This led to a whole new branch of cosmology, in which branes could be island universes floating in an extra-dimensional space like leaves in a fish tank, colliding and otherwise interacting with each other through a higher dimension.
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The term 'Pinwheel nebula' is an antiquated misnomer used by observers before Edwin Hubble realized that many of these spiral shaped nebulae were actually 'island universes' or what we now call galaxies.
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In 1912, Vesto Slipher measured the first Doppler shift of a "spiral nebula" (the obsolete term for spiral galaxies), and soon discovered that almost all such nebulae were receding from Earth. He did not grasp the cosmological implications of this fact, and indeed at the time it was highly controversial whether or not these nebulae were "island universes" outside our Milky Way.
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As a result, he was able to come up with a distance estimate of . He became a proponent of the so- called "island universes" hypothesis, which held that spiral nebulae were actually independent galaxies. Andromeda Galaxy above the Very Large Telescope. The Triangulum Galaxy is visible on the top. In 1920, the Great Debate between Harlow Shapley and Curtis took place concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the universe.
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Thomas Wright and Kant first speculated that fuzzy patches of light called nebulae were actually distant "island universes" consisting of many stellar systems. The shape of our own galaxy was expected to resemble such "islands universes." But "scientific arguments were marshalled against such a possibility," and this view was rejected by almost all scientists until Edwin Hubble's measurements in 1924. William Herschel's model of the Milky Way, 1785 In 1783, amateur astronomer William Herschel attempted to determine the shape of the galaxy by examining stars through his handmade telescopes.
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This firmly established the spiral nebula as being objects well outside the Milky Way galaxy. Determining the distance to "island universes", as they were dubbed in the popular media, established the scale of the universe and settled the Shapley-Curtis debate once and for all."Island universe" is a reference to speculative ideas promoted by a variety of scholastic thinkers in the 18th and 19th centuries. The most famous early proponent of such ideas was philosopher Immanuel Kant who published a number of treatises on astronomy in addition to his more famous philosophical works.
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The Indian cyclic model assumes the existence of countless island universes, which go through their own periods of development and destruction. The conception of cyclicity is taken to be recursive. For an early exposition of these astronomical and cosmological ideas, one may like to read al-Bīrūnī’s classic history of Indian science, composed in 1030 AD, and for an even earlier, popular, view of Indian ideas, one may consult the great Vedantic text called the Yoga Vāsiṣṭha (YV), which at 32,000 shlokas is one of the longest books in world literature.
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Most 18th to 19th century astronomers considered them as either unresolved star clusters or anagalactic nebulae, and were just thought as a part of the Milky Way, but their true composition and natures remained a mystery. Observations using larger telescopes of a few nearby bright galaxies, like the Andromeda Galaxy, began resolving them into huge conglomerations of stars, but based simply on the apparent faintness and sheer population of stars, the true distances of these objects placed them well beyond the Milky Way. For this reason they were popularly called island universes, but this term quickly fell into disuse, as the word universe implied the entirety of existence. Instead, they became known simply as galaxies.
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Sharp view of the Andromeda Galaxy American astronomer Edwin Hubble included M31 (then known as the Andromeda Nebula) in his groundbreaking 1923 research on galaxies. Using the 100-inch Hooker Telescope at Mount Wilson Observatory in California, he observed Cepheid variable stars in M31 during a search for novae, allowing him to determine their distance by using the stars as standard candles. The distance he found was far greater than the size of the Milky Way, which led him to the conclusion that many similar objects were "island universes" on their own. Hubble originally estimated that the Andromeda Galaxy was 900,000 light-years away, but Ernst Öpik's estimate in 1925 put the distance closer to 1.5 million light-years.
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As a result, Curtis became a proponent of the so-called "island Universes" hypothesis, which held that objects previously believed to be spiral nebulae within the Milky Way were actually independent galaxies. In 1920, the Great Debate between Harlow Shapley and Curtis took place, concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the Universe. To support his claim that the Great Andromeda Nebula (M31) was an external galaxy, Curtis also noted the appearance of dark lanes resembling the dust clouds in our own galaxy, as well as the significant Doppler shift. In 1922 Ernst Öpik presented an elegant and simple astrophysical method to estimate the distance of M31.
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Shapley participated in the "Great Debate" with Heber D. Curtis on the nature of nebulae and galaxies and the size of the Universe. The debate took place on April 26, 1920, in the hall of the United States National Academy of Sciences in Washington DC. Shapley took the side that spiral nebulae (what are now called galaxies) are inside our Milky Way, while Curtis took the side that the spiral nebulae are 'island universes' far outside our own Milky Way and comparable in size and nature to our own Milky Way. This issue and debate are the start of extragalactic astronomy, while the detailed arguments and data, often with ambiguities, appeared together in 1921."The Scale of the Universe" Shapley, H. and Curtis, H. D., Bulletin of the National Research Council, 2, 169, pp.
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Mount Wilson astronomer Harlow Shapley championed the model of a cosmos made up of the Milky Way star system only; while Heber D. Curtis argued for the idea that spiral nebulae were star systems in their own right as island universes. This difference of ideas came to a climax with the organization of the Great Debate on 26 April 1920 at the meeting of the U.S. National Academy of Sciences in Washington, D.C. The debate was resolved when Edwin Hubble detected Cepheid Variables in the Andromeda Galaxy in 1923 and 1924. Their distance established spiral nebulae well beyond the edge of the Milky Way. Subsequent modelling of the universe explored the possibility that the cosmological constant, introduced by Einstein in his 1917 paper, may result in an expanding universe, depending on its value. Thus the Big Bang model was proposed by the Belgian priest Georges Lemaître in 1927 which was subsequently corroborated by Edwin Hubble's discovery of the redshift in 1929 and later by the discovery of the cosmic microwave background radiation by Arno Penzias and Robert Woodrow Wilson in 1964.
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