Solar Spectrum
Solar Spectrum
[A chart shows the visible colored spectrum of the sun from deep violet to deep red. Along the spectrum are shown 28 black spectral lines of different thickness. Above the chart is a caption:] The Sun's spectral lines
[Above the chart there are four and below the chart there are two labels, each label has one or more arrows pointing to different black lines. The two that has only one arrow points to two close lines marking them both. Only 22 lines are labeled like this, the other 6 are not labeled. The labels in reading order, with the number of arrows noted behind in square brackets:] Calcium [3] Iron [5] Sodium [1] Oxygen [2] Hydrogen [3] Magnesium [1] Those giant sunglasses [5]

This comic's release day was postponed from the scheduled Wednesday release to a Thursday release because Randall noticed the extreme popularity of the previous comic on Monday: 1732: Earth Temperature Timeline . Randall even explained this in the header text; see this trivia item from the previous comic.
This comic depicts the Fraunhofer lines , i.e. the spectral lines seen when sunlight is split in a spectrometer . These appear as black gaps in the rainbow of light, caused by light being absorbed by elements in the Sun . The frequencies of light that an atom absorbs depend on the exact arrangement of electron orbitals around it - because each element has a different pattern of orbitals, each one has a distinctive pattern in the absorption spectrum. The chart shows most of the main lines in the visible spectrum and identifies the elements linked to them.
The image of the Fraunhofer lines from Wikipedia is shown below in the section with a table of these lines. Here it can be seen that all the lines that are labeled with elements are correctly labeled. Also all lines shown in the part of the spectrum included in the comic are included. Ten of the lines included are not labeled in the picture on Wikipedia (at least not with an element; two of the three "h" labels are not in the table on Wikipedia). Six of these also have no label in the comic. The other four lines' label Those giant sunglasses constitutes the joke of the comic. There seems to be only one clear error in the comic and that is the fifth line labeled Sunglasses, the middle of the lines, which is actually a Hydrogen line (C in the picture below). But the line next to it to the right is one of those not labeled in either picture and it seems likely that it was this line Randall meant to be a Sunglass line...
All ten extra lines (including both the labeled and unlabeled ones) seem to correspond to the spectrum of silicon , and the joke then refers to the silicon dioxide (aka glass) used in the lenses of the Sun's sunglasses. Of course, this means that the glasses have been ionized and turned into plasma by the heat of the sun.
The idea of a sun with sunglasses is a reference to pictures/clipart of the sun wearing sunglasses, often used to denote good weather. Randall has specifically used this picture in at least two what if? posts:
In Into the Sun it is seen in the fourth image. The title text of that image even references the fact that those sunglasses will block the light to Earth: A partial solar eclipse is when the Earth moves across the part of the Sun blocked by its sunglasses.
So this comic is a direct callback to this what if? post.
In Black Hole Moon it is in the first image also including a banana as the mouth. Both the image and the title text of that image references the fact that those sunglasses will block (eclipse) some the light to Earth: Doctors warn that even sunglasses that block UVB will only protect you from the part of the Sun covered by them.
There is another joke in drawing a sun with sunglasses because sunglasses are meant to protect your eyes from the sun, so what should they protect the Sun's eye from, Star light...? Also, any glasses worn by the sun, would they not become sun glasses?
Transitions ® is a brand of photochromic lenses ; however, photochromic lenses are often referred to as "transition lenses", so the title text does not necessarily refer to the brand. Photochromic lenses are a type of plastic lens used in prescription spectacles that allow the lens to turn dark when exposed to UV light such as that found in sunlight. The sun choosing to get transition lens would prove a waste of money as the lenses would be permanently transitioned to be dark, so a pair of ordinary sunglasses would likely have proved more cost effective. (Always assuming they do not turn into plasma when getting close to the sun...)
This is the official image for Fraunhofer lines (solar spectrum) on Wikipedia: The graph is a typical spectral lines chart, with a long rainbow band (from ultraviolet to the left to infrared on the right both colors appearing black as they are not visible.) The black lines in it, indicating the traces of different elements. Noe that the comic only covers the visible part of this spectrum.
[A chart shows the visible colored spectrum of the sun from deep violet to deep red. Along the spectrum are shown 28 black spectral lines of different thickness. Above the chart is a caption:] The Sun's spectral lines
[Above the chart there are four and below the chart there are two labels, each label has one or more arrows pointing to different black lines. The two that has only one arrow points to two close lines marking them both. Only 22 lines are labeled like this, the other 6 are not labeled. The labels in reading order, with the number of arrows noted behind in square brackets:] Calcium [3] Iron [5] Sodium [1] Oxygen [2] Hydrogen [3] Magnesium [1] Those giant sunglasses [5]
