Methods
Nothing here was measured by us. Every value was downloaded from a published catalogue, and between that catalogue’s file and the page you were reading it sat in, something was done to it — a unit converted, a redshift turned into a distance, a fit refused for failing its own quality test. This page is the whole of what was done, per upstream, with the assumptions written out.
It describes the rows this database is serving right now, so the counts below are queried rather than quoted. If you need a state of the catalogue that cannot move under a citation, take a frozen release instead — the conventions on this page are recorded inside it.
THREE RULES BEHIND ALL OF IT
] READ FIRST- A number whose provenance is not stored is not published
Every value carries the catalogue it came from; every distance carries the method that produced it; every model fit says it is a model fit. Where an upstream gives a quantity without saying what it is, the honest label is the column it came from, and that is what the record says.
- A missing value stays missing
Nothing is filled in, averaged, or carried over from a similar object. Sentinels are not measurements: an unmeasured mass published as zero, a distance parked at a placeholder, a proper motion sitting on the ceiling of a four-digit column — all read as empty. So does a published uncertainty of exactly zero, which is the same trap wearing the strongest possible claim.
- A derived value falls with what it was derived from
If a distance is rejected, the absolute magnitude and the luminosity computed from it go too, and an error bar never outlives the value it qualified. Two catalogues describing one object are never blended into a third reading that neither of them published.
WHERE A DISTANCE COMES FROM
] 183 022 OF 223 448 OBJECTSA distance from Earth is the number readers quote most and the number most often quoted without its ladder. Each row here is one rung, the count is how many objects stand on it, and the last column is how many of those publish an uncertainty. The record says which rung it is on in distance_method, in words, not as a code.
| RUNG | OBJECTS | WITH A BAR |
|---|---|---|
| By definition | 1 | — |
| Trigonometric parallax | 109 399 | — |
| The system distance an exoplanet inherits | 10 137 | 6200 |
| Cluster membership | 318 | — |
| The globular-cluster scale | 103 | — |
| The planetary-nebula statistical scale | 106 | — |
| Redshift, converted here | 9804 | 9804 |
| Spectroscopic redshift from a galaxy survey | 21 518 | 21 518 |
| Photometric redshift from a galaxy survey | 31 437 | 31 437 |
| A distance the survey measured directly | 176 | 171 |
| A literature value, entered by hand | 23 | 4 |
| No distance from Earth | 40 426 | — |
The last row is not a gap in the data. A planet, a moon, an asteroid or a comet has no fixed distance from Earth — what it has is an orbit, and the orbit is what the record stores. The rest are the stars and galaxies whose catalogue carries no usable parallax and no redshift this page would convert.
- By definition1
The Sun. Its distance is the astronomical unit, which since 2012 is a defined number of metres rather than a measured one.
- Trigonometric parallax109 399
The angle the Earth's orbit subtends at the star, inverted to a distance by the catalogue that measured it — Hipparcos, and Gaia where the two could be matched to one star. It is the only rung that assumes nothing about the object itself, which is why it is the rung everything else is calibrated against. These rows carry no error bar: the upstream publishes the distance without one, and the bar it deserves is not this site's to invent. A parallax distance is also not symmetric in the way a naive inversion suggests — the further the star, the more the inversion favours the near side — and no correction for that is applied here either.
- The system distance an exoplanet inherits10 137
A planet is as far away as its star, so both carry the distance the exoplanet archive publishes for the system — with the column it came from and the paper the archive names for it written into the method, because it is not one kind of measurement. Most are inferred from Gaia parallaxes rather than being one, and the microlensing systems are a Bayesian estimate from a galactic model with no parallax measured anywhere in them.
- Cluster membership318
An open cluster is nearer than any single one of its stars can prove: the members share a parallax, and averaging over the ones a membership analysis accepts beats every one of them (Cantat-Gaudin et al. 2020).
- The globular-cluster scale103
Harris's catalogue (2010 edition), which fixes each cluster's distance from its horizontal branch and its reddening — a standard candle, calibrated, not a geometric measurement.
- The planetary-nebula statistical scale106
Stanghellini et al. (2008), which reads a distance out of a nebula's angular size and surface brightness against a calibrating sample. It is a scale rather than a measurement, and it is the weakest rung the catalogue publishes: individual objects can be wrong by tens of per cent.
- Redshift, converted here9804
A galaxy's heliocentric redshift, corrected into the frame of the cosmic microwave background and run through the cosmology below to a luminosity distance. Only above the floor, and only where both frames agree there is a Hubble flow to measure.
- Spectroscopic redshift from a galaxy survey21 518
The same conversion, over a redshift a survey measured from spectral lines. The redshift is the survey's; the distance is this catalogue's, so that two galaxies in one cluster are not divided by two different expansion rates.
- Photometric redshift from a galaxy survey31 437
The same conversion again, over a redshift fitted to colours rather than measured from lines. The survey gives such a redshift a standard deviation of about 20% of itself against 2.9% for a spectroscopic one, so these distances are good to a fifth and their error bars say so — a photometric distance and a spectroscopic one on neighbouring pages are not comparable numbers.
- A distance the survey measured directly176
The handful of galaxies whose distance a survey published from a measurement of its own rather than from a redshift, kept exactly as published. There is no expansion rate inside these, so none is divided out of them.
- A literature value, entered by hand23
The objects no bulk catalogue carries — the black holes among them — whose distance is transcribed from the paper that measured it.
THE COSMOLOGY
] H0 70 · ΩM 0.3153Turning a redshift into a distance takes three choices, and all three are made here rather than inherited from whichever survey the redshift came from.
- The expansion rate: H₀ = 70 km/s/Mpc
The round number between the two measurements that do not agree — 67.4 ± 0.5 from the microwave background and 73.0 ± 1.0 from the distance ladder, which are five standard deviations apart. Choosing either would take a side in a live argument on every galaxy page. The ±4% it costs is not left implied by the digits: it is in the error bar on every distance derived this way.
- Flat ΛCDM with Ωm = 0.3153
The full luminosity distance, not cz/H₀, which is only its first term and runs 13% short at the furthest galaxy this catalogue holds.
- The rest frame of the microwave background
A heliocentric redshift carries the Sun’s own 370 km/s through the local universe, which at these distances is the same size as the signal. Redshifts are corrected into the background frame through the measured dipole before anything is computed from them.
- A floor at z = 0.0015, tested in both frames
That is 450 km/s of recession, against the 200–400 km/s of infall toward Virgo and the Great Attractor that nobody removes from an individual galaxy. Below it a “distance” maps the local velocity field rather than the expansion. The test runs in both frames, because a redshift that 370 km/s of correction can push across the floor is one that 300 km/s of the galaxy’s own motion can push back — and where the two frames disagree about whether there is a Hubble flow to measure, no distance is published in either.
One consequence is worth stating plainly: the expansion-rate systematic is the same sign on every distance derived from a redshift. It cancels when you compare two of them and it does not cancel when you compare one against a distance measured any other way. That is also why a survey’s own published distances are recomputed here rather than republished — two galaxies in one cluster divided by two different expansion rates differ by several per cent for no astronomical reason, and no error bar removes a relative offset.
TEMPERATURES, RADII AND MASSES
] 59 890 MODEL FITSAlmost no star has a measured radius or mass. What 59 890 of them carry here is a model fit to a low-resolution spectrum, published by the survey that fitted it, and the record says so in stellar_parameters_method rather than presenting it as a measurement.
A fit the fitter’s own numbers distrust is not published at all. Where the astrometric solution does not fit a single star — the standard threshold for an unresolved companion or a bad fit — the whole set goes, mass, luminosity and age with it, because all of them are derived through that solution’s parallax and the companion that broke it sits inside the aperture the spectrum was taken through.
Two populations are kept and told on themselves instead, because they are wrong in a stated direction rather than unusable: 11 917 fits above 7,500 K, where temperature and interstellar reddening trade off against each other, and 7174 sources the survey’s own classifier calls binaries and its parameter pipeline fitted as one star. Those two sets overlap: 17 874 stars carry a caveat at all, and it is in the same string as the method, so a reader taking the value takes the warning with it.
A further 3853 stars are exoplanet hosts whose parameters are the exoplanet archive’s composite values — assembled per system from the literature, so the numbers on one page may come from more than one paper.
POSITIONS, EPOCHS AND TIME
] ICRS- Coordinates are J2000 equatorial, as the catalogues published them
ICRS, epoch J2000.0 in every row. Solar-system bodies are the exception everywhere they appear: their coordinates are apparent and of date, computed for the instant you asked.
- The sky map draws stars where they are now, not where they were
The map mixes a catalogue of bright stars with a survey whose positions are for epoch 2016.0, so the catalogue’s stars are carried forward to that epoch with their own proper motions before being drawn — otherwise the fastest of them land arcminutes from the survey source underneath. The readout names the frame and the epoch it is quoting. The stored coordinates do not move: this is a rendering epoch, and the record and the API still answer J2000.
- Transit epochs are quoted as published
An exoplanet transit epoch is published on a dynamical time scale and is used here as though it were civil time. The difference is under about nine minutes — smaller than the transit predictions on this site are useful to, and stated rather than silently absorbed.
CONVENTIONS
] ONE COLUMN, ONE QUANTITY- Which radius
A radius says which radius it is. Planets and moons carry the volumetric mean radius — for an oblate giant the equatorial one is thousands of kilometres larger — and small bodies carry an effective radius, the radius of the sphere with the same projected area, which is not a claim that the body is round. Where the upstream does not say, neither does the record.
- Which magnitude, in which band
Magnitudes are apparent unless labelled otherwise and always carry the passband they were measured in. An absolute magnitude means absolute visual for a star and the small-body H for an asteroid or comet; those are different quantities and are labelled as such rather than sharing a column heading.
- Units, once, everywhere
Masses in kilograms, lengths and orbits in kilometres, distances from Earth in light-years, angles in degrees. Conversions happen at ingest, so nothing on a page is a unit a reader has to guess at, and an uncertainty is converted by the same factor as the value it qualifies.
- Error bars are two numbers
Uncertainties are stored under the field they qualify as
[minus, plus], both positive, because the archives publish them separately and a fit is not obliged to be symmetric. A source that publishes one σ gets two equal halves. They are the upstream’s own, except where a distance was derived here — then the bar is what this arithmetic propagated, and the section above says which terms are in it. - Star colour is computed, and it excludes reddening
The colour of a star on the map and on its own page is Planck’s law at the star’s temperature through the standard colour matching functions, taken from the spectral type where there is one and from the colour index otherwise. Interstellar dust reddens the light that arrives, and no correction for it is applied — so the colours are what the star is, not what a telescope would record. Luminosity class and brightness are deliberately not modelled into the hue.
WHAT THE CHECKS REFUSE
] 20 RULES · 95 ROWS FLAGGEDEvery merged row is run past a set of physical plausibility rules before it can be published: Kepler’s third law against the orbit, the distance modulus against the two magnitudes and the distance, bulk density against mass and radius, albedo and inclination within their ranges, an eccentricity against the sign of the semi-major axis, a mass against the hydrogen-burning limit, a luminosity against what a star can emit.
Where this catalogue caused a violation, it is fixed at the source and the rule stays behind as a guard matching nothing. Where an upstream publishes two numbers that cannot both be right — most of the 95 flagged rows are a planet whose mass and radius imply a density greater than a white dwarf’s — the row keeps the published values and gains a sentence saying what is wrong with them, on the page and in data_quality. Quote it if you quote the number.
What each rule matched when it was last reviewed is recorded, and a rebuild in which one of them matches more rows than that does not ship. The number going up is the alarm, not the inconvenience. Where two catalogues describe one object differently beyond the tolerance for that quantity, the disagreement is written down and shipped inside the release rather than resolved in silence.
EVERY UPSTREAM, AND WHAT HAPPENED TO ITS NUMBERS
] 16 CATALOGUESLicences and retrieval dates are also listed on the about page; what is here is the treatment. The three groups are three different jobs: a catalogue creates objects, an enrichment adds fields to objects it did not create, and a survey is stored whole, drawn on the map and given no pages of its own.
CATALOGUES — THEY CREATE THE OBJECTS
- TAKEN
- Positions, proper motions, Johnson V magnitudes and B−V colours, spectral types, parallax distances and the Bayer, Flamsteed, HD, HR, Gliese and variable-star designations, for every star down to about ninth magnitude.
- DONE TO IT
- A distance parked at 100,000 parsecs is the compilation's way of saying it has no parallax, and it is dropped — along with the absolute magnitude and the luminosity computed from it, which are arithmetic on a placeholder rather than measurements. A proper motion sitting exactly on the column's four-digit ceiling goes the same way. Greek letters are restored for display (ε Eri, not 'eps Eri'); the designations stay in the ASCII a person would type.
- TAKEN
- The galaxies of a 23-million-row survey compilation that carry a name, a distance and a magnitude: positions, redshifts in both frames, B and W1 magnitudes, stellar masses and the flag saying which kind of distance each row has.
- DONE TO IT
- The redshift is the survey's and the distance is this catalogue's: it is recomputed from the survey's own microwave-background-frame redshift under the cosmology above, rather than republished, because the survey used a different expansion rate and two galaxies in one cluster cannot be divided by two different numbers. Everything derived from the distance moves with it — the absolute magnitude by its definition, the stellar mass as the square of a luminosity. The error bar is rebuilt from three independent terms: the redshift's own measurement error propagated through the distance–redshift relation, the survey's peculiar-velocity residual, and the ±4% on the expansion rate.
- TAKEN
- Asteroids and comets — everything large and bright, every near-Earth object down to the kilometre class, and all known comets — with orbital elements, absolute magnitudes and diameters.
- DONE TO IT
- A diameter becomes an effective radius, which is the radius of the sphere with the same projected area and not a claim that the body is round. An unmeasured mass published as zero is read as missing, not as zero. An unbound orbit keeps no mean distance, and a value dropped takes its uncertainty with it.
- TAKEN
- The corrected NGC and IC catalogue: galaxies, nebulae and clusters with positions, sizes, magnitudes, morphologies, redshifts and the cross-identifiers that make M31, NGC 224 and Andromeda one page.
- DONE TO IT
- Redshifts are heliocentric, so they are corrected into the frame of the microwave background before anything is computed from them, and a distance follows only above the floor described above. Morphologies are read on the de Vaucouleurs scale in both its notations — the letters and the numeric stage — which are the same classification and used to disagree here about where the spirals end.
- TAKEN
- The archive's composite-parameters table: one best-available row per confirmed planet, with its orbit, mass, radius, equilibrium temperature and host, plus the published uncertainties on all of them.
- DONE TO IT
- Masses and radii arrive in Earth and Jupiter units and are stored in kilograms and kilometres, uncertainties converted by the same factor. The system distance keeps the archive's own reference rather than being called a parallax, because for most rows it is not one. A host star that this catalogue already holds is merged onto its existing page rather than given a second one.
- TAKEN
- Every known natural satellite: orbital elements, physical parameters where they have been measured, and the discovery year and discoverer.
- DONE TO IT
- Radii are mean radii and say so. A moon's body code is an internal key of the ephemeris system — and 219 of them are also asteroid numbers — so it is stored as a property rather than as a name search can resolve.
- TAKEN
- The Sun, the eight planets and the black holes, transcribed one object at a time from NASA fact sheets and from the papers that measured them, because no bulk catalogue carries them.
- DONE TO IT
- A planet's radius is the volumetric mean radius, which for an oblate giant is several thousand kilometres from the equatorial one — the record says which it is. A planet's headline distance is the semi-major axis of its orbit about the Sun, stored as that and not as a distance from Earth, which changes daily. This layer also carries descriptions and discovery years for catalogued objects, and it is checked against the catalogue on every run: an entry matching no object fails the build rather than disappearing.
ENRICHMENTS — THEY FILL FIELDS ON OBJECTS THEY DID NOT CREATE
- TAKEN
- Distances for the open clusters, the globular clusters and the planetary nebulae, from three published tables of a few hundred rows each.
- DONE TO IT
- The clusters and nebulae are matched by name, which is the only key these tables and this catalogue share, so a name two objects claim is dropped rather than given to whichever was reached first, and every run prints what did not match. A redshift distance would have been meaningless for all of them: inside the Local Group a Hubble-flow distance is not a distance, it is a peculiar velocity.
- TAKEN
- Temperatures, radii, masses, luminosities and ages for the stars carrying a Hipparcos number, from the mission's own astrophysical-parameter tables, joined through the cross-match the archive publishes.
- DONE TO IT
- These are fits to low-resolution spectra, not measurements, and every enriched star says so. A source whose astrometric solution does not fit a single star (RUWE above 1.4, Lindegren et al. 2021) loses the whole set, mass and age included, because all of them are derived through that solution's parallax. Fits above 7,500 K, where temperature and extinction trade off against each other, and sources the mission's own classifier calls binaries, are kept and carry the reason they may be wrong.
- TAKEN
- Variability type, period and epoch for the stars that vary.
- DONE TO IT
- Matched on the variable-star designation, case-sensitively and outside the site's own search index — that index lower-cases, and in this catalogue 'MU Tau' and 'mu Tau' are two different stars in one constellation.
- TAKEN
- The discovery circumstances of the numbered minor planets: the night, the observatory and who is credited.
- DONE TO IT
- The orbits in the same file are deliberately not taken. They are a second fit of the same observations already ingested from elsewhere, and two fits of one dataset are not two facts.
- TAKEN
- Spectral classifications for the whole Hipparcos population, and morphologies and angular sizes for the galaxies.
- DONE TO IT
- This is the one upstream that overwrites rather than filling holes: a spectral type is one astronomer's reading of a spectrum, and a catalogue whose stars were all read by the same authority is worth more than one where each row kept whichever of two compilations happened to say more. The two strings are never blended — that would leave the catalogue unable to say who classified the star. A morphology written in a notation with no rule for it costs the row its family and nothing else; the angular size in the same record is a separate measurement and is kept.
- TAKEN
- The double and multiple stars: separations, position angles and component magnitudes.
- DONE TO IT
- The pairs publish no identifier this catalogue holds, so they are matched on position — and a position alone would hang a companion on whichever star happened to be nearest when the real primary is not catalogued here at all. The magnitudes have to agree too, and an entry that two stars fit equally well is dropped. That a pair is one system is a fact about two objects rather than a column of either, so it is stored as a relation between them.
- TAKEN
- Cross-identifiers, alternate names, discovery credit for the deep-sky objects, and the link to an encyclopaedia article.
- DONE TO IT
- Matched on a catalogue designation wherever there is one. Where only a name matches, the item has to agree with this catalogue about the body the object orbits or the constellation it lies in, and an ambiguity is refused rather than guessed — without that rule 'Io' resolves to an asteroid, 'Moon' to an astrology article and 'Polaris' to a radar satellite. A misspelling is corrected only when no object anywhere carries the name as written, exactly one object of the same parent is one character away, and that name is free.
SURVEYS — STORED WHOLE, DRAWN, NEVER GIVEN A PAGE
- TAKEN
- Every source in the mission's third data release brighter than G = 15, held exactly as published: a position, a proper motion, a brightness and a colour.
- DONE TO IT
- These are drawn on the sky map and reachable by their own identifier, and they have no page: 'a source at these coordinates' is not an article. Positions are carried forward to the survey's own 2016.0 epoch using the stored proper motions, and a source that lands on a catalogued star is marked so the map does not draw the same star twice.
- TAKEN
- All 23,181,758 rows of the galaxy compilation, including the quasars and the rows with no name — positions, redshifts in both frames, magnitudes and distances.
- DONE TO IT
- Distances are converted to the same cosmology the catalogued galaxies use, and the floor is applied in both frames here too, so a galaxy that has a page and the survey row it came from cannot print two different distances.
WHAT IS DELIBERATELY NOT HERE
] AND WHY- No measurement of our own
This is a compilation. Nothing here was observed, fitted or estimated by the site; the only arithmetic applied is the arithmetic described above, and it is applied to somebody else’s numbers.
- No blended values
Where two catalogues disagree, one wins the field and the record names it. An average of two readings is a third reading that nobody published, and it would leave the page unable to say who measured what.
- No artist’s impressions
Every photograph is an observation, under a licence that permits reuse, credited to the person who took it. A rendering, a simulation or a concept painting is refused by a gate in the pipeline rather than by editorial taste — which is why some objects have no picture at all.
- No pages for survey rows
Tens of millions of survey sources are stored and reachable by their own identifier, and none of them has a page or is offered to a search engine. A row that is a position, a brightness and a redshift can be looked up; it cannot be read.
If something on this page is wrong, that is worth more to us than a compliment — the about page says where to write.