the Great Debate is the name given to the discussions which had it in the current of the Années 1920 on nature of what at the time was called “nebulas”, and which are in fact of the Galaxie S located outside the Milky Way. The debate related to nature and the distance from these objects, and in consequence of their galactic or extragalactic nature. The most famous date of this debate is that of the April 26th 1920 with the National Museum off Natural History of the Smithsonian Institution .

The two main characters of this debate were Harlow Shapley and Heber Curtis. The first defended the idea that the observable Univers did not extend beyond the Milky Way, the second defending the contrary idea, being based in particular on the observation of Nova E in what is today called the Galaxie of Andromède.

The debate profited from the observations of Vesto Slipher which in 1914 had detected a certain number of shifts towards the red in some of these nebulas, of which the amplitude, connected by Doppler effect at their rate of travel, seemed to indicate that these objects were not gravitationally related to our galaxy, when well even they would have been inside this one today.

Shapley as for him announced detection of movements within the nebula of Andromède, tending to think that this object was of relatively modest extension, and thus near to the Solar system.

It is known today that these measurements were erroneous and that it is not possible to detect notable movements within the galaxy of Andromède on scales of time and the order of the duration of an human life. The elements which have lets Shapley think that it had observed movement within these objects are on the other hand not known.

The Great Debate was closed in 1925 or 1926 by Edwin Hubble which detected Céphéide S and many other variable stars in several of these nebulas (NGC 6822, M33, M32 and M31 in particular), making it possible to measure the distance of it and thus to prove the extragalactic nature of these objects.

See too

• Galaxy

• Céphéide
• Edwin Hubble
• Harlow Shapley
• Heber Curtis

External bonds

• The Shapley - Curtis Debate in 1920, on the site of the Center of space flights Goddard