<p>I'm trying to get a database of about 20 million of the brightest stars in the sky for a project I'm working on, and I want to reconcile it with the Hipparcos data set.  Currently, I'm only using the HIP dataset for everything, especially the constellations, additionally, as far as I understand, the DR2 data set doesn't have the brightest stars. </p>

<p>My confusion is that for many stars, there is an easy one-to-one correlation, i.e. at 104.304832, 57.563355, I see a magnitude 4.613 star.  This star appears in the HIP dataset as a 6.150 magnitude star.  Roughly the same parallax.  But, if I look at 102.733851, 57.649038, at the 7.44 magnitude star, I see nothing at all in the HIP dataset. </p>

<p>Honestly, <em>most</em> of the stars I'm trying to correlate between the two data sets do not appear.  Is RA/DEC defined differently between the two data sets?  Are they intended to be used as concatenated data sets?  I'd <em>really</em> like to use DR2 to refine the parallax mappings from the HIP data set because for my project estimated distance is so critical.</p>

CC BY-SA 4.0
2019-10-20T00:06:00.537Z
Looking through the data sets more, the problem doesn't look quiet as bad as I thought, I'm finding more situations with easily reconcilable stars.  Just odd that so many stars don't seem to correlate.
Charles Lohr
CC BY-SA 4.0
2019-10-20T00:07:26.873Z
Have you read both the Hipparcos and DR2 data release papers?
ProfRob
CC BY-SA 4.0
2019-10-20T00:15:48.883Z
@RobJeffries - Nope.  I am going to go do that, now.  I'm guessing this is a really nuanced answer that won't be particularly useful for people to just "google the answer" in the future.
Charles Lohr
CC BY-SA 4.0
2019-10-20T08:24:26.103Z
I think it is just that there are bright stars missing from *both* catalogues and that stars that have strong evidence of binarity in their astrometric data may also not be in DR2. Also, what correlation radius are you using? Nearby, bright stars can have large proper motion.
ProfRob
CC BY-SA 4.0
2019-10-22T01:09:41.410Z
Oh man, you guys have such good input!!! I never even thought about the proper motion issue!  I will read that paper.  I can't believe there is literally a paper _on_ my question!
Charles Lohr
<data-analysis><gaia><hipparcos>
Why is it hard to reconcile Gaia DR2 with the HIP data set
Charles Lohr
# Why is it hard to reconcile Gaia DR2 with the HIP data set

(Asked by: username_0 on 2019-10-19)

I'm trying to get a database of about 20 million of the brightest stars in the sky for a project I'm working on, and I want to reconcile it with the Hipparcos data set. Currently, I'm only using the HIP dataset for everything, especially the constellations, additionally, as far as I understand, the DR2 data set doesn't have the brightest stars.

My confusion is that for many stars, there is an easy one-to-one correlation, i.e. at 104.304832, 57.563355, I see a magnitude 4.613 star. This star appears in the HIP dataset as a 6.150 magnitude star. Roughly the same parallax. But, if I look at 102.733851, 57.649038, at the 7.44 magnitude star, I see nothing at all in the HIP dataset.

Honestly, *most* of the stars I'm trying to correlate between the two data sets do not appear. Is RA/DEC defined differently between the two data sets? Are they intended to be used as concatenated data sets? I'd *really* like to use DR2 to refine the parallax mappings from the HIP data set because for my project estimated distance is so critical.

**Tags:** data-analysis, gaia, hipparcos
> **Comments:**
> **username_0**: Looking through the data sets more, the problem doesn't look quiet as bad as I thought, I'm finding more situations with easily reconcilable stars.  Just odd that so many stars don't seem to correlate.  
> **username_1**: Have you read both the Hipparcos and DR2 data release papers?  
> **username_0**: @RobJeffries - Nope.  I am going to go do that, now.  I'm guessing this is a really nuanced answer that won't be particularly useful for people to just "google the answer" in the future.  
> **username_1**: I think it is just that there are bright stars missing from *both* catalogues and that stars that have strong evidence of binarity in their astrometric data may also not be in DR2. Also, what correlation radius are you using? Nearby, bright stars can have large proper motion.  
> **username_0**: Oh man, you guys have such good input!!! I never even thought about the proper motion issue!  I will read that paper.  I can't believe there is literally a paper _on_ my question!  

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## 0 answers

