ChView data files are created from catalogs of nearby stars assembled by astronomers, past and present, that are now available in machine-readable format as well as in print. These files include the 1991 ("preliminary") versions of the Yale Bright Star Catalogue and the Third Catalogue of Nearby Stars (CNS3), which are compiliations of older and smaller star catalogues. In addition, ChView files are being updated and expanded using new trignometric parallax measurements of distances from Earth made available by the European Hipparcos satellite mission. (See References for further information on data sources.)
Several files of increasing size with updated Hipparcos distances are now available from Downloads:
In addition, two older files using non-Hipparcos parallax data (mostly Yale trigonometric measurements) have been retained:
STAR NAMES & CATALOGUE NUMBERS. The hard work of many astronomers is often memorialized in the name of a star or its catalogue number. In general, a star without a known proper name is assigned a catalogue number -- preferably a relatively old one -- in the star name area. Alternative names and catalogue numbers are provided in the"Notes" section of each star's record in the data files. Brief explanations of stellar nomenclature, a key to abbreviations of source catalogue names, and other information available for each star can be found in Stars.
DISTANCE. Distances estimated using the trigonometric parallax method are inherently less accurate for stars that are further away from Earth. Earth-based measurements become particularly unreliable for stars that are beyond 200-250 ly from Earth, although distances to some particularly interesting stars have been revised using other methods. However, parallax data collected by the spaced-based, European Hipparcos satellite mission from 1989 to mid-1993 have added, as well as improved many previous, distance estimates for relatively bright stars by avoiding the problems of atmospheric distortion. (In the current files, the distances of all Hipparcos catalogue stars within 100 ly and of some other stars within 13 ly have been given two decimal places for easier differentiation from other stars possibly lacking similarly precise parallax measurements. In general, ChView users should round up at least one decimal place for distance and positional values.)
Accurate parallax measurements are particularly difficult to obtain for intrinsically faint stars, and Hipparcos lacks data for very close by but extremely dim stars such as Wolf 359. Although repeated observations over longer periods of time can improve precision, only Earth-based telescopes can provide such data today. As a result, some of the stars within 13 ly in the data files use trigonometric measurements from the 1995 Yale parallax catalog (van Altena et al) although Hipparcos data is available.
While the Hipparcos mission does improve distance estimates significantly for thousands of nearby bright (and many dim stars) not previous measured or not repeatedly measured over a long period of time, some bright stars known to be relatively close to Earth were not observed at all, or were observed not well enough for calculating accurate parallax. This may have occurred because the rocket that launched the Hipparcos satellite failed to insert it into the planned geostationary orbit. As a result, some nearby bright stars not listed in the Hipparcos catalogue can be found in the less precise Tychos Catalogue. For example, BD+37 3812 (listed as a G8V, main sequence dwarf in CNS3 but was once classified as a K1IV subgiant) was relegated to the Tychos catalogue -- also noted in Chview as Tyc 3151-2837-1 with its Tycho parallax and standard error (TycPlx=34.60, e=4.40 milli-arcseconds).
One interesting nearby, multiple star system that appears to lack Hipparcos parallax altogether -- that is not even listed in the Tycho catalogue -- is Alula Australis, or Xi Ursae Majoris. This star system is listed in the 1991 Yale Bright Star catalogue as HR 4375 (a variable G0Ve and spectroscopic double) and HR 4374 (a F8.5Ve and spectroscopic double) as only being about 24 ly away, with four apparent visual (optical?) companions. Recently, astronomers detected a brown dwarf companion of HR 4374 (also listed in the Henry Draper catalogue as HD 98230) which has been designated by its discoverers as HD 98230 B; this substellar object has a minimum mass equivalent to at least 37 Jupiter-sized planets but moves in a very close orbit of only 0.06 astronomical units (closer than Mercury) to its host star with a period (or local "year") of less than four days.
POSITION. All stars in the data files with a Hipparcos number (Hip #) and/or a bright star number (HR #) have x,y,z equatorial coordinates derived from Star Date (J) 2000 Right Ascension (RA) and Declination (Dec) positions, as well as recent distance measurements from Earth. In general, the position of relatively brighter stars (excluding most M and some more distant K and G dwarf stars) and most stars within 13 ly are based use Star Date 2000 RA and Dec. On the other hand, the position of many dim M dwarfs and white dwarfs farther than 13 ly away, and of some more distant GK dwarfs, are calculated from Star Date 1950 (and other Star Date) RA and Dec positions, and so do not account for small positional changes caused by the Earth's precession and by net stellar movements relative to Sol up to Year 2000.
MASS. Relatively few stars in the data files have a value for mass (as a ratio of Sol's) derived from orbital calculations for binary systems and other methods. For some of the brighter main-sequence stars, a rough estimate of stellar mass derived from absolute visual magnitude -- not the more complete wavelength accounting of bolometric magnitude -- or luminosity from a print source was used, if it seemed reasonably close to theoretical values for particular main-sequence spectral types. Those seeking more precise estimate of mass should consult the latest astronomical study on the star of interest.
SPECTRA & LUMINOSITY. Where differing spectral types and luminosity classes were assigned to a star by recent catalogues, a range is indicated (e.g., F8-G0V-IV) to avoid the loss of possibly useful information -- for example, indicating variability or subgiant pulsation rather than simple observational error or the imprecision of older equipment and methods. However, the spectral type of 92 late-type dwarfs (K5-M8) visible from the northern hemisphere and believed to be located within 26 ly (8 parsecs) -- plus another nine suspected nearby dwarfs -- were revised to a standard system presented in 1994 (Henry et al). Some stars lack any spectral type information at all (including many binary companions that are dimmer than the primary star). At the request of some ChView users, the relatively minor share of stars within 100 ly of Earth that lack an assigned luminosity class (mostly M and some K dwarfs) were marked main sequence (V) by default, since few nearby stars are likely to be subdwarfs or to have evolved off the main sequence at such a relatively close distance without noticeably high luminosity or obscuring dust. On the other hand, stars lacking luminosity class information that are believed to be located farther away than 100 ly were not given a default designation, since they include many FGK stars which may be subgiants or even more highly evolved, giant and supergiant stars.
OTHER DATA. The "Notes" field of each star's record in ChView files may contain other available information including: off-main-sequence status, variability, rare stellar population type, heavy elements in the star's atmosphere, mass, diameter, optical and gravitationally-bound companion objects and orbital characteristics, and alternative names and catalogue numbers.
FILE COMPLETENESS. A 1997 paper by astronomers (Henry et al) associated with the Research Consortium on Nearby Stars (RECONS) suggests that the sample of stars known to lie within 10 parsecs (32.6 ly) of Earth is "woefully incomplete," particularly in faint red (M) dwarfs and "white" dwarfs. Assuming that the density of star systems within five parsecs extends to 10 parsecs, 130 star systems (over half) were estimated to be missing from the total of 234 found by May 1998, including four nearby stars or star systems recently found to be closer to Earth by RECONS -- one in the published literature. The under-representation of M dwarfs in the known sample is "particularly evident in the southern sky where few follow-up observations of candidates have been done." On the other hand, small, faint, red M dwarfs dominate the solar neighborhood, account for at least 70 percent of all stars, and comprise nearly half of our galaxy's estimated total stellar mass. [The 1997 paper and related studies can be found at the RECONS home page.]
Although the updated data files contain nearly 200 individual stars within 25 ly from Earth in a 50 ly-diameter sphere, the 1997 RECONS paper suggests that the actual number of nearby stars may be roughly 50 percent more -- over 300. Geometrically, the volume of space increases roughly eight times for every doubling of the distance from Earth. Hence, assuming the same stellar density at increasing distances from Earth, there may be over 2,400 stars within 50 ly of the sun, roughly 20,000 within 100 ly, over 150,000 within 200 ly, and so on.
Let's assume that there are actually around 300 individual stars located within 25 ly of Earth. Then, the 50 ly file (with over 1,100 stars) may actually contain less than half of the stars that actually exist within 50 ly from Earth. One could safely presume that virtually all of these missing stars are main sequence M dwarfs and some relatively faint white dwarfs that remain to be discovered, or assigned more accurate distances. At over 4,600 stars, however, the 100 ly file has less than one fourth of the stars that should lie with a sphere of 100 ly from Earth; at this point, some G and more K stars may be missing -- particularly, in the spherical shell between 50 and 100 ly from Earth. With roughly 8,700 stars, the new 150 ly file may contain less than a sixth of the stars that may actually lie within that vast space -- a 300 ly-diameter sphere. By comparison, the old 250 ly file, which does not contain many main sequence stars (including many FGK dwarfs that could have Earth-like planets) that may now have Hipparcos parallax measurements within that distance, probably contains less than 1/30th of the stars that may be located within that distance.
Without a doubt, the more distant stars (beyond 100 ly) that have been included in the data files tend to be unusually large, luminous, and rare. Due to their extreme mass or late stage of evolution, many also have been targets of special studies by astronomers. Distances to these bright stars, moreover, are often easier to measure with telescopes because of their higher luminosity. Most are visible to the naked eye -- many with ancient names -- and many can serve as guide stars ("landmarks") for imaginary voyages among the vast majority of stars too dim to be seen without a telescope.
An update of the older 250 ly data file -- 250ly.chv -- will likely include more main sequence FGK dwarfs previously lacking parallax measurements (and recently discovered extra-solar planets and brown dwarfs) that are now believed to lie with 250 ly of Earth. However, Hipparcos measurements probably revise distances for many subgiants, giants, supergiants, and hypergiants somewhat farther out than indicated by recent bright star catalogues. The 250 ly file currently contains:
|1||Possible Wolf-Rayet star - Suhail (Gamma2 Velorum).|
|3||Possible Hypergiants - CD-41 4507 and V810 and Omicron1 Centauri.|
|93||Possible Bright Giants.|
|5,100+||Main Sequence Dwarf stars.|
|10||Stars with noticeable dust and/or dust disks.|
|35||Possible extrasolar planets (mostly gas giants) or brown dwarfs; many confirmed since summer 1995.|
|7||Nearby flare stars.|
|2||Stars with an x-ray source that may be a neutron-star or black-hole companion. (Unfortunately, these two red dwarf stars lack a catalogue number -- Not Numbered or "NN" -- in the 1991 Catalogue of Nearby Stars.)|
Examples of all these celestial objects are discussed in Stars, Extrasolar Planets, and Glossary.
FILE TYPES & CONVERSION. Information on ChView ASCII and binary file structure and file conversion is provided in APPENDIX A - ASCII File Format and APPENDIX B - Binary File Format.