Digital Universe Atlas

Digital Universe Guide

• Contents
• Overview
  • What is the DU?
  • Getting Help
  • Digital Universe License
  • Acknowledgments
• Getting Started
  • Installing
  • Launching the Milky Way
  • Learning to Fly Partiview
  • Partiview's User Interface
  • Digital Universe Files
• Milky Way Atlas
  • Overview
  • Tutorial
  • Data Groups
  • Optimizing
• Extragalactic Atlas
  • Overview & Primer
  • Tutorial
  • Data Groups
  • Optimizing
• Appendix
  • Light-Travel Time
  • Parallax and Distance
  • Redshift and Distance
  • Electromagnetic Spectrum
  • Constants and Units
• Index

• Home
• Digital Universe
• Guide
• AMNH Stars

AMNH Stars

The stars are among the richest, most complex data group in the Atlas. Calculating the distance to stars is exceedingly difficult unless they are very close. For the AMNH star catalog, we set tolerances on the amount of acceptable uncertainty in a star's distance, which then determines the size of our final catalog. How do we determine these distances, though?

Astronomers use something called the trigonometric parallax to determine a star's distance. We discuss this technique in detail in “Parallax and Distance.” The short version is that for nearby stars, astronomers observe a small apparent motion visible only through telescopes. This motion results from Earth's orbit around the Sun, and the angle formed from this motion is called the parallax angle. With a simple geometric argument, the distance to the star can be calculated as one side of a triangle.

Source Catalogs

The AMNH catalog is derived from several catalogs, but the bulk of our stars come from the Hipparcos catalog. The Hipparcos satellite was launched from French Guyana by the European Space Agency in August 1989. It collected data for four years before it was shut down in August 1993, having fulfilled its mission goals. The satellite still orbits Earth at a very high altitude.

Hipparcos was named after the Greek astronomer Hipparchus, who lived in the 2nd century B.C. and who is credited with creating the first star catalog (and inventing trigonometry). The Hipparcos mission's goal was to measure the trigonometric parallax of more than 100,000 stars and the photometric properties (brightness) of half a million stars. The mission was successful, measuring parallaxes for 118,218 stars and photometric data for more than 1 million stars. Hipparcos remains the most successful astrometric mission to date and will likely remain at the top for some time.

Hipparcos was a “targeted mission,” meaning the target stars were determined before the satellite was launched into space. For this reason, Hipparcos did not observe all the nearby stars, as they already had good parallax measurements. Since 1838, astronomers have been measuring parallaxes from ground-based telescopes. The data from these telescopes were compiled in 1969 by Wilhelm Gliese (1915-1993) from the Astronomisches Rechen-Institut, in Heidelberg, Germany, and updated in 1991 by Gliese and Hartmut Jahriess for the third edition of the Gliese Catalog.

The Gliese Catalog contains all known stars (as of 1991) within 25 parsecs (81.5 light-years) of the Sun. The catalog contains more than 3,800 stars and has many that Hipparcos did not observe. We have almost 50 such stars from Gliese in our AMNH star catalog.

Finally, we integrate data from the General Catalog of Mean Radial Velocities. The catalog contains mean radial velocities of more than 36,000 stars. The radial velocity is the velocity of the star along the line of sight, measuring how fast the star is moving toward or away from Earth in the radial direction. This value is measured from the Doppler shift in the star's spectrum.

We can also measure the star's tangential velocity, or proper motion, across the sky. For fast-moving stars, we can see a shift in the star's position over the course of years. The fastest star in the sky is Barnard's Star. The second-closest star, Barnard's is about 6 light-years away and moves 10.3 arcseconds per year (1 degree across the sky every 350 years). This velocity, combined with the radial velocity, yields the star's overall velocity, or space velocity. We then break this velocity into its vector components and include these, in addition to the star's speed, in our catalog.

The AMNH Catalog

We call our catalog the AMNH Catalog because we use a unique method for computing the stellar distance, setting it apart from distances used in other star catalogs.

Distances in the AMNH Catalog result from trigonometric and photometric parallaxes. (Photometric parallax uses a star's brightness and luminosity to derive its distance.) If the star has a measured trigonometric parallax with an uncertainty of less than 20%, the distance is based solely on that measurement. If the uncertainty is greater than this value but less than 75%, we compute the trigonometric parallax distance and the photometric parallax distance, then compute a weighted mean distance based on the uncertainties of each. If the measured trigonometric parallax uncertainty is more than 75%, then we reject the star. There are no stars in our catalog with pure photometric parallax distances.

The distance determination method is traced for each star via the dcalc data variable. If dcalc = 1, the star's distance is derived purely from its trigonometric parallax. If dcalc = 2, the distance is calculated with the weighted mean value of the trigonometric and photometric parallaxes.

All the data variables for the stars data group are listed below. As always, the first three columns of any Partiview file are x, y, and z. Column 4 in the file corresponds to data variable zero, column 5 is data variable 1, and so on. Partiview will generate a report of all data variables for the active data group with the command datavar or the shortened dv command.

Where mag is the magnitude, ly stands for light-year, mas is milli-arcsecond, and pc/Myr is a speed expressed in parsecs per million years.

Labels for the stars include the star names, often Arabic in origin. Names like Betelgeuse, Rigel, and Aldebaran are found in the label file that accompanies this data group. We also provide Greek and Flamsteed labels in the alternate star names data group.

Selection Expressions

Using Partiview selection expressions, we have preset some “scenes” in the mw.cf file. Selection expressions allow you to display data based on some selection criteria. For example, if you want to see only those stars within 100 light-years, you could write a selection expression to remove all the stars outside this radius using Partiview's thresh, only, and sel commands.

In the following table, we list our preset selection expressions. To use these expressions, type see [alias] at the Partiview Command Line (i.e., type see blue).

Boxes

In addition to the selection statements, we have defined a few boxes in the stars group. Boxes are useful for highlighting a subset of data within the group (using the boxes command) or separating data from the group (using the clipbox command). The table shows predefined boxes in the mw.cf file.

Pressing the Box Toggle Button in Partiview will produce two boxes around the Hyades and Pleiades; however, the box around Gould's Belt will be invisible. Use the showbox and hidebox commands to alter which boxes are displayed. showbox 1 will turn on the Gould's Belt box, for example. (Gould's Belt is a grouping of bright stars inclined to the Galactic plane that demonstrate recent star formation.)

The last entry in the table is a clipbox rather than an ordinary box. It removes all data outside the box from the display. Type cb on while stars is the active group to see this box.

© 2002-2005 American Museum of Natural History
Last Modified: 2007-12-19 by Brian Abbott

© 2001-2008 American Museum of Natural History.

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Last modified: 19 December 2007