Digital Universe Guide - Hayden Planetarium

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132 3. THE MILKY WAY ATLAS 3.3.34 Local Group of Galaxies Group Name lgrp Reference Students for the Exploration and Development of Space (SEDS) Prepared by Brian Abbott (AMNH/Hayden) Labels Yes Files localgroup.speck, localgroup.label Dependencies none Census 46 galaxies and labels The Local Group refers to the few dozen galaxies in our Galactic neighborhood. While these are outside the Milky Way, we include them here to provide a transition to the Extragalactic Atlas portion of the Digital Universe. The group consists of three large spiral galaxies (in order of size): Andromeda, the Milky Way, and M33. The remaining members of the group are dwarf spheroidals, dwarf ellipticals, and irregulars. The galaxies occupy a space of about 6 million light-years (1 Mpc) in diameter. The Andromeda Galaxy and the Milky Way dominate, each with a host of small dwarf galaxies huddled around them. Colliding Galaxies Several galaxies in our neighborhood are interacting with our Milky Way. Astronomers believe that galaxy interactions and mergers are far more common than once thought. For some time, we have understood the involvement of the Large and Small Magellanic Clouds (LMC and SMC) with our Milky Way. These galaxies are irregular galaxies that are about 160,000 light-years from the center of the Milky Way. Both galaxies contain active star formation as well as older stars and globular clusters. Radio observations also show the possibility of a stream of gas connecting them to the Milky Way. The LMC was considered to be our nearest neighbor until 1994, when the Sagittarius dwarf elliptical galaxy (SgrDEG) was discovered. The SagDEG lies opposite the center of the Milky Way and is interacting with our Galaxy. As it orbits the Milky Way, it is stretched apart, leaving streams of stars in its wake. It’s not entirely accurate representing the Sagittarius spheroidal as one point in the Atlas, since these streams are believed to stretch across the Galactic plane over tens of degrees in our sky. The SgrDEG lost the title of closest galaxy in November 2003, when the Canis Major dwarf galaxy was discovered. The Canis Major dwarf is about 25,000 light-years away from the Sun in the constellation Canis Major.
3.3. MILKY WAY DATA GROUPS 133 3.3.35 Planetary Orbits Group Name solsys Reference Jet Propulsion Lab’s HORIZONS Prepared by Brian Abbott, Ryan Wyatt (AMNH/Hayden) Labels No Files solsys-mw.speck Dependencies halo.sgi Census 9 orbits Because the Solar System is among the most familiar and widely understood aspects of astronomy, we have included the paths of the planetary orbits in the Milky Way Atlas. This is a bit unnatural, since the scale of the Solar System is so small when compared with the nearest star to the Sun. But with a few modifications, you can see how the Solar System fits in the overall scale of the Milky Way Atlas. This is not a “working” Solar System. There are no planets that move along these trajectories. Only the orbits themselves are present for context within the Atlas. The orbits’ color coding is loosely based on the planets’ colors. Mercury is gray, Venus is yellow, Earth is blue, Mars is orange, and so on for the outer planets. The data set also includes what we call the mini-sun. This is a smaller Sun exactly like the Sun in the stars group but far less luminous. Recall that the average Earth-Sun distance is defined to be 1 astronomical unit (AU) and equals 149.6 million kilometers (93 million miles). (Yes, that small distance between the Sun and the blue orbit is 93 million miles.) We can also express this distance in light-travel time as 8.3 light-minutes. Light-travel time is the time it takes light to traverse some distance in space. It takes the Sun’s light a little more than 8 minutes to reach Earth. It takes more than 4 hours to reach Neptune. For more on light-travel time, see “Light-Travel Time and Distance.” Seeing the Solar System Group The scale of the Solar System is so small compared with the parsec-scaled Milky Way Atlas (Earth is 0.00000485 parsecs from the Sun) that you must alter the display characteristics of Partiview to see the solsys (and probes) data group. Once you bring up the Milky Way Atlas, follow these steps to view the orbits: 1. Turn on the solsys data group. 2. Change Partiview’s clipping planes using the command clip 1e-5 1e5. You should see the orbits appear. If you see nothing, see the “Troubleshooting” paragraph below.
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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 75 3.3.4
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Page 145 and 146: 4 The Extragalactic Atlas The word
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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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