In connection with my
recent Andromeda image I did some reading and found out that you can actually resolve the Cepheid famously known as Hubble's variable star V1.
In the 1920s the nature of the spiral nebulae now known as galaxies has still been unclear. Were they galactic nebulae or were they 'island universes'? Edwin Hubble tried to settle the question by examining the Andromeda nebula as it was called at the time and found what he thought was a nova. Soon he noticed that the star's brightness varied in faster cycles than usual for a nova. By determining the period he saw that his nova was actually a variable star of the Cepheid type. Thus he crossed out the handwritten 'N' (for 'nova' ) and wrote 'VAR!' for 'variable' instead on the glass plate. By measuring its apparent brightness and by knowing the period of the Cepheid he could estimate the distance of the Cepheid. The result has been about 1 million light years. The result was wrong at least by a factor of 2 but it definitely put the Andromeda nebula well outside of the Milky way.
Using the data of the middle panel of my
Andromeda image I did a complete re-edit in full resolution of a cropped region and highlighted the Cepheid V1, which in 1923 enormously increased the size of the Universe. It's faint but visible.
Revision D: Monochrome negative for better visibility. And some fun additions. I couldn't resist...
For further reading see for example
Hubble Views the Star That Changed the Universe.
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