# Calculating the distance of objects using the Gaia DR1

The Gaia DR1 was released middle of September with ~1.1 billion objects. The schema contains right ascension, declination and parallax, which could be used to calculate Cartesian co-ordinates (xyz).

The overwhelming majority (>99.998%) of objects do not contain an entry for parallax. I suspect this needs to be computed using other columns (e.g. phot_g_mean_mag?).

How do you compute the parallax for an object when its not included?

The data model is documented here .

• Have you considered using NOMAD? usno.navy.mil/USNO/astrometry/optical-IR-prod/nomad (fewer objects, only a billion, but perhaps more useful). – user21 Sep 19 '16 at 17:37
• Disregard previous comment. Apparently NOMAD, unlike its predecessors, does NOT include absolute magnitude or anything that can help find distance. – user21 Sep 19 '16 at 17:42
• You can't "compute" a parallax, it is a quantity that is measured. You mean, can you calculate a distance? Theanswer is no, unless you know what kind of a star it is. Even then, theanswer is rough - that is the whole point of Gaia (eventually) producing a parallax for all those stars. – ProfRob Sep 19 '16 at 18:12

You could use phot_g_mean_mag (apparent magnitude), but you would have to assume an absolute magnitude for each star or galaxy you were interested in. From the data in that table, there's no way to do that. (Some of the matched tables might give you a starting point: e.g., if you decided that Gaia source X was matched by SDSS source Y, then you could track down the SDSS measurements, which might let you decide what kind of star it was and thus guess at its absolute magnitude.)

• Will the Gaia science team be able to use the associated photometric data to identify the type of star, and therefore its absolute magnitude? Or will new distances produced only by he gaia instrument using measuring parallax? – soupy_norman Sep 20 '16 at 9:55
• Not the current photometric data, no. Gaia is also taking spectroscopy, and there are campaigns using ground-based telescopes to get spectroscopy of many Gaia targets. Taken all together, that will (in the future) allow stellar-type identifications and photometric distance estimates. Of course, as Mike G pointed out, Gaia DR1 is very much a preliminary catalog; the final Gaia data release will have parallaxes for as many as a billion stars. – Peter Erwin Sep 20 '16 at 13:36

The early 1990s Hipparcos mission yielded parallaxes for 118000 stars (Hipparcos catalog) and positions without parallax for another 2.4 million (Tycho-2 catalog). The Tycho-Gaia astrometric solution (TGAS) combines those data with preliminary Gaia observations to get 2 million parallaxes, "only" 17 times as many as Hipparcos, with better precision. If this is a small fraction of the total number of star positions, it's not a deficiency; it's just easier to get a position than a parallax. Lindegren et al. 2016 explain what they could or could not do with the first 14 months of Gaia observations. Future Gaia data releases should have still more parallaxes.

• With a title like "Gaia Data Release 1: Astrometry - one billion positions, two million proper motions and parallaxes" I'd expect something like 99.8% 'missing' parallax values, not 99.998% as mentioned in the question. Is it just a question of timing - or are most of them proper motions but not parallaxes? – uhoh Sep 21 '16 at 3:00
• 99.8% is correct. My queries at the Gaia Archive show that all of gaiadr1.tgas_source, and 0.18% of gaiadr1.gaia_source, has parallax and proper motion. – Mike G Sep 21 '16 at 4:57

What will be included in the first public data release?

Gaia Data Release 1 includes the positions and G magnitudes – a broad, visible light passband spanning 330 nm to 1050 nm – for more than one billion stars using observations taken between 25 July 2014 and 16 September 2015.

In addition, for a subset of data – about 2 million stars in common between the Tycho-2 Catalogue and Gaia – there will be a five-parameter astrometric solution, giving the positions, parallaxes, and proper motions for those objects. This is referred to as the Tycho-Gaia Astrometric Solution (TGAS).

By measuring the amount of displacement – the parallax angle – and knowing the distance between the Sun and the Earth, astronomers can determine the star’s distance using simple trigonometry.

Even for the nearest stars the amount of apparent movement is tiny: less than one arcsecond. Yet Gaia will measure the positions of one billion stars to microarcsecond accuracies.

Assuming "will measure the positions" means "will measure the parallaxes", this appears to be referring to the final version of GAIA which will be published in the early 2020s.

To make things easier, http://cdn.gea.esac.esa.int/Gaia/ has a "tgas_source" subdirectory which just includes the 2,057,066 stars for which there is additional data (so you don't have to download the entire much larger GAIA archive).