The Astronomy Nexus

The current version of the database is now hosted at Github as well as here.

The HYG database (v3.5) and Augmented Tycho-HYG (v1.0) catalogs are compilations of interesting (to me, anyway) stellar data from a variety of catalogs. They are useful for background information on all sorts of data: star names, positions, brightnesses, distances, and spectrum information. A version of the HYG catalog also powers the charts elsewhere on this site.


Choose the version of the database that best serves your needs:

  • AT-HYG 1.0: Current version, containing all valid stars in Tycho-2, with Gaia DR3 distances when available, as well as many fields from HYG for stars that have either HIP or Henry Draper IDs (2.55 million stars, 140 MB)
  • HYG 3.5: Current version, containing all stars in Hipparcos, Yale Bright Star, and Gliese catalogs (almost 120,000 stars, 14 MB)
  • HYG 3.0: Previous major version, containing the same stars but with somewhat less data (especially proper names).
  • HYG 2.0: Older version of the full Hipparcos/Yale/Gliese database (almost 120,000 stars, 9 MB)
  • HYG v 1.1 database: Database containing all stars brighter than magnitude +9.0, or closer than 50 parsecs.(87476 stars)

About the HYG and AT-HYG Databases

HYG (Hipparcos, Yale, Gliese)

The HYG database is a subset of the data in three major catalogs:

  1. the Hipparcos Catalog
  2. the Yale Bright Star Catalog (5th Edition)
  3. and the Gliese Catalog of Nearby Stars (3rd Edition).

Each of these catalogs contains information useful to amateur astronomers:

  • The Hipparcos catalog is, up until the late 2010s, the largest collection of high-accuracy stellar positional data, particularly parallaxes, which makes it useful as a starting point for stellar distance data.
  • The Yale Bright Star Catalog contains basic data on essentially all naked-eye stars, including much information (such as the traditional Bayer Greek letters and Flamsteed numbers) missing from many other catalogs
  • The Gliese catalog is the most comprehensive catalog of nearby stars (those within 75 light years of the Sun). It contains many fainter stars not found in Hipparcos.

The name of the database comes from the three catalogs comprising its data: Hipparcos, Yale, and Gliese.

All told, v 2.x of this database contains ALL stars that are either brighter than a certain magnitude cutoff (magnitude +7.5 to +9.0) or within 50 parsecs (about 160 light years) from the Sun. The current version, v. 3.5, has no magnitude cutoff: any star in Hipparcos, Yale, or Gliese is represented.

The database is a comma separated values (CSV) file that can be imported into most database and spreadsheet programs. On this web site it is stored as a Zip file or a GZ file, which most popular unzippers can open.

AT-HYG (Augmented Tycho - HYG)

The Augmented Tycho catalog is a project I've been working on for a few months. It's HYG, plus more ... a lot more.

The primary goal was to take advantage of the spectacular data sets from Gaia, especially the Data Releases 2 and 3 (DR2 + DR3), which contained over 1 billion exceptionally precise distance measurements to stars throughout our galaxy. I also wanted something larger than the older HYG catalog, since I first developed it over 15 years ago, when limits on computer storage and processing ability were more significant.

The Tycho-2 catalog was a suitable happy medium: at 2.5 million stars, it has more than enough for typical charting applications, and more than 20 times as many as HYG, but the data set is still manageable by fairly small applications in the 2020s.

Details of the construction of the catalog are on Github, both at a summary level and a more detailed level. The TL;DR version is:

"all of Tycho-2, which is essentially complete to V = 11.0 and has many fainter stars down to about V = 12.5, with Gaia DR3 distances for nearly all of them, and interesting historical and cultural information from HYG for the ones with a HIPPARCOS or a Henry Draper number."

Version Information

AT-HYG Database: Version 1.0

AT-HYG is in its first full release: v1.0. Previous alpha versions (0.x) had the same fields, but did not have the same level of data quality or comprehensiveness, and are not being supported further (they are currently available on Github if you're curious).

Version 1.0 contains the following fields:

  1. `id`: A numeric ID for each star, after sorting all entries by right ascension.
  2. `tyc`: The Tycho-2 ID, with leading zeros removed from the first and second portion (for consistency with Gaia linking tables)
  3. `gaia`: The Gaia Data Release 3 ID.
  4. `hyg`: The HYG main catalog ID from HYG v3.
  5. `hip`: The HIPPARCOS ID, from HYG if known, otherwise Tycho-2.
  6. `hd`: The Henry Draper (HD) catalog ID, from HYG if known, otherwise Tycho-2.
  7. `hr`: The Harvard / Yale Bright Star Catalog ID, from HYG.
  8. `gl`: The Gliese ID, from HYG
  9. `bayer`: The Bayer (Greek letter) designation, from HYG
  10. `flam`: The Flamsteed number, from HYG
  11. `con`: The three-letter constellation abbreviation, from HYG
  12. `proper`: A proper name for the star, from HYG
  13. `ra`: Right ascension (epoch + equinox 2000.0), in hours, from HYG or TYC
  14. `dec`: Declination (epoch + equinox 2000.0), in degrees, from HYG or TYC
  15. `pos_src`: Indicator of source for the position fields `ra` and `dec` (see below)
  16. `dist`: Distance from Sol in parsecs. From Gaia if known, otherwise HYG.
  17. `x0`
  18. `y0`
  19. `z0`: These three fields are Cartesian coordinates. The directions are such that x is towards RA 0, Dec 0, y towards RA 6 hr., Dec 0, and z towards Dec 90 degrees.
  20. `dist_src`: Indicator of source for the distance fields `dist`, `x0`, `y0`, `z0` (see below). `x0`, `y0`, and `z0` also depend on `ra` and `dec`, so they will also be determined by the position source. An extremely common combination is raw distance from Gaia but the position from TYC.
  21. `mag`: V or VT magnitude for the star
  22. `absmag`: Corresponding absolute magnitude
  23. `mag_src`: Indicator of source for the magnitude field `mag` (see below). `absmag` depends on both apparent magnitude and distance, so may be determined by values from two sources.

Fields from HYG are from HYG v3.5 in this version.

AT-HYG Database Construction

This is outlined in very great detail in the official Github repo. A capsule summary is:

  • Start with the following catalogs:
    1. HYG v3.5, from this site.
    2. The full Tycho-2 catalog available here, and the first Tycho-2 Supplemental catalog, available here, combined into a single catalog.
    3. A link table catalog between Tycho-2 and Henry Draper, available here.
    4. A link table catalog between Tycho-2 and Gaia DR3. I used the query facility at this site for the task. The detailed query is at the Github repo, in case you are curious, but the short version is: "link the table gaiadr3.gaia_source_lite to the table gaiaedr3.tycho2tdsc_merge_best_neighbour" to associate Gaia DR3 with Tycho-2 source IDs.
  • Merge the Tycho-2 catalog against the Gaia link table to get Gaia distances.
  • Merge the Tycho-2 catalog agains the HD link table to get a bunch of Henry Draper cross-references.
  • At this point you have the Augmented Tycho or AT catalog, leaving the merge with HYG data to complete.
  • Merge as many HYG catalog records as possible via either the HIP or the HD IDs.
  • Merge as many non-HIP/non-HD items (mostly from Gliese) as possible by using SIMBAD to find Tycho-2 or GAIA IDs for those entries.

HYG Database

Version 3.x: The field content is very nearly the same as in Version 2, but the column headers are somewhat different, and a few extra fields (for variable star range and multiple star info) have been added to the end of each record. For a full list of the updated column names, see the official database documentation on Github.

En route to version 3.5, there have been some significant changes, which are outlined in more detail on Github. The main ones are:

  • In v3.1, all the proper names for stars determined by the IAU's Working Groups on Star Names have been included. Versions of HYG prior to v3.1 had only a fairly small subset.
  • In v3.1, the deprecated "NN" and "Wo" prefixes for stars in the Gliese catalog were replaced with "GJ" (Gliese-Jahreiss), the preferred designation for the catalog's later entries.
  • In v3.1 through v3.4, a few additional star catalog IDs or designations were added as proper names to some very nearby stars, such as "Ross 128" and "EZ Aqr".
  • In v3.2, a few HIP IDs that were found to be invalid were deleted (without renumbering StarIDs, to preserve the remaining IDs in 3.2 and later versions)
  • In v3.3, a few errors with HIPPARCOS - Henry Draper ID cross-references were found, and corrected.
  • In v3.4, a few errors with HIPPARCOS - Gliese ID cross-references were found and corrected. In addition, a few Gliese stars were found to be incorrectly associated with a HIP ID. Since the HYG catalog is supposed to be as complete as possible, the Gliese stars in question were added as entirely new entries at the end of HYG as of v3.4.
  • In v3.5, a few more errors in HIP->HD cross-references were found and corrected.
Fuller details about all these changes are detailed at the Github repo in the "hyg" section.

Currently, versions v3.1 through v3.4 are only available at the Github repo. I have retained version 3.0 on this site because it was the most current version for over 8 years, and it may make sense for some older applications, but new applications should use v3.5 (or later, if applicable).

  1. StarID: The database primary key from a larger "master database" of stars.
  2. HD: The star's ID in the Henry Draper catalog, if known.
  3. HR: The star's ID in the Harvard Revised catalog, which is the same as its number in the Yale Bright Star Catalog.
  4. Gliese: The star's ID in the third edition of the Gliese Catalog of Nearby Stars.
  5. BayerFlamsteed: The Bayer / Flamsteed
    designation, from the Fifth Edition of the Yale Bright Star Catalog. This is a combination of the two designations. The Flamsteed number, if present, is given first; then a three-letter abbreviation for the Bayer Greek letter; the Bayer superscript number, if present; and finally, the three-letter constellation abbreviation. Thus Alpha Andromedae has the field value "21Alp And", and Kappa1 Sculptoris (no Flamsteed number) has "Kap1Scl".
  6. RA, Dec: The star's right ascension and declination, for epoch 2000.0. Stars present only in the Gliese Catalog, which uses 1950.0 coordinates, have had these coordinates precessed to 2000.
  7. ProperName: A common name for the star, such as "Barnard's Star" or "Sirius". I have taken these names primarily from the Hipparcos project's web site, which lists representative names for the 150 brightest stars and many of the 150 closest stars. I have added a few names to this list. Most of the additions are designations from catalogs mostly now forgotten (e.g., Lalande, Groombridge, and Gould ["G."]) except for certain nearby stars which are still best known by these designations.
  8. Distance: The star's distance in parsecs, the most common unit in astrometry. To convert parsecs to light years, multiply by 3.262. A value of 10000000 indicates missing or dubious (e.g., negative) parallax data in Hipparcos.
  9. Mag: The star's apparent visual magnitude.
  10. AbsMag: The star's absolute visual magnitude (its apparent magnitude from a distance of 10 parsecs).
  11. Spectrum: The star's spectral type, if known.
  12. ColorIndex: The star's color index (blue magnitude - visual magnitude), where known.
  13. * X,Y,Z: The Cartesian coordinates of the star, in a system based on the equatorial coordinates as seen from Earth. +X is in the direction of the vernal equinox (at epoch 2000), +Z towards the north celestial pole, and +Y in the direction of R.A. 6 hours, declination 0 degrees.
  14. * VX,VY,VZ: The Cartesian velocity components of the star, in the same coordinate system described immediately above. They are determined from the proper motion and the radial velocity (when known). The velocity unit is parsecs per year; these are small values (around 10-5 to 10-6), but they enormously simplify calculations using parsecs as base units for celestial mapping.

Fields labeled with "*" exist only in version 2.0 or higher. Also, since I used a larger set of data for this version, the StarID differs from versions 1.*

HYG Database Construction

I came up with this database while creating the 3D Universe web site. I needed a reference that would let me search for groups of stars by magnitude or distance, while giving me more information than was contained in any one catalog.

I started with the Hipparcos data. The Hipparcos data set represents by far the most comprehensive collection of stellar distance and brightness data in existence, except for very low-luminosity stars. Essentially all naked-eye stars (in fact, most stars down to about apparent magnitude +9 and many others down to about +11) are represented in the Hipparcos catalog.

In older versions of the dataset, I first prepared a subset of the Hipparcos data. I did this for boring technical reasons that no longer apply, so version 2.0 uses the entire Hipparcos catalog.

I next consulted the Gliese catalog to fill in gaps in the Hipparcos catalog, and to add various Gliese data to the catalog. In particular, the Gliese catalog ID is a common reference for nearby stars, and the Gliese catalog contains radial velocity data, which Hipparcos lacks. Additionally, though Hipparcos distances are generally superior to Gliese data, the Hipparcos catalog missed many nearby stars that were below its magnitude cutoff.

To cross-reference stars, I used the Henry Draper catalog number, whenever present, to add Gliese data to the Hipparcos catalog. Many of the faintest stars lacked this catalog number, so I compared the positions and brightnesses of Gliese stars to those in Hipparcos, and if they matched to within a certain tolerance, I assigned the appropriate Gliese data to the Hipparcos star. Stars that failed both references were then added to the end of the Hipparcos list.

I also converted Hipparcos apparent magnitudes to Gliese values for all components of known multiple stars in the latter catalog. Again, the Hipparcos magnitude measurements are often superior, but the Hipparcos catalog treats multiple stars inconsistently. In particular, it breaks some out as separate stars (e.g., Alpha Centauri) but merges others (such as Capella, 70 Ophiuchi, and many others). By contrast the Gliese catalog breaks all known multiple stars, excluding those too close to be separated optically, into their components, and gives each one a magnitude.

I then calculated absolute magnitudes for all stars, added those to the database, and added about 250 proper names. Then, again using Henry Draper as a cross reference, I added data from the Yale Bright Star Catalog: HR numbers, radial velocities (if not already added from Gliese), and the Bayer and Flamsteed designations. Finally, I added a number of radial velocities from the Wilson Evans Batten catalogue to stars that didn't already have that information.

These steps resulted in the full database. To make the various subsets, I took the resulting database and extracted subsets of the data.

HYG Database Quality Issues

With over 100,000 stars to worry about, I generally couldn't go in and edit suspect records by hand. Consequently, there are some issues that serious users may want to be aware of:

  • The spectral types, in general, come from the Hipparcos catalog. A few stars -- those found only in Gliese, have a spectral type from that catalog. The spectral types from Hipparcos have not been closely vetted and I have already found some probable errors. For example, the spectral type of 36 Ophiuchi B (a double star that was merged in Hipparcos) is given as K2 III (giant), when its luminosity clearly indicates K2 V (main sequence). Also, the star HIP 84720 (Gliese 666 A) is listed as M0 V, whereas its luminosity and color index are more consistent with a late G-type star (about G8 V). M0 V appears to be the spectral type of Gliese 666 B, a companion to this star. Use the spectral types with caution.
  • There may be errors in the Henry Draper numbers in one or more catalogs, leading to false cross-references.
  • There may be errors in the matching of Gliese stars to Hipparcos stars by position and magnitude. In general, this is likely to be an issue only for multiple stars with highly uncertain magnitudes in both catalogs, as the position constraints were fairly severe (stars had to have positions matching to +/- 0.15 degrees, less than the radius of the full Moon). I have not seen any apparent errors on scanning the database thus far, but this is one area that could be a problem.
  • Radial velocity information can be quite uncertain. Uncertainties of a few km/second are not unusual. There are 3 primary sources: the values in the Gliese catalog, the values in the Yale catalog, and the Wilson Evans Batten catalog mentioned earlier, in that order. I do not yet have a detailed breakdown of the uncertainties in these sources.

In short, though I have done what I can, I can't warrant the database to be error-proof. If you need to launch probes to all the stars in the database, you might want to give it a more thorough going-over before doing so :-)

Other Issues

The data in this database are subject to change. I may add or delete stuff as I feel like. If there are any really large changes, I will post copies to this site.