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On December 30th 1924, Edwin Hubble's paper was presented to a meeting of the American Astronomical Society in Washington D.C. on his findings, regarding the distance of the "Spiral Nebulae".
Hubble's observations of Cephid Variable stars in the Andromeda Galaxy led him to theorize that the spiral nebulae observed across the sky were in fact external objects to the Milky Way.
These findings resolved an almost century old debate regarding the nature of the Spiral Nebulae.
Hubble had estimated that the Andromeda Galaxy was approximately 900,000 light years distant. Though this distance was incorrect, this discovery dramatically changed the way we viewed the Universe at that time.
Edwin Hubble was born in Marshfield Missouri in the year 1889. Although he showed promise as a boxer, he turned down the sport to study at Oxford University in England. At Oxford he studied Law, while representing the school in boxing tournaments. Upon his return to America he practiced Law for only a few months before he went back to University to study Astronomy. Four years later Hubble was given his Ph.D. whilst working as a research assistant at Yerkes Observatory. In 1919 Hubble arrived at the 100" telescope at Mount Wilson. It was there at Mount Wilson that Hubble was to discover that the Universe was a lot larger than first thought.
Several years earlier in 1917 a nova was found in the Andromeda galaxy. Shapley had discounted this star as being a Nova because he believed that the Andromeda spiral was within our own Galaxy. Heber Curtis on the other hand used this star to explain his view of the Universe. Curtis believed that the spiral nebulae seen across the sky were Island galaxies like our own Milky Way. This difference of opinion became known as the Shapley-Curtis debate. This debate raged through the astronomical community for several years.
This was not the first time that the universe was envisioned as being populated with individual galaxies. The term " Island Universe" was put forward by Immanuel Kant. Kant had proposed that the Milky Way was a flattened disk of stars and that the spiral nebulae were other "island Universes".
Hubble sided with Curtis, believing that the spirals were external galaxies. Hubble thought that the easiest way to resolve the debate would be to photograph the galaxies so that they would show individual stars.
In 1912 Henrietta Leavitt had found that a type of variable star could be used to judge astronomical distance. Included in the Harvard Circular Number 173 (1912) Henreitta Leavitt quoted:
"Since the variables are nearly the same distance from the Earth, their periods are apparently associated with their actual emission of light, as determined by their mass, density, and surface brightness.... A number of brighter variables have similar light curves, as UY Cygni, and should repay careful study. The class of spectrum ought to be determined for as many objects as possible. It is hoped, also, that the parallaxes of some variables of this type may be measured.
Miss Leavitt had been working on the SMC another member of the Local Group.
Hubble tried to photograph some of these variable stars in the 100" scope.
He spent several years taking photographs of the Galaxies M31, M33, and NGC6288. Hubble noted "Dense swarms of images in no way differing from those of ordinary stars".
On the night of October 6th, 1923 he succeeded in showing the variation of several stars in the Andromeda galaxy.
On the 19th of February 1924 Hubble wrote a note to Shapley who read: "You will be interested to know that I have found a variable star in the Andromeda Nebulae".
Shapley's reply was: "the most entertaining piece of literature I have seen in a long time".
Hubble then went on to discover not only variable stars in the Andromeda Galaxy but also several novae, and supergiant stars.
Andromeda and other Clouds through History.
(This section is currently under construction)
Expanding an idea
By the late 1920's most Astronomers accepted a Universe that was populated by Galaxies, fixed in space like pins on a board. This static view of the universe had been accepted since Newton’s time. The distribution of matter in the universe was such, that the combined effects of gravity would keep everything in place.
Einstein's theory of relativity stated that the universe should be expanding. No observations at that time had shown this, so Einstein had to adjust part of his theory to include a correction to compensate for this.
Einstein stated in 1917:
"We admittedly had to introduce an extension to the field equations of gravitation which is not justified by our actual knowledge of gravitation...That term is necessary only for the purpose of making possible a quasi-static distribution of matter, as required by the fact of the small velocities of the stars."
1917 also saw the first paper published that showed direct observational data to support the idea of an expanding universe. The paper was published by Vesto Slipher from the Lowell Observatory in Flagstaff, Arizona. Percival Lowell, the director of the Lowell observatory at this time, believed that the spiral nebulae were primordial solar systems. Lowell employed Vesto Slipher to look for changes in the spiral nebulae's spectra in order to check for rotation in these nebulae. As well as seeing the effects of rotation, he discovered that the spectral lines of common elements were red shifted.
Having established that the spiral nebulae were actually external galaxies to the Milky WAy, Hubble and his assistant Milton Humason started collecting the spectra of these galaxies. Hubble and Humason found that nearly every galaxy they obtained a spectra for contained redshifts of varying degrees. Using the redshift data and the distance estimates that Hubble had obtained from his observations, he was able to find a correlation between the distance of the galaxy and the speed of it's recession.
One of the first galaxies noted without a redshift was the Andromeda galaxy. This galaxy actually showed a blue shift which implied that the galaxy must be approaching the Milky Way.
Discovering the Local Group
MILKY WAY Has been known to man since he first looked up into the night sky.
LARGE MAGELLANIC CLOUD Has been refered to by many ancient peoples inhabiting the Southern hemisphere.
SMALL MAGELLANIC CLOUD Has been refered to by many ancient peoples inhabiting the Southern hemisphere.
M31 THE ANDROMEDA GALAXY May have been known to the ancient people of the Northern hemisphere. First recorded sighting was during the 10th century by Al Sufi a Persian astronomer, and was refered to as the "little cloud"