The image below is a closeup of the Andromeda galaxy core. The area photographed includes the Cepheid variable star – M31_V1 – that Hubble used to calculate the first reliable distance to the “Andromeda Nebula”. His result proved conclusively that “nebula” were distant objects that were not part of the Milky Way.
The wide field view of Andromeda below shows the section of the galaxy captured in the close up image as well as locators for M31_V1.
Hubble used the recently commissioned 100″ Hooker telescope on top of Mount Wilson to capture a 1hr photographic plate of M31 on October 5/6, 1923. A crop from the original glass plate H335H (Hooker 335 Hubble) is shown below along with the same area from my image. Perhaps the plate has faded over the years or the technicians had keen eyes, but M31_V1 is barely visible.
Hubble had originally identified the star as a Super Nova and marked it “N”. Nova had been seen on plates before, so it was reasonable to assume that a star that had not been seen before was a super nova. However, reexamining previous plates and finding a fainter star in the same location, he then decided it was a variable star and marked it “VAR!”. This was significant because Henrietta Leavitt had earlier discovered a method for accurately calculating distances to Cepheid variable stars. Hubble’s discovery of M31_V1 was the first time a variable star had been observed in a nebula.
Cepheid variables are a class of variable stars that can be used as a “standard candle”. That is, their absolute (actual) brightness can be calculated, so their apparent brightness is then a measure of their distance (the dimmer the star, the further away it is). Today, Cepheid variables are still an important tool for determining distances to galaxies.
In 1908 Henrietta Swan Leavitt discovered a Period-Luminosity relationship for Cepheid variables that allows their absolute luminosity (brightness) to be calculated from their period – the time the star takes to vary from minimum to maximum intensity. At the time, she had been working for the Harvard College Observatory as a “computer” and assigned the task of identifying variable stars from photographic plates. Although the women computers were low paid and not even considered astronomers, they still made significant contributions to astronomy. She writes in her 1912 article defining the P-L relationship: “A remarkable relation between the brightness of these variables and the length of their periods will be noticed.”
Prior to Leavitt’s ground breaking discovery, various techniques had been tried to determine the size of the Milky Way and the distance to M31 in particular, but none were conclusive. The prevailing view prior to Hubble’s measurement was that our Milky Way galaxy was about 30,000 light years in diameter (current estimates range from 100,000 to 180,000ly). And the Milky Way was the entire extent of the universe. The fuzzy objects visible in telescopes were described as “nebula” and thought to be within the boundaries of the Milky Way. There were some suggestions that these nebula lie on the periphery and could be as far away as 300,000ly, but the evidence was unconvincing.
Hubble’s H335H image was the first time a variable star had been identify in a “spiral nebula”, so this was the first opportunity to conclusively state the distance to M31. Hubble then took a series of plates to characterize the period and light curve of M31_V1 and confirmed it as a Cepheid variable. Using Leavitt’s P-L relationship rule and work Shapley had done to calibrate the rule to absolute distances, Hubble was able to calculate the distance to M31. He announced the results in a January 1925 paper presented to the American Astronomical Society stating definitively that M31 was 1,000,000ly away and clearly not contained within the Milky Way. (Current estimates put M31 at a distance of 2.5m light years.)
Hubble would then go on to locate variable stars in other nebula at even greater distances. Again he used Leaveitt’s P-L relationship to calculate the distances as well as the Doppler red-shift affect to determine their relative speeds. His observations showed that the further away a galaxy was, the faster it was receding from us. His now famous article in the 1929 Proceedings of the National Academy of Sciences declared for the first time that the universe was expanding!
And if it was expanding into the future, it must have been smaller in the past. This would eventually lead to the idea of the “Big Bang!”