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

https://files.mastodon.social/media_attachments/files/111/188/814/071/512/183/original/c76443d049196338.jpeg