Selecting Target Stars
Life as we know it developed on a planet orbiting a G2 V star, the Sun. The cryptic “G2 V” designation is the Sun’s “spectral type.” Based on a star’s spectrum, astronomers group stars by temperature. From hottest to coolest, the spectral classes are O, B, A, F, G, K, and M. Each class is subdivided into ten and numbered from 0 to 9. The Sun is a G2 star. The “V” is the Roman numeral for five and designates the Sun’s luminosity class. Stars in luminosity classes I, II, and III are “giant” stars, very luminous and nearing the end of their life as a star. Class IV stars are “sub-giant” stars that are just entering “old age” and as the name implies, large but not giant. Stars in luminosity class V, like the Sun, burn only hydrogen in their cores and are relatively stable.
It is generally agreed that stars with spectral types from about F5 through K5 may be suitable hosts for habitable planets. Some recent studies indicate the some cooler stars, perhaps to spectral type M4, also may host habitable planets.
The HabCat Catalogs
In 2003, Margaret Turnbull and Jill Tarter published two lists of selected stars. The Nearby Habitable Systems (HabCat1) was created from the Hipparcos Catalogue by examining the information on distances, stellar variability, multiplicity, kinematics, and spectral classification for the 118,218 stars contained therein. They also made use of information from several other catalogs containing data for Hipparcos stars on X-ray luminosity, Ca II H and K activity, rotation, spectral types, kinematics, metallicity,
and Stromgren photometry. Combined with theoretical studies on habitable zones, evolutionary tracks, and third-body orbital stability, these data were used to remove unsuitable stars from HabCat, leaving a residue of stars that, to the best of our current knowledge, are potentially habitable hosts for complex life. The resulting HabCat1 catalog contains 17,133 well-selected “habstars”.
Since we need about one million target stars to fully utilize the capability of the ATA, a second catalog of stars was derived from the Tycho-2 Catalogue of 2.5 million stars. Unlike the Hipparcos stars, the Tycho stars did not have distance measurements. The approximately 250,000 stars of HabCat2 were selected primarily by their colors (brightness in blue and “visual” filters) and proper motion (motion across the sky).