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Energy from the Stars : The ElectroMagnetic Spectrum


This is second in a series of articles on the basics of astronomy and observational techniques. you can read the first article here. The articles will broadly cover the different kinds of telescopes used for the different energetic phenomenon associated with astronomy. In the previous article, the reader was hinted at the possibility of observations being carried out outside the visible spectrum using specific instruments. In this article, i will describe the different energy levels observed in astronomy and their associated phenomenon. I will mention the new telescopes which are observing emissions in these exotic bands and leave you to read further. 

ElectroMagnetic Spectrum - as we all know, the electromagnetic spectrum can be roughly divided up into - 
  • Gamma Rays 
  • X Rays 
  • UV rays 
  • Visible Light 
  • IR rays 
  • Microwave radiation 
  • Radio waves
this should help you better recollect the frequencies and wavelengths of the corresponding waves and also get an idea of the respected energies involved with these waves. 


a more concise look at the same.  


one more simple calculation which i want you to look at is the relation between energy of a system and the temperature of the system. 

as many of you might know, 
E = Kb * T
where E - energy of the system in eV
T - temperature of the system in Kelvin
Kb - the boltzmann constant
= 8.62 * 10^(-5) eVK^(-1)

Just crunch some numbers and see what the corresponding temperatures of systems should be for them to emit at frequencies mentioned above i.e gamma rays, UV, visible, IR and radio. 

You will see that any system whose temperature is ~300K i.e room temperature will be emitting MIR radiation at ~26meV. Bodies that are at ~3000K will be emitting at NIR. The sun, which we know has a temperature of ~6000K will be emitting at the visible range. 
~10^5 K - UV rays. 
~10^7 K - X rays and then at some unfathomable temperatures, Gamma rays are produced! 

This is to try and give you a hint of the kind of temperatures a system should have to be able to emit such waves. 

The other thing i haven't mentioned explicitly here is that the energy mentioned is the peak emission from the body at that particular temperature. It means that other rays with much less energies will also be produced in this process. For example, any object at ~300K will not only produce MIR waves but also micro and radio waves. 

To be more accurate, the energy distribution and the peak emission wavelength can be determined by looking at the planck spectrum of the body. 


refer to the planck's law and black body radiation for a better understanding of the emission continuum. 

So, as you can see, there is an energy continuum coming from any body i.e continuum radiation assuming the system to be a black body at a certain temperature. But when we look at any object in space, it's spectrum isn't exactly continuous. There are small dark lines in the middle of this spectrum which baffled people for quite a while. 

So, when people were looking at the spectrum of our sun, people saw such dark lines spread throughout the spectrum and nobody knew what half these lines corresponded to. By this time, people already knew Hydrogen gas produces a specific emission spectrum when heated i.e along with the continuum of the background, people could see spikes at certain frequencies. 

The wavelengths at which these spikes occurred could be accounted using this formula - 


the  Rydberg Formula

and using the same formula with corrections for atomic number, people could see that the new lines observed from the spectrum of the sun correspond to an atom with atomic number 2. hence the name Helium, derived from Helios - the sun! 

An interesting thing to do now would be to work out all of the spectral lines i.e the Lyman series, Balmer series and so on and so forth for H and check what the temperature of a H cloud should be to be able to emit such characteristic emissions! Believe me, you will be amazed at the kind of energies and temperatures which come out of these calculations. If you are still interested, you could continue to do the same for all of the elements with atomic number <10. Do it, it's awesome. 

And best of all, if you know any programming language, write a code to calculate all of these values at one go using a simple program. That isn't too much work, is it?! 

So, not only does every object in space have a continuum background emission, it also has a characteristic spectrum corresponding to the elements present on the object. 

So far, we've seen the different parts of the electromagnetic spectrum, the energies that correspond to these parts, the temperatures that correspond to these parts, continuum and characteristic emission from an object at some temperature. 

So, there's a butt load of energy being dumped from the cosmos onto earth every second. But as most of you already know, most of it gets reflected! You guys already know that the ozone layer is responsible for reflecting *harmful* UV rays. well, there's more that the earth's atmosphere reflects that just the UV rays. The water vapor and the CO2 in the atmosphere absorbs the heat from the surface of the earth being radiated and then reflects it back. You've probably heard of this if you know what global warming is. Yeah, it's not some crazy government conspiracy. It's a scientific fact! 

To get a complete picture, take a look at this picture 


So, as you can see, all kinds of emissions till the visible range are reflected by the earth's atmosphere back into space. most of the IR spectrum is reflected back by the atmosphere as well and then there's all of this internal heat from the surface of the earth! 

So, now you know why the pictures from the Hubble Telescope are wayyyy more awesome than any of the pictures ground based telescopes have been able to take though the hubble telescope's primary mirror is just 1m wide. in comparison, we have ground based telescopes whose primary mirrors are as large as 10m! There are a couple of other things involved here as well, but let's talk about them at a later point in time! 

This is the main reason why NASA and the ESA keep sending more and more awesome telescopes into space. A couple of interesting space telescopes would be the Kepler, COBE, WMAP...

There you go. For now, i will stop. You should read up more from the wiki pages i linked to, the satellites i mentioned, the basic physics i referred to and about astronomy in general. 

Until next time...

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