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Therefore, the discovery of the spiral nebula remove the Hubble Space Telescope, from their original position of humility in our common and as a member of a different galaxy and the galaxy is enthroned them their rights, the vast broadening the universe. It has been clear to see these relics and completely normal galaxies (such as Orion), which is really only a large cloud of dust floating in interstellar space. Many of the spiral galaxy billions of stars in each configuration, which is blurred, a faintly luminous not only for the masses, they are very large distance from the observer to attend the meeting. This conclusion is the last astronomer from another direct evidence - Wilson, and Walter Bard, who is solved photo Andromeda's central institutions and its two companions needed a lot of small dots of stars on behalf of individual emission of these remote from the system being built. Preferred term changes to the old approach, instead of a spiral nebula discussion on spiral galaxy.           Discovery of Galactic Proportions     Astronomers have long been acquainted with certain rather peculiar-looking celestial objects which had come to be called "spiral nebulae." In contrast to other known nebulosities, which usually have irregular shapes and look more or less like clouds in the sky, spiral nebulae always have well-developed structures, consisting of a lenticular central body with a pair of spiral arms winding around it. Until about a quarter of a century ago, the spiral nebulae were more or less generally believed to be located among the stars of our Milky Way system and were thought to be possible example of stars giving birth to their own planetary systems according to the classical picture of Kant and Laplace.   However, in 1925, all these views were completely overthrown by a great discovery made by Edwin P. Hubble, an astronomer at Mount Wilson Observatory. Studying the Great Spiral Nebula of Andromeda, which is visually the largest of them all and therefore the most accessible to observation, he noticed that its spiral arms contain a number of extremely faint stars whose brightness changes periodically, following a simple sine law. Such stars, called "Cepheid variables" (after Delta Cephei, the first star in which such variability was noticed), are well known in our Milky Way system, and their periodic changes in luminosity are explained as the result of periodic pulsations of their giant bodies. A simple correlation exists between the period of these pulsations and the absolute luminosity of the star in question: the brighter the star the longer the period of pulsation. This so-called "period-luminosity relation" established by the Harvard astronomer Harlow Shapley is a powerful tool for measuring the distances of pulsating stars which are too far away to show a parallax displacement. By measuring directly the pulsation period of a given star, we can arrive at a definite conclusion about its absolute brightness. This, in combination with the visual brightness, tells us the actual distance of the star.   The observed pulsation periods of the Cepheids found by Hubble in the spiral arms of the Andromeda nebula indicated that they must possess very high absolute luminosities. On the other hand, they are so faint visually as to be at the limit of visibility. The inevitable conclusion was that they—and also the nebula itself—must be extremely far away. The distance worked out to almost 1 million light-years, that is, about a hundred times the diameter of the entire Milky Way system! Other spiral nebulae, which are visually smaller and fainter than the one in Andromeda, must be farther away. If the spiral nebulae are really that far off, they must also be much larger than originally suspected; in fact they must be about as large as the Milky Way system itself!   Thus Hubble's discovery removed the spiral nebulae from their former humble position as common members of our galaxy and enthroned them as independent galaxies in their own right, scattered through the vast expanse of the universe. It became clear that these objects have nothing to do with ordinary nebulae (like the one in Orion), which are really only large clouds of dust floating in interstellar space. The spiral nebulae are formed by many billions of individual stars which blur together into one faintly luminous mass only because of their exceedingly great distance from the observer. More recently this conclusion was proved directly by another Mount Wilson astronomer, Walter Baade, who was able to resolve photographically the central body of the Andromeda nebula and those of its two companions into myriads of tiny luminous dots representing the individual stars from which these distant systems are built. It seems advisable to change the old terminology, and instead of talking about spiral nebulae to talk about spiral galaxies.   Hubble's discovery proved to be the germ of still more remarkable progress in our knowledge about the nature of the universe. It had been known for some time that spectral lines in the light emitted by spiral nebulae show a shift toward the red end of the spectrum. Interpreted in terms of ordinary Doppler effect, this meant that these objects were moving away from the observer. As long as these objects were believed to be members of our stellar system, one had to conclude that they had some peculiar motion among the stars, being driven from the central regions of the Milky Way toward its periphery. With the new broadening of horizons a completely new picture emerged: the entire space of the universe, populated by billions of galaxies, is in a state of rapid expansion, with all its members flying away from one another at high speed.           — George Gamow, The Creation of the Universe (1952), Chapter 2 – The Great Expansion                             Indexes/21