July 19th 2026

Things from Today:

• The Coronal Heating Problem - The Solar Corona (the outermost layer of the suns atmosphere) has puzzled astronomers since as far back as the 19th century. In 1869, astronomers were able to isolate light from the Corona during a total solar eclipse; and discovered a shade of green light that did not correspond to emissions from any known element. This led astronomers of the time to hypothesise the existence of a new element, 'Coronium', present in the suns Corona.
  In the early 1900's scientists started to learn more about the structure of atoms, leading Walter Grotrian and Bengt Edlén in 1939 to realise that this particular shade of green light corresponds exactly to the kind of light that would be emitted by an atom of iron with thirteen of its electrons stripped away.
  Stripping away just one electron from an atom requires a lot of energy.. and removing thirteen requires a really quite extreme environment. Calculations show that an iron atom could only lose this many electrons, if the temperature of the corona was around a million degrees.
  The puzzle now is that the outermost part of the Sun's atmosphere is far hotter than the surface of the sun itself (which is a measly few thousand degrees). This is very counterintuitive - you would quite reasonably expect that the sun will be hottest in the middle (it is), cooler further towards the edge (it is), and coolest at the furthest extent of its atmosphere (it is not!).
  This bizarre mismatch in temperatures is called 'the Coronal Heating Problem', and it remains an unsolved problem for solar physicists & astronomers today.
  A number of mechanisms have been proposed; most involving the strong magnetic field of the sun depositing energy into the Corona.
  This is a really interesting problem, which brings together scientists of many different skillsets: theorists, programmers/simulation writers, engineers, and observational astronomers. It would be fascinating to see this problem finally solved!

• Photographic Plates - After our conversation with Dawid on June 28th about the wonders of modern astrophotography, Peter kindly brought in some comparatively ancient astronomical technology to share with the group (see photos and text in the section below).

• The Four Colour Theorem - We discussed the Four Colour Theorem in mathematics, and the slightly disappointing (for mathematicians...) way in which it was initially proven.
  The theorem itself is simple enough to state (although, quite counterintuitive).
  It states that any map, no matter how complicated, can be coloured in such a way that no regions of the map with the same colour are touching along a common border, using only four colours.
  You can try this yourself; draw a complicated map (with narrow stretched out regions, counties inside other counties, whatever you like...) and try to colour this in such that no like colours touch along a border - you will never need more than four colours (or.. if it looks like you do need more than four colours - you may need to go back and revise some of your earlier colouring).
  The initial proof of this theorem in 1976 was quite unsatisfying to mathematicians. Effectively, it involved testing every single combination of possible map borders using a computer.
  This was the first mathematical proof ever completed in this way with the aid of a computer.
  Now in the age of AI, mathematicians are using large language models to help them prove / disprove theorems on a regular basis. See this article I posted after the May 24th Spacetime Sunday.

Extra Reading:

• [Article] Discovery of giant ‘exosatellite’ challenges our notion of what makes a moon - Keith Cooper (Physics World)

• [Article] The Organisms That Make Earth’s Harshest Places Home - Jake Buehler (Quanta Magazine)

Extra Watching / Listening:

• [Podcast] How Fast Is the Universe Really Expanding? - Adam Riess  (The Joy of Y Podcast)

• [Video] Is Entanglement the Key to a Theory of Quantum Gravity? - Sean Hodgman (StarTalk) 

Homework

This 'homework' is recurring, and one I like to set my own students (and try to stick to myself!):

1. Learn something new everyday.

2. Share what you know with someone else.

3. Look at the world and "think like a Martian".

If you learn something interesting, I would love to hear about it! Tell me about it next time or drop me an email!

spacetime-sundays@science-on-sea.com