Robert McNees
Astronomer Edwin Hubble was born #OTD in 1889.
Hubble is best known for combining galactic distance measurements with Slipher’s redshift data to obtain a linear relation between distance and recession velocity. This is now understood as evidence for an expanding universe.
Before that, Hubble used Henrietta Swan-Leavitt's period-luminosity relation for Cepheid variable stars to establish spiral nebulae as separate galaxies situated outside the Milky Way.
Robert McNees
In 1923, Hubble took a photographic plate of the spiral nebula M31. It showed what he at first thought was a nova. Over subsequent observations he realized it was actually a Cepheid variable star. He crossed out “N” and excitedly wrote “VAR!”
Using Henrietta Swan-Leavitt's relationship between the period and brightness of Cepheids, Hubble established that M31 was situated outside the Milky Way. Spiral nebula were in fact distinct galaxies.
IMAGE: Carnegie Observatories
Robert McNees
This momentous result is only 99 years old. There are people alive today who were born before we understood that the Milky Way is just one of many galaxies throughout the Universe.The current oldest living person, Lucille Randon of France, was 19 years old when Hubble announced this discovery.
Robert McNees
By 1929, Hubble had collected (with the help of Milton Humason's careful telescope work) distance measurements for several galaxies. Combining these with Vesto Slipher's redshift measurements, he proposed a relationship between distance and recession velocity. More distant galaxies were moving away from us more rapidly.
Hubble's paper "A relation between distance and radial velocity among extra-galactic nebulae" appeared in January, 1929.
Figure: PNAS March 15, 1929 vol. 15 no. 3 168-173
Hifilarry
@mcnees wow. History.
Robert McNees
@rjme Yeah I was really pulling for Betty White for this.
Richard Easther
@mcnees I sometimes do a thing in class "Put up your hand if you have a relative who was alive in 1923, 1929, 1965..." -- that person was born before... But the 1923 one has got a bit harder :-)
Robert McNees
Hubble was hesitant to speculate too much about the meaning of his result. The words "recession" and "expansion" don't actually appear anywhere in the paper!
But the implications were clear to Hubble. At one point, he says "For this reason it is thought premature to discuss in detail the obvious consequences of the present results."
Robert McNees
The "cosmological principle" is the assumption that, on very large scales, most places in the Universe are more or less like most other places, and things look more or less the same regardless of what direction one points their telescope.
Combining this with Hubble's observation, you conclude that no matter where you go, it must seem like all the galaxies are moving away from you, in all directions. The simplest explanation is that the Universe is expanding.
Robert McNees
It's important to note that there were other physicists and astronomers exploring these ideas. Both Lemaître (1926-27) and Robertson (1928) proposed models in which distant objects moved away from us with a velocity proportional to their distance. Robertson even used some of Hubble's earlier data to support his proposal. Hubble must have been aware of their work, though as far as I know he never cited it.
Robert McNees
@rdviii All I know is that it has a greek root, the term that was used for the Milky Way. Beyond that I don't really know! But if anyone knows some interesting etymology I'd like to hear it!
Rod Van Meter
@mcnees thank you! Can you tell us a little about how the word "galaxy" came to be accepted for these nebulae?
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