne University.

Thermal radiation therefore is dependent solely on the temperature of the emitter source. I would like to jump straight in with some examples at this point, but in order to do so I need to go back a few steps and explain the processes by which we show our measurements.

Imagine we are looking at a star and want to measure it. There are many ways in which we can measure it, from size to density to age to… well lots and lots of ways. So lets start with its luminosity.

Our star is emitting energy in all directions, this energy per second is called its luminosity L. We measure its luminosity in Watts W. Our measuring tool is our radio telescope which has a collecting surface area measured in Metres squared, M₂. Our telescope can detect photons in a finite range in units of Hertz, Hz. Our position from our star will determine how much luminosity we can actually measure.

Flux F is defined as the total radiation energy crossing a unit area per unit time within a particular narrow frequency range. We can think of the energy from the source as spreading out through space in series of concentric spheres.

A sphere of radius R from the source with luminosity L, the radiant flux, F, passing through this sphere is:

F=L / 4πR2

The units of flux would be W/m2, which is an example of the inverse square law.

I would love to go on, but the idea of this blog is for beginners like myself to understand what we can do with a radio telescope. If I just start throwing lots of number and symbols at you then I defeat the purpose my own object.

We have a basic understanding of thermal and non thermal radio sources and know that we can take measurements with our radio telescope. We also know our equation for measuring flux. Hopefully my next blog will uncover a few more gems to help our understanding.




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