Digital Universe Guide - Hayden Planetarium

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62 3. THE MILKY WAY ATLAS To confirm the shift, increase the center size using the Censize Slider. You will see Cartesian axes appear at the Galactic center, as well as a half-axes element at the Sun. Upon confirmation, you can reduce the center size back to zero to hide it from view. Now you can orbit the center of the Galaxy. Bring the Galactic center to the middle of your Graphics Window using the Fly Flight Mode if necessary, then switch back to Orbit Flight Mode. The Galactic Center The center of our Galaxy is tumultuous. Within the bright region on the image are billions of stars with densities approaching 50,000 stars per cubic parsec (compare that with the density around the Sun of 0.05 stars per cubic parsec). These high densities ensure stellar interactions and even collisions. Astronomers cannot see the Galactic center in the visible spectrum because of the intervening gas and dust. However, these regions can be probed in the infrared and radio, which are less susceptible to light scattering from dust. At the core of the Galaxy, astronomers observe a swirling ring of gas only a few parsecs across with a strong magnetic field and extended filaments. The energy required to sustain this activity must come from a massive object. But because the gas features are so small, the object driving this activity must be small too. Astronomers currently believe that a super-massive black hole exists at the Galactic center. A black hole is a region of space where gravity is so high that nothing escapes, not even light. Our Barred Galaxy Surrounding the core is the Galactic bar. Astronomers observe noncircular orbits among the stars near the Galactic center and therefore believe our Galaxy is a barred spiral. Turn on the Galactic bar to see the size and shape of this feature. The bar is an ellipsoid with a size of (ax, ay, az) = (5500, 2050, 1350) light-years and its long axis tipped 14 ◦ with respect to the Sun. Inside this ellipsoid are billions of stars and the most active star formation in the Galaxy. This is the Galactic powerhouse, and much of the Galaxy’s activity is driven from this area. Beyond the bar, there is the Galactic disk. The image shows spiral arms with bright H ii regions (star-forming regions like the Orion Nebula) sprinkled throughout them. Depending on what kind of object you consider, the disk varies in thickness. If you consider young stars, gas, and dust in the vicinity of the Sun, the disk is only 300 parsecs (978 light-years) thick. However, if you look at older disk stars in the range of 7 billion to 10 billion years, the disk is around 2 to 3 kpc (6,500 − 9,800 light-years) thick. These stars formed in the disk but over time found their way out of the disk from interactions with
3.2. MILKY WAY ATLAS TUTORIAL 63 molecular clouds and other stars. These are the main elements in the disk component of our Galaxy. Let’s now explore the spherical components of the Galaxy in the next tutorial. Tutorial: The Spherical Component Goals: Explore the spherical component of the Galaxy, including the Galactic bulge and its halo. Before starting, turn on: galaxy You will be using: bulge, halo As you’ve just seen, the disk component of the Galaxy is the lifeblood of the system. This is where stars are forming and where they explode into clouds of glowing hydrogen. The spherical component is from an era when the Galaxy was forming. The stars in this part of the Galaxy are older, cooler stars that formed long ago and have long lifetimes. Let’s explore the structural elements that make up this component of the Galaxy. The Galactic Bulge The innermost component is the Galactic bulge (bulge button). The bulge is the brightest part of the Galaxy and contains the Galactic bar (bar button) and nucleus. While it contains many older stars, it also contains the active Galactic center, where stars form and orbit in the disk component. Overall, the stars in the bulge are more metal-rich and younger than those found in the halo, the region surrounding the bulge. The Galactic Halo The Galactic halo (halo button) is a large, roughly spherical volume that encompasses the entire Galactic disk. The halo is filled with old, faint stars and globular clusters. Recall that the globulars are densely packed clusters of 100,000 to 1 million stars. The stars in the halo and inside globular clusters are metal-poor, older stars that formed close to the era of Galaxy formation. The shape is thought to be slightly flattened like the bulge but is spherical to first order. The size of the halo remains under study. In the Milky Way Atlas, we set the radius to 41,000 pc (about 130,000 light-years). If you have the globular clusters on, you’ll see that most of the clusters lie within the halo but that a handful do not. Do these clusters belong to our Galaxy? Is the halo significantly larger, as some astronomers suggest? The answers to these questions will be found in two areas: in the
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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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3.3. MILKY WAY DATA GROUPS 141 this
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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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