I understand that differentiating between celestial bodies and noise due to equipment is a fundamental problem in astronomy. I also understand that we solve this problem by looking at something called flux density and continuum and line spectra but how do I understand these terms ??

Please consider I'm well versed in university level physics so I understand how spectra will help but don't understand which quantity's flux density do we look for in astronomy. Please help me understand these terms and hence solve the noise vs star/planet problem.


1 Answer 1


You can differentiate between planet (Did you mean asteroid? Planets are quite bright.) and noise quite easily. Just take three pictures of the sky and they will be probably quite noisy. If some spot changes its position on all three pictures, then this is some near object (astronomically near). You can additionally verify it if its trajectory is line or not. If it is not, then it is probably just noise, but that isn't always the case.

Differentiating between a star and noise is harder. You have to take more pictures and hope that the noise will cancel out.

About flux density: this is the measure for light over some surface and it is proportional to the number of photons that hit the sensor.

  • $\begingroup$ Since, you mentioned number of photons does it mean the measure of energy of respective wavelengths from each object reaching the sensor?? And if yes what signatures are we looking for in particular ? high Gamma intensity or UV or some other part of the spectrum?? $\endgroup$ Jan 16, 2021 at 4:41
  • $\begingroup$ Also if you do find a source which explains this physics please share, that will be a lot of help! $\endgroup$ Jan 16, 2021 at 4:42
  • $\begingroup$ Electric density is given by electric flux over surface. Similarly, flux density is given by light flux over surface. Unfortunately, stars are very different between each other. Some can be red, some can be yellow, blue, etc. But this is true for noise, too. Some pixel can be red, some can be yellow, blue, etc. In most cases, we take two photos and the noise cancels out. And more photos -> less noise. By the way, such images for detecting stars are usually white and black to get most of the camera. Hubble has a camera with only black or white, for example. $\endgroup$
    – User123
    Jan 16, 2021 at 9:36

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