<p>You <em>could</em> 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 <em>matched</em> 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.)</p>

CC BY-SA 3.0
2016-09-20T09:55:08.350Z
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
CC BY-SA 3.0
2016-09-20T13:36:35.160Z
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
CC BY-SA 3.0
2016-09-19T15:59:19.807Z
2016-09-19T15:59:19.807Z
Peter Erwin
<p>The early 1990s <a href="http://www.cosmos.esa.int/web/hipparcos" rel="nofollow">Hipparcos</a> 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.
<a href="http://arxiv.org/abs/1609.04303" rel="nofollow">Lindegren et al. 2016</a> 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.</p>

CC BY-SA 3.0
2016-09-21T03:00:59.637Z
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
CC BY-SA 3.0
2016-09-21T04:57:10.773Z
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
CC BY-SA 3.0
2016-09-19T19:08:11.510Z
2016-09-19T19:08:11.510Z
Mike G
<p>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).</p>

<p>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?).</p>

<p>How do you compute the parallax for an object when its not included?</p>

<p>The data model is documented <a href="http://gaia.esac.esa.int/documentation/GDR1/datamodel/Ch1/gaia_source.html" rel="nofollow">here</a> .</p>

CC BY-SA 3.0
2016-09-19T18:12:29.203Z
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
<astrometry><gaia>
Calculating the distance of objects using the Gaia DR1
soupy_norman
# Calculating the distance of objects using the Gaia DR1

(Asked by: username_0 on 2016-09-19)

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](http://gaia.esac.esa.int/documentation/GDR1/datamodel/Ch1/gaia_source.html) .

**Tags:** astrometry, gaia
> **Comments:**
> **username_1**: 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.  

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## 2 answers

### Answer by username_2 on 2016-09-19. Score: 2 | Accepted answer

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.)

> **Comments:**
> **username_0**: 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?  
> **username_2**: 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.  

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### Answer by username_3 on 2016-09-19. Score: 4

The early 1990s [Hipparcos](http://www.cosmos.esa.int/web/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](http://arxiv.org/abs/1609.04303) 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.

> **Comments:**
> **username_4**: 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?  
> **username_3**: 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.  

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