VAR!
In the early morning hours #OTD in 1923, Edwin Hubble took a photo plate of M31 showing a Cepheid variable star.
Using Henrietta Swan Leavitt’s distance-luminosity relationship, Hubble concluded that M31 is located outside the Milky Way.
This observation established that there are other galaxies besides our Milky Way, that our little island is not the whole Universe.
Image: Carnegie Observatories
You can see three marks "N" on the plate where Hubble identified novae.
Comparing with a plate taken the night before, he realized that the brightness of one was changing. Recognizing a Cepheid variable star, he crossed out the "N" and wrote ‘VAR!’
Here are the plates side-by-side. H331H was taken the previous night, a 40-minute exposure with poor seeing conditions. Plate H335H, which revealed the Cepheid, was a slightly longer exposure under better conditions.
Image: Carnegie Observatories
I like the fact we still use the term 'plates' to refer to images in science, even in some papers. Is this term derived from the fact that early cameras used plates, covered in ligh sensitive chemicals to take the photos. ?
@zleap Yes, that's exactly what these were.