Digital Universe Guide - Hayden Planetarium

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190 4. THE EXTRAGALACTIC ATLAS The data appear to form lines that point toward Earth. Astronomers call this radial stretching effect a “finger of god.” Throughout history, astronomers have shown just how ordinary we Earthlings are by disproving our position in the center of the Solar System, the Galaxy, and even the Universe. When, in the 1980s and 1990s long lines of galaxies pointing radially toward Earth were discovered, we had reason to believe we were special again. However, it was quickly apparent that this was an artificial result. Astronomers realized these radial effects are a product of velocity dispersions that differ from the overall expansion of the Universe. Within a cluster of galaxies, an individual galaxy could be heading toward Earth (blueshifted), away from Earth (redshifted), or there may be no radial component at all in its motion (no shift in the light). These dispersions cause the distances of galaxies within clusters to be stretched radially. In fact, the mapping of large-scale structure is stymied by this effect—the more concentrated the area, the more uncertainty there will be in the galaxy’s distance. For a more realistic view of the Universe, this stretching must be removed. For large clusters in 2MASS, the stretching has been removed, resulting in a statistically accurate representation of a galaxy cluster. In the mid-range densities, the radial stretching remains—you will see many long lines of galaxies that point toward Earth. As the density decreases, the stretching will not be as prominent. Let’s go back to our normal view for 2MASS by typing see lss. Labels The 2MASS data group has two sets of labels. One set is a local label, showing the names of individual galaxies when you fly close to them (with the default label size). Another set of labels in the 2MASSclusters.label file, identify galaxy clusters and structure. These labels are 20 times larger than the individual galaxies, so they will be visible from greater distances. Adjust the Labelsize and Labelminpixels Sliders to suit your label taste.
4.3. EXTRAGALACTIC DATA GROUPS 191 4.3.12 Abell Galaxy Clusters and Nearby Superclusters Group Name Abell Reference A Catalog of Rich Clusters of Galaxies (Abell+ 1958, 1989) With alterations by R. Brent Tully (U Hawaii) Prepared by R. Brent Tully (U Hawaii) Stuart Levy (NCSA/U Illinois) Labels Yes Files abell.speck, abell.label Dependencies abell.cmap Census 2,246 galaxy clusters The Abell group is composed of all the nearby galaxy clusters. The northern hemisphere survey, published in 1958, was compiled by George Abell (1927–1983) from the Palomar Sky Survey plates. A subsequent southern hemisphere catalog was published posthumously in 1989. Further data analysis was conducted by R. Brent Tully of the Institute for Astronomy, to determine the distance and three-dimensional distribution of these galaxy clusters. Each point in this data set represents a cluster of tens to hundreds (possibly even thousands) of galaxies. You will notice some points are assigned colors while most are gray. The data set also has an arbitrary cut-off, resulting in the rectangular shape of the data set. Clusters of Clusters Galaxies group together to form the large-scale structure of the Universe. Dense clusters of galaxies are connected by filaments, or strands, of galaxies. Between, vast voids resemble the inside of a bubble and are occupied by less dense material. Beyond these structures, astronomers have found larger-scale constructs called “superclusters.” Larger than a cluster of galaxies, superclusters are made from many galaxy clusters. In the Abell data, the non-gray colors represent these superclusters. These mammoth objects are on the order of 300 million light-years in diameter. Compare that to the size of one cluster, Virgo, which is only 15 million light-years across, or our Galaxy, which is a scant 100,000 light-years across. The Shapley concentration, named after the American astronomer Harlow Shapley (1885–1972), is one of the closest rich superclusters to us. Six hundred million light-years distant, the supercluster is composed of about 25 rich galaxy clusters, amounting to 10,000 Milky Ways.
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The Digital Universe Guide Brian Ab
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Contents Contents 3 List of Tables
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List of Tables 2.1 Log and Linear F
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1.2. GETTING HELP 7 1.2 Getting Hel
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1.5. ACKNOWLEDGMENTS 9 1.5 Acknowle
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2.2. LAUNCHING THE MILKY WAY ATLAS
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2.3. LEARNING TO FLY PARTIVIEW 13 T
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2.3. LEARNING TO FLY PARTIVIEW 15 M
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2.4. PARTIVIEW’S USER INTERFACE 1
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2.5. DIGITAL UNIVERSE FILES 23 File
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3.1. MILKY WAY ATLAS OVERVIEW 25 3.
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3.2. MILKY WAY ATLAS TUTORIAL 27 3.
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3.3. MILKY WAY DATA GROUPS 67 Earth
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3.3. MILKY WAY DATA GROUPS 69 Data
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Page 145 and 146: 4 The Extragalactic Atlas The word
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Page 209 and 210: Appendix B Parallax and Distance On
Page 211 and 212: absolute magnitude M, can be calcul
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Page 215 and 216: Appendix D Electromagnetic Spectrum
Page 217 and 218: Appendix E Constants and Units In a
Page 219 and 220: Metric Prefixes These prefixes are
Page 221 and 222: INDEX 221 center command, 50, 61 cl
Page 223 and 224: INDEX 223 Earth’s radio signals,
Page 225 and 226: INDEX 225 irregular galaxy, 147, 17
Page 227 and 228: INDEX 227 entering, 21 every, 206 j
Page 229: INDEX 229 stellar distance uncertai
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