Digital Universe Guide - Hayden Planetarium

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84 3. THE MILKY WAY ATLAS 3.3.8 Open Star Clusters Group Name oc Reference Optically visible open clusters and Candidates (Dias+ 2002-2012) Prepared by Brian Abbott (AMNH/<strong>Hayden</strong>) Labels Yes Files oc.speck, oc.label Dependencies none Census 1,041 clusters with labels An open star cluster is a loose assemblage of stars numbering from hundreds to thousands. Unlike in an OB association, the stars in an open cluster are bound by their mutual gravitation. Astronomers know from the stellar spectra that stars in open clusters are typically young. (With the star’s spectrum, we can determine the spectral type and the luminosity class, revealing the age of the star.) Since these are young stars, we expect to see them in the star-forming regions of our Galaxy, namely in the spiral arms. For this reason, open clusters exist, for the most part, in the plane of the Galaxy where we view the arms edge-on as the band of light in the night sky. Because of this, open clusters were originally known as Galactic clusters, but this term fell out of favor once astronomers began to understand that the Galaxy includes objects beyond the Milky Way’s disk. Source Catalog The open cluster catalog was compiled by Wilton Dias and collaborators in Brazil and Portugal. It is a comprehensive collection of data from other catalogs coupled with the latest science and data from ground- and space-based observatories. Data Variables for Open Star Clusters Number Name Description Units 0 diam Angular diameter arcminutes 1 logage Log of the cluster’s age years Exploring the Data As previously mentioned, open clusters are good tracers of local spiral structure. We have included a parameter in the data file to emphasize this point visually. The logage data variable is an indication of where the most recent clusters are forming. A preset selection
3.3. MILKY WAY DATA GROUPS 85 expression (in mw.cf), called young, will remove the older clusters from view, leaving those clusters younger than 20 million years. To see this, type see young at the Partiview Command Line (be sure oc is the active group). The remaining clusters highlight the local spiral arms nicely. From outside the Galaxy, you should see three distinct arms: the Sagittarius Arm toward Galactic center; the Orion Arm, where we live; and the Perseus Arm, toward the outer edge of the Galactic disk. To emphasize the point further, turn on the OB associations. These are in exact agreement with the open clusters, each providing a measure of the Galactic structure in our part of the Galaxy. Selection Expressions for the Open Clusters Alias Partiview Command Description young thresh logage 6 7.3 Show clusters that are less than 20 million years old
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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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3.2. MILKY WAY ATLAS TUTORIAL 29 Wi
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3.3. MILKY WAY DATA GROUPS 135 3.3.
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3.3. MILKY WAY DATA GROUPS 141 this
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3.3. MILKY WAY DATA GROUPS 143 Not
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4 The Extragalactic Atlas The word
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4.1. EXTRAGALACTIC ATLAS OVERVIEW 1
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4.2. EXTRAGALACTIC ATLAS TUTORIAL 1
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4.3. EXTRAGALACTIC DATA GROUPS 169
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4.4. OPTIMIZING THE EXTRAGALACTIC A
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Appendix A Light-Travel Time and Di
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Appendix B Parallax and Distance On
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absolute magnitude M, can be calcul
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discovered that there was a linear
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Appendix D Electromagnetic Spectrum
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Appendix E Constants and Units In a
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Metric Prefixes These prefixes are
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INDEX 221 center command, 50, 61 cl
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INDEX 223 Earth’s radio signals,
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INDEX 225 irregular galaxy, 147, 17
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INDEX 227 entering, 21 every, 206 j
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INDEX 229 stellar distance uncertai
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Digital Universe Guide - Hayden Planetarium

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