National Aeronautics and Space Administration Home Edition Age of the Universe: Cosmic Times Size of the Universe: 2 Billion Years 1929 280 Million Light Years Andromeda Nebula lies outside the Milky Way Galaxy Spiral Nebulae are indeed “Island Universes” stronomer Edwin Hubble, inch reflector partially resolved a of the Mount Wilson few of the nearest, neighboring AObservatory at Pasadena, [spiral] nebulae into swarms of California, has solved the mystery stars.” One of the nearby nebulae of the spiral nebulae. The spiral Dr. Hubble photographed was the nebulae look like hazy pin-wheels Andromeda Nebula. He estimates in the sky. He has determined it is as large as the Milky Way that these objects are much more and holds as much matter. It may distant than previously thought. contain some three to four billion Therefore, they are distant galaxies stars that produce one-billion times and not part of our own Milky Way the light of the Sun. galaxy. In the process, Dr. Hubble was also able to determine the These photographs showed there distance to the spiral Andromeda were individual stars in the nebula. Nebula. They also showed some of the stars changed in brightness over Dr. Hubble’s observations support time. Known as Cepheid variable Image credit: Hale Observatories, courtesy AIP Emilio Segre Visual Archives the views Dr. Heber Curtis stars, these stars were the key Edwin Hubble at Mount Wilson Obser- expressed in a debate with Dr. to determining distances to the vatory Harlow Shapley in 1920. Curtis nebulae. The true brightness of stated that bright diffuse nebulae the Cepheids in the nebulae Swan Leavitt of the Harvard College are fairly close to Earth and are Hubble studied was known from Observatory and by Dr. Harlow part of the Milky Way, while spiral how the Cepheid changes its Shapley of the Mount Wilson nebulae are at great distances and brightness. Scientists had already Observatory. not part of the Milky Way. known exactly how light dims over distance. The distance to the star, In 1912, Miss Leavitt was the On December 30, 1924, Hubble and the nebula it is located in, first to recognize the importance announced that he had taken can be found by comparing the of Cepheid variables. They are photographs of a few bright spiral apparent brightness of these stars giant stars, and each varies in nebulae with the Mount Wilson to their true brightness. brightness over time. Cepheids Hooker telescope, the largest are named after the first such star reflecting telescope in the world. Dr. Hubble’s work builds on earlier of its type found: Delta Cephei in According to Dr. Hubble, “The 100 observations by Miss Henrietta “Andromeda” continued on page 2 “Andromeda” continued from page 1 2 Classifying Nebulae the constellation Cepheus. While further subdivided according to studying Cepheids in the Small or over a thousand years, astronomers wondered about shape (spherical to elongated Magellanic Cloud, Miss Leavitt the nature and development ellipses, for example) and structure noticed that the Cepheids would F of nebulae. Nebulae appear to be (hazy to distinct spiral arms, barred brighten, then fade, and then faint clouds of gas and dust in the spirals, etc.). brighten again. The length of time distant universe. Until recently, there it took for the star to go through have not been enough observations Hubble’s system shows a sequence this cycle (the period) was directly to classify nebulae based on their of evolutionary change but was related to its true brightness: the features or qualities. “based primarily on the structural longer the period, the brighter the forms of photographic classification star. The Small Magellanic Cloud During his studies of the spiral which should be entirely independent is a large group of stars visible in nebulae, Dr. Hubble proposed of theoretical considerations”. the southern hemisphere. Since a system to classify all nebulae. Future astronomical studies and all of these stars were in the Small He sorted them into three basic evidence will be the test for this new Magellanic Cloud, they were at categories: elliptical, spiral, and classification system. ♦ irregular. These categories were roughly the same distance from the Earth. Each Cepheid’s true brightness was directly related to NASA its period. Elliptical Galaxies Spiral Galaxies Barred Spiral Galaxies Soon after Miss Leavitt’s discovery, Images From & STScI Dr. Shapley began searching for “Great Debate” Resolved Cepheids in globular clusters in our own Milky Way galaxy. Globular r. Hubble’s discovery settles Milky Way was less than 30,000 clusters are sphere-shaped the “Great Debate” over the light years in diameter and 8,000 groups of tens of thousands of size of our own Milky Way light years in thickness. densely packed stars. He used D galaxy. It also settles the question the period-brightness relationship of the distance to spiral nebulae Dr. Hubble’s recent observations to determine the distance to more and what they are. support Curtis’ views. While bright, than 230 globular clusters. He diffuse nebulae are relatively close assumed that Cepheids in distant Held on April 26, 1920 at the and part of the Milky Way, spiral globular clusters act the same as National Academy of Sciences nebulae are separate systems nearby Cepheids. Based on that in Washington, D.C., the debate located far away from it. He assumption, he found the most focused on the opposing views of estimates the spiral Andromeda distant clusters in the Milky Way astronomers Harlow Shapley and Nebula is as large and holds as Galaxy are about 200,000 light Heber D. Curtis. much matter as the Milky Way. years away. However, Hubble’s findings also Shapley’s studies led him to describe support Shapley’s ideas about the By studying the periods of the the Milky Way as an enormous Milky Way’s size and the location of Cepheids in the Andromeda galaxy of stars some 300,000 light the solar system. Nebula, Dr. Hubble was able to years across. This is much larger determine the true brightness (or than most previous estimates. He Shapley’s work had increased also said the solar system is far the size of the Universe by about absolute magnitude) of each. He from the galaxy’s center and that all ten times, but Hubble’s recent then made observations of their nebulae, including spiral nebulae, discoveries have multiplied it apparent brightness (or apparent are located within the Milky Way. by at least another ten. Shapley magnitude). Once he knew the observed a historical progression, difference between how bright a Using photographs he had taken from belief in a small universe star appeared and how bright it as evidence, Dr. Curtis presented with man at its center, to a larger truly was, he was able to calculate his idea that spiral nebulae were one with Earth further from the its distance from the Earth. He “island universes,” or distant star center: “The significance of man found Andromeda to be 900,000 systems similar to the Milky Way. and the Earth…has dwindled light years away – the most distant They were not a part of the Milky with advancing knowledge of the object known to-date. ♦ Way galaxy. He also believed the physical world…” ♦ 3 The Universe is Expanding “Red Shift” is Proof of Einstein’s General Theory sing the 100-inch Hooker Telescope at Mount Wilson UObservatory, Dr. Edwin Hubble has studied many spiral nebulae. He has discovered they are moving away from us at a rapid pace. This is strong evidence of an expanding universe. He has further determined that the farther the Image Credit: Hubble, PNAS, 15, 3 (1929) nebula, the faster it moves away. Hubble’s diagram showing the relationship between galaxy distance and velocity Hubble and colleague Milton Hubble’s findings build on the work with a 200-inch telescope. This Humason measured the speeds of Dr. Vesto M. Slipher of Lowell new telescope will collect four and distances for twenty-four of Observatory in Flagstaff, AZ. In times more light than the 100 inch these nebulae. Because they 1912, Dr. Slipher first recorded the telescope Hubble currently uses. are moving so quickly, their light electromagnetic spectra of spiral waves (electromagnetic spectra) nebulae. All but a few of the forty are stretched out. Because red Einstein’s General Theory spectra Slipher later gathered were light has the longest wavelength, very red-shifted. This means almost Holds True this phenomenon is known as all of them were moving away from “redshift.” The team noticed that us. However, his studies lead him The past decade has been an dimmer, more distant objects have to a conclusion that speeds for the exciting and challenging time a larger redshift than objects closer nebulae might be closer to only 600 in the development of scientific to Earth. As reported in a recent miles per second. understanding of the Universe. paper, Hubble’s measurements During this time scientists have led him to a useful speed-distance Although Hubble’s work presents been testing their ideas about the relationship: redshifts increase in a big breakthrough in our Universe against Einstein’s 1916 direct proportion to their distance understanding of the Universe, one Theory of General Relativity. The from us. big question remains; how far out theory describes the universe as into the Universe does his model three dimensions of space (length, Dr. Hubble determined distances hold? The 100-inch telescope can width, and depth) and one of time. It to the twenty-four nebula using clearly show Cepheids in only the says that gravity curves this space­ Cepheid variable stars. These nearest nebulae. In very distant time and that the curvature controls are stars that astronomers use nebulae, where Cepheids are the natural motions of objects in to determine the distance to the barely visible, Hubble uses the space.