Would someone please explain this graph. I am not an expert and with my limited understanding of physics, I am unable to correlate this graph between the light received with its wavelength.

I don't remember where exactly I found this; I was surfing online and came across it, and found it puzzling. Now I'm at a loss to find the original source. Can someone else recognize what's being illustrated here?

enter image description here

  • $\begingroup$ Where is this from? Can you credit the source and provide a link? What is the context? Possibly using an exoplanet transit to look for sunspots on other stars? (1, 2, 3, 4) and youtu.be/hBU1yFbGHB8 $\endgroup$
    – uhoh
    Dec 6, 2019 at 10:39
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    $\begingroup$ In that case the word "Wavelength" might be an error, but it's hard to tell without the context. However, if it's from a crackpot site, then the question won't have a good answer. So please link to the original source and describe the context. Thanks! $\endgroup$
    – uhoh
    Dec 6, 2019 at 10:41
  • $\begingroup$ @uhoh I don't remember from where I get this. I was surfing online and come across. $\endgroup$ Dec 6, 2019 at 10:45
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    $\begingroup$ It might relate to using the fine detail of the reduction in light from a transiting exoplanet in front of a spinning star to image starspots and similar. The general idea is that light from the side of the star spinning towards us is blue-shifted, and the other side red-shifted, so the effect of the planet on the exact shape of a spectral line from the star depends on which side of the star it's blocking and whether that bit of the star is "full brightness" or a starspot $\endgroup$ Dec 6, 2019 at 12:27
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    $\begingroup$ @SteveLinton I think if this were the case the graph would be comparing exoplanet's relative position in front of the star (x distance or time) to wavelength rather than "light" vs "wavelength". If the position of the planet plays a role in this animation, then there are too many data points for a line graph visualization (time/position, "light", wavelength) $\endgroup$
    – Dragongeek
    Dec 6, 2019 at 15:01

1 Answer 1


While this animation is somewhat misleading as the wavelength line seems to be following the position of the exoplanet as it passes the star, I think this is an example of Exoplanet Spectroscopy.

Simply put, scientists measure the light output from a star. When an exoplanet passes between the star and our observation point, the total light level drops. This dropping of the light level is the primary way that we detect exoplanets currently. This method is called "Transit photometry".

However, if the planet blocking the star has an atmosphere, some of the light that we observe can pass through the atmosphere. Depending on what wavelengths of light are absorbed, we can compare the star's spectral output without and with a planet occluding it to preform spectral analysis of the planet's atmosphere. This type of process generates Light(Flux)/Wavelength graphs that look similar to what is shown in the GIF.

Graph demo

  • $\begingroup$ This really sounds like the right answer. So the choreography of the line in the spectrum moving back and forth in unison with the planet's motion across the disk is a red herring and a distraction, as are the funny dots that are dancing in the center of the star's disk? $\endgroup$
    – uhoh
    Dec 6, 2019 at 14:38
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    $\begingroup$ @uhoh I'm pretty sure those are supposed to be sunspots but ¯\_(ツ)_/¯ $\endgroup$
    – Dragongeek
    Dec 6, 2019 at 14:57

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