Jansky measurements

I last blogged about thermal and non thermal radio sources. As we saw it is a complicated topic so I am trying to break it down into easily understood parts. We found that thermal sources depended on the heat emitted from our astral body where non thermal sources covered all other types of signal, including magnetic field strength and relative proportions of excited atom states. Our equation F=L / 4πR2 was examined and the relative parts explained.

Radio astronomy uses many terms, with flux being commonly quoted in Jansky’s. Jansky was covered in an earlier blog being the engineer who identified radio signals from space. The Jansky equation is

1 Jy = 10−26 W m−2 Hz−1

• W is the total amount of watts which is the number of Joules per second collected.           • m−2 is per square metre.
• Hz−1 is the number of Hertz over which the measurement is made.


In this equation the Hz is actually the detector bandwidth of the receiver.

Continuum sources are radiation emissions heated solid materials. They emit a wide range of wavelengths and are extremely broadband It is difficult to exactly quantify their bandwidth, but we consider that they are broader than the detector bandwidth of the receiver being used to detect them, and assume that the received energy is more or less uniformly distributed across the receiver bandwidth. The detector will thus pick up more total power the wider its bandwidth is.

The Third Cambridge Catalogue of Radio Sources lists up to 400 radio sources in the northern hemisphere brighter than 9Jy at 159 MHz with the brightest astronomical radio sources having flux densities between 1-100 Janskys.

Gravitational waves also carry energy, so their flux density can also be expressed in terms of janskys. Typical signals on Earth are expected to be 1020 Jy or more, though  such signals are difficult to detect due to the poor coupling of gravitational waves to matter.

The following orders of magnitude are observed from the earths surface.

Screenshot 2018-07-11 14.32.24
Image from Wikipedia


There is much more than can be said about units of measurement but this blog is not about physics, its about radio astronomy. Now we have our basis to work from we can move onto the sources themselves and what they tell us about the universe around us.


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