Digital Universe Guide - Hayden Planetarium

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144 3. THE MILKY WAY ATLAS 3.4 Optimizing the Milky Way Atlas While technology is quickly catching up with the graphics power our Atlas demands, some people still find that the Atlas runs sluggishly on their computers. There are a few things you can do to reduce your computer’s strain. Threshold the Stars The most common culprits in the Milky Way Atlas are the stars. We distribute a star catalog with more than 100,000 stars, mostly from Hipparcos and a few from the Gliese Catalog. Many computers cannot handle all of these stars with their points and textures. Using the Partiview command thresh, you can remove many of the stars from view, improving the performance. We have placed many preset threshold statements for the stars in the mw.cf file. Some threshold distance, others the star’s color. One threshold useful for eliminating most of the stars without altering the night sky view is a threshold on the stars’ brightness, called the apparent magnitude. The system of stellar magnitudes has its roots in ancient times. The scale is such that the brighter the object is, the smaller its apparent magnitude is. The brightest object in the sky, the Sun, has a magnitude of −26; the moon’s is −12. The brightest star in the sky other than the Sun, Sirius, has a magnitude of −1. Our eyes can see to about +6.5 magnitude, so thresholding the stars that are dimmer than this will preserve the night sky while cutting out many of the dimmer stars. We have set up a threshold statement in the mw.cf file that removes all stars over 7.5 apparent magnitude. To activate it, type see m75 at Partiview’s Command Line. Alternatively, you could enter the command by typing thresh appmag < 7.5. This leaves you with about 25,000 stars and, we hope, improved performance. If you are still having troubles, though, please read on. Save Your Changes Once you have the settings configured for your system, open the mw.cf file in a text editor and alter the file to suit the improved custom configuration.
4 The Extragalactic Atlas The word “extragalactic” means everything outside the Milky Way, our home galaxy. Extragalactic space consists mainly of galaxies, the building blocks of the Universe. Within the observable Universe (that part of the Universe visible to us), astronomers estimate there to be 40 billion to 100 billion of them. Imagine, 40 billion galaxies with billions of stars each. If even a tiny fraction of those stars are host to planets, there would be trillions and trillions of planets in the Universe. The Digital Universe allows us to view these galaxies in three dimensions, revealing the weblike distribution of galaxies in our Universe. In the Milky Way Atlas we adopted the notion of a solar neighborhood. In the Extragalactic Atlas we expand on that idea and discuss our Galactic neighborhood. Beyond the neighborhood, you may wonder whether we are located in an action-packed region of the Universe or on the outskirts looking in. You may even contemplate some age-old questions: How big is the Universe? Where are we in the Universe? Is there other life in the Universe? These and other questions will be addressed in this chapter. Some will be answered, while others remain beyond our reach. We will start this chapter with a review of the objects found in extragalactic space and briefly discuss the origin and structure of the Universe. We will then turn to a series of tutorials in “Extragalactic Atlas Tutorial,” where we discuss the issues specific to the Atlas. Finally, we present an overview of each data set in “Extragalactic Data Groups.” 145
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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 63 mo
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3.2. MILKY WAY ATLAS TUTORIAL 65 fa
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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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3.3. MILKY WAY DATA GROUPS 71 The l
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3.3. MILKY WAY DATA GROUPS 73 3.3.3
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3.3. MILKY WAY DATA GROUPS 77 If yo
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3.3. MILKY WAY DATA GROUPS 79 lumin
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3.3. MILKY WAY DATA GROUPS 83 Two p
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4.3. EXTRAGALACTIC DATA GROUPS 195
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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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