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Cosmic Times National Aeronautics and Space Administration Early Edition Age of the Universe: Cosmic Times Size of the Universe: 2 Billion Years 1929 280 Million Light Years Andromeda Nebula Lies Outside Milky Way Galaxy Spiral Nebulae are indeed “Island Universes” stronomer Edwin Hubble of the Mount Wilson Observa­ Atory at Pasadena, Califor­ nia has solved a mystery. Spiral nebulae in our universe have been a mystery to astronomers. Nebu­ lae look like fuzzy pinwheels in the sky. Astronomers weren’t sure if these nebulae were in our own galaxy or outside of it. Hubble figured out that these objects are much farther away than other as­ tronomers thought. That means they are distant galaxies and not a part of our own Milky Way Galaxy. While figuring this out, Dr. Hubble also figured out how far away the Image credit: Hale Observatories, courtesy AIP Emilio Segre Visual Archives Andromeda Galaxy is from us. Edwin Hubble Dr. Hubble’s observations support Dr. Heber Curtis’ views. In a de­ reflecting telescope in the orld.w showed that there were individu­ bate with Dr. Harlow Shapley in Hubble said that the telescope fo­ al stars in the nebula. They also 1920, Dr. Curtis said that bright, cused a few of the closest, neigh­ showed that some of the stars spread-out nebulae are fairly boring spiral nebulae into swarms changed in brightness over time. close to earth and are a part of of stars. One of the nebulae was These changing stars, known as the Milky Way. Spiral nebulae, on the Andromeda Nebula. Hubble Cepheid variable stars, were the the other hand, are much farther estimates that the Andromeda key in figuring out distances to away and are not part of the Milky Nebula is as large and holds as nebulae. In the nebulae Hubble Way. much matter as the Milky Way studied, the true brightness of Galaxy. The Andromeda Nebula the Cepheids was known based On December 30, 1924, Dr. Hub­ might contain three to four billion on how Cepheid stars change ble announced that he had taken stars that make one billion times in brightness. Scientists had al­ photographs of a few bright spiral the light of the Sun. ready known how light dims over nebulae with the Mt. Wilson Hook­ a distance. That means the dis­ er telescope, which is the largest The photographs Hubble took tance to the star and the nebula “Andromeda ” continued on page 2 2 “Andromeda” continued from page 1 the star is located in can both be Classifying Nebulae found by comparing the apparent brightness of the Cepheid stars to or over a thousand years, their true brightness. astronomers have won­ Fdered about nebulae. Spiral galaxies Dr. Hubble’s work builds on the Nebulae appear to be faint earlier work of Miss Henrietta clouds of gas and dust far off in Swan Leavitt of the Harvard Col­ the universe. Until recently, there lege Observatory and the work of haven’t been enough observa­ Dr. Harlow Shapley of the Mount tions of nebulae to put them into Wilson Observatory. categories. Images From NASA & STScI In 1912, Miss Leavitt was the first During his study of spiral nebu­ astronomer to see the importance lae, Dr. Hubble came up with a of the Cepheid variable stars. The Elliptical galaxies giant stars are named after the first system to classify all nebulae. star of its kind found—Delta Ce­ He put them into three basic cat­ phei in the constellation Cepheus. egories. These categories are When Miss Leavitt was studying elliptical, spiral, and irregular. the Cepheid stars in the Small The three categories are then di­ Magellanic Cloud, she saw that vided further based on the nebu­ the Cepheids would get brighter, lae’s shapes (like spherical to Images From NASA & STScI then fade, then get brighter again. elongated elliptical, for example) She also noticed that the length and structure (hazy to distinct of time, or period, it took for the spiral arms, barred spirals, etc.) Barred spiral galaxies star to go through this cycle was related to its true brightness. The Although Dr. Hubble’s system longer the period of the star, the shows an order of change based brighter the star. on the evolution of the nebu­ The Small Magellanic Cloud, lae, it was mainly based on the where Miss Leavitt was studying nebulae’s structural forms. Clas­ Cepheid stars, is a large group sification through photographic Images From NASA & STScI of stars visible in the southern classification should be the inde- hemisphere of the Earth. Since pendent consideration, Hubble Hubble classification of different all of the Cepheid stars she was said. Future study of the nebu­ types of galaxies (also called nebu- studying were in the Small Mag- lae will be the test for the new lae) ellanic Cloud, they were all about classification system. ♦ the same distance from Earth. So each Cepheid’s brightness was directly related to its period. clusters. He assumed that Ce­ or absolute magnitude, of each of pheids in distant globular clusters the stars. He then observed their Soon after Miss Leavitt’s discov­ act the same as closer Cepheids. apparent brightness, or apparent ery, Dr. Shapley started looking Based on that, he found that the magnitude. Once he knew the for Cepheids in globular star clus­ most distant clusters in the Milky difference between how bright a ters in our own Milky Way galaxy. Way galaxy are about 200,000 star looked and how bright it truly Globular clusters are ball-shaped light years away. was, he was able to calculate its groups of tens of thousands of distance from the Earth. He found densely packed stars. He used Dr. Hubble studied the periods of that the Andromeda nebula was Miss Leavitt’s period-brightness the Cepheid variable stars in the about 900,000 light years away. relationship to figure out the dis­ Andromeda nebula. He was able This is the most distant object tance to more than 230 globular to figure out the true brightness, known so far. ♦ 3 Universe is Expanding – “Red Shift” is Proof of Einstein’s General Theory sing the 100-inch Hooker nebulae are being pulled along as certain amount. Astronomers will Telescope at Mount Wilson space expands. need larger telescopes and new UObservatory, Dr. Edwin standard candles in order to mea­ Hubble has studied many spiral Hubble’s discoveries build on the sure even greater distances. nebulae. He has discovered that work of Dr. Vesto M. Slipher of they are moving away from us at Lowell Observatory in Flagstaff, A new instrument may help. Last a fast pace. This is strong evi­ AZ. In 1912, Dr. Slipher first re­ year, the Rockefeller Foundation dence of an expanding universe. corded the electromagnetic spec­ agreed to give 6 million dollars to He has also figured out that the tra of spiral nebulae. Almost all of pay for the construction of a new farther away a nebula, the faster it the 40 spectra Slipher gathered observatory with a 200-inch tele­ moves away. were red-shifted. This means that scope. This new telescope will most of them were moving away collect four times more light than Hubble and another astronomer from us. However, his study of the 100-inch telescope Hubble and colleague, Milton Humason, nebulae led him to the conclusion uses right now. measured the speeds and dis­ that they were moving away at tances of 24 of these nebulae. only 600 miles per second. Einstein’s General Theo­ Because they are moving so ry Holds True quickly, their light waves (electro­ Hubble’s work is a large break­ magnetic spectra) are stretched through in our understanding of The past decade has been an out. Because red light has the the universe. But there is still a exciting and challenging time for longest wavelength, this is known big question. How far out into the the development of scientific un­ as “redshift.” The team noticed universe does his model hold up? derstanding of the universe. Sci­ that dimmer, more distant objects The 100-inch telescope can show entists have been testing their have a larger redshift than objects Cepheid variable stars clearly in ideas about the universe against closer to Earth. Hubble said in a the nearest nebulae. In far off Einstein’s 1916 Theory of General recent paper that his measure­ nebulae, where the Cepheids are Relativity. Einstein’s theory de­ ments led to a useful speed-dis­ barely visible, Hubble uses the scribes the universe as three di­ tance relationship. Hubble found brightest individual stars, some mensions of space (length, width, that redshifts increase as they get of them 50 to 100 times brighter and depth) and another dimen­ farther from us. than the Cepheids, as standard sion of time. The theory says that candles. gravity curves this space-time and Dr. Hubble figured out the dis­ that this curve controls the natural tances to 24 nebulae using Ce­ Today’s telescopes, and the stars motions of objects in space. pheid variable stars. Astronomers they see, can only show Hubble a use these stars to figure out the “Expanding ” continued on page 4 distance to the nebula they are found in. Once Hubble knew how far away the nebulae were, he could then compare their distance to their redshift in order to look for a pattern. He found that the most distant objects are speeding away from us at perhaps thousands of miles per second. Dr. Hubble’s recent discoveries show that the volume of space itself is expanding. Spiral nebu­ lae seem to be moving away from each other at a speed that increases with distance, but the nebulae aren’t just moving.
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