Digital Universe Guide - Hayden Planetarium

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48 3. THE MILKY WAY ATLAS thresh distly 0 100 Now you see a nice sphere of stars surrounding the Sun. Turn on the ecliptic coordinates, eclip, which are traced on a 100-light-year-radius sphere. As you orbit, notice the midplane of the sphere (the sphere’s “equator”). This is called the ecliptic, the plane that contains Earth’s orbit around the Sun and generally defines the plane of the Solar System. Stellar Distances Are Determined by Parallax Return all stars to view (see all) and let’s now look at the parallax. A detailed definition of parallax and how astronomers calculate a star’s distance can be found in “Parallax and Distance.” Briefly, parallax is an angle astronomers measure that is used to solve the lengths of a right triangle, thereby yielding a star’s distance (which is one side of the triangle). This angle comes from Earth’s path around the Sun. If a star is close, it will appear in one place with respect to the background stars (those stars that surround it in the sky but are farther away). Six months later, it will appear to have moved ever-so-slightly relative to the background stars. This motion is so small that astronomers need powerful telescopes to detect it. In the Atlas, we can look at the distance of stars in terms of the parallax angle. The range of values shown in the report generated by the datavar command is 0.64 to 722.33 milliarcseconds (mas) with a mean of 8.35 mas. The closer the star, the larger this angle will be, so let’s look at stars with large parallax angles: thresh plx 100 800 There are 187 stars in this range resembling our view of the stars within 25 light-years. Parallax Uncertainty and the Plane of the Solar System Associated with each of these parallax measurements is an uncertainty. These uncertainties come from the methods and instruments used to measure the parallax, for no measurement is free from uncertainty. To see how this uncertainty propagates through the stellar catalog, let’s look at those stars with small errors. The parallax uncertainty ranges from 0.19 − 76.92 mas, with a mean of 24.7 mas. Look at the stars with very little uncertainty using the command thresh plxerr 0 3
3.2. MILKY WAY ATLAS TUTORIAL 49 This shows stars with highly accurate distances. As you might expect, these are nearby stars, but you’ll notice they are not distributed in a sphere. The reason for this is somewhat obscure. If you still have the ecliptic coordinates on, you’ll notice that the long axis of symmetry of the star distribution intersects the ecliptic poles. Think about what astronomers observe on Earth. As Earth orbits the Sun, the parallax of a star near the ecliptic pole (not the celestial pole) will make a circle in the sky. Conversely, if you’re looking at a star near the ecliptic (in the same plane), it will appear to move back and forth along a line. Between these extremes, the parallax motion is in the shape of an ellipse. It’s easier to measure the width of an angle formed by a circle than a line, so the uncertainty for a star in the ecliptic will be more than that of the same star at the ecliptic poles. Therefore, our distances are more accurate at the poles of the ecliptic coordinates than near the ecliptic itself. Let’s see how this uncertainty in the measured parallax angle translates into distance uncertainty in the Atlas. Tutorial: Distance Uncertainty Goals: Be aware of the underlying uncertainties in the stellar catalog. Before starting, turn on: stars, mwVis You will be using: altLbl, center and censize commands, 10ly, err You’ve seen how the measured parallax angle determines the distance to a star and how the uncertainty in that measurement propagates into the stellar data. Now you’ll see a visual representation of these uncertainties for select stars. Let’s go home in the Milky Way Atlas (press the Home Button). (If you’re continuing from the last tutorial, reset the stars back to their normal brightness and use see all to return all stars to view.) Instead of backing away from the Sun, let’s fly forward toward the Hyades star cluster. If you don’t know where the Hyades is, turn on the open clusters, oc, for a moment to find the cluster. Once you’ve centered the cluster, fly forward by moving the mouse to the right (in Orbit Flight Mode) while pressing the right mouse button. It may take a few seconds to get going since you’re so close to the Point of Interest. The Hyades forms a “V” shape. As you fly out, you’ll see what appeared to be the brightest star in the cluster, Aldebaran, pass by long before reaching the cluster. At 65 light-years, it’s not actually in the cluster but sits between the cluster and Earth. Once you get to the cluster, stop. You’re going to change
Page 1 and 2: The Digital Universe Guide Brian Ab
Page 3 and 4: Contents Contents 3 List of Tables
Page 5 and 6: List of Tables 2.1 Log and Linear F
Page 7 and 8: 1.2. GETTING HELP 7 1.2 Getting Hel
Page 9 and 10: 1.5. ACKNOWLEDGMENTS 9 1.5 Acknowle
Page 11 and 12: 2.2. LAUNCHING THE MILKY WAY ATLAS
Page 13 and 14: 2.3. LEARNING TO FLY PARTIVIEW 13 T
Page 15 and 16: 2.3. LEARNING TO FLY PARTIVIEW 15 M
Page 17 and 18: 2.4. PARTIVIEW’S USER INTERFACE 1
Page 23 and 24: 2.5. DIGITAL UNIVERSE FILES 23 File
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Page 67 and 68: 3.3. MILKY WAY DATA GROUPS 67 Earth
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3.3. MILKY WAY DATA GROUPS 99 3.3.1
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3.3. MILKY WAY DATA GROUPS 101 3.3.
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3.3. MILKY WAY DATA GROUPS 103 heat
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3.3. MILKY WAY DATA GROUPS 107 righ
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3.3. MILKY WAY DATA GROUPS 121 Othe
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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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