Here's a breakdown of the key steps:
1. Cepheid Variable Stars: Hubble used Cepheid variable stars, which are stars whose brightness fluctuates with a predictable period. The period of a Cepheid's brightness variation is directly related to its intrinsic luminosity (its actual brightness). By measuring the period of a Cepheid in a galaxy, astronomers can determine its luminosity.
2. Distance Calculation: By comparing the apparent brightness of a Cepheid (how bright it appears from Earth) to its known luminosity, astronomers can calculate the distance to the Cepheid, and therefore, the galaxy it resides in.
3. Redshift and Distance: Hubble observed that the further a galaxy was, the faster it was moving away from us, as indicated by the redshift of its light (the light was shifted towards the red end of the spectrum). He established a direct relationship between the distance of a galaxy and its redshift.
4. Extending to Andromeda: While Hubble didn't directly measure the distance to Andromeda using Cepheid variables, he used other astronomers' observations of Cepheids in Andromeda to estimate its distance. Using the relationship between redshift and distance he established, he could then confirm the distance to Andromeda based on its redshift.
It's important to note:
* The distance to Andromeda has been refined over time with more accurate measurements and techniques.
* Hubble's original estimate of Andromeda's distance was close but not as precise as modern measurements.
* His work on the redshift-distance relationship revolutionized our understanding of the universe and the expansion of space.
