What was the exact method by which Hubble discovered the red-shift of distant galaxies?

Edwin Hubble discovered the redshift of distant galaxies through a combination of observational and analytical methods. Here are the key steps involved in his discovery:

Spectroscopic Observations

Hubble used spectroscopy to examine the light emitted by galaxies. By analyzing the spectral lines of the galaxies, he observed that these lines were shifted towards the redder end of the electromagnetic spectrum, a phenomenon known as redshift35.

Redshift and Doppler Effect

The redshift was interpreted using the Doppler effect, which indicates that the galaxies are moving away from the observer. The extent of the redshift was found to be directly related to the velocity of the galaxies35.

Distance Measurements

To correlate the redshift with distance, Hubble used Cepheid variable stars to measure the distances to a sample of galaxies. Cepheid variables have a known relationship between their brightness and period, allowing Hubble to estimate their distances25.

Combining Redshifts and Distances

Hubble combined the redshift measurements (which were related to the velocities of the galaxies) with the distance measurements obtained from Cepheid variables. By plotting the velocity (inferred from redshift) against the distance, he found a direct proportionality between the two quantities. This relationship is now known as Hubble's Law235.

Previous Contributions

Hubble's work built upon earlier measurements by Vesto Slipher, who had measured the redshifts of many galaxies in 1917, and Henrietta Swan Leavitt's methodology for calculating intergalactic distances. These contributions were crucial in allowing Hubble to make precise correlations between redshift and distance13.