Digital Universe Guide - Hayden Planetarium

3.3. MILKY WAY DATA GROUPS 67
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 BC and
who is credited with creating the first star catalog (and inventing trigonometry, among other things). 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 only be surpassed by Gaia, which is set to launch
in a couple years.
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 fourth-closest star, Barnard’s is about 6 light-years away and moves
10.3 arcseconds/year (1 degree across the sky every 350 years). This velocity, combined with the radial