<p>Could you give a source or example: The Tycho-II catalogue paper by <a href="http://adsabs.harvard.edu/abs/2000A&amp;A...355L..27H" rel="nofollow">Hog et al. (2000)</a> suggests there are about 19,000 stars in Hipparcos or Tycho-I that are not in Tycho-II. This can be because they are too bright (but that is not very many stars) or for other reasons that are not clearly specified (binarity is an issue I think).</p>

<p>Looking at <a href="http://www.astro.ku.dk/~erik/Tycho-2/faq.html" rel="nofollow">this website</a>, it suggests 4% of stars did not have proper motions and could not be assigned epoch 2000 positions in Tycho II but I don't think these are <em>missing</em>.</p>

CC BY-SA 3.0
2016-09-27T16:37:28.203Z
2016-09-27T16:37:28.203Z
ProfRob
<p>Both use the same data. But at least, 90k stars are lost in Tycho-2. 
Why?</p>

CC BY-SA 3.0
2016-09-27T15:26:09.773Z
@barrycarter There are 90k Hippacos sources in tgas catalog released by Gaia recently.
questionhang
CC BY-SA 3.0
2016-10-11T14:09:55.367Z
@barrycarter  The question is 93k >> 18374.
questionhang
CC BY-SA 3.0
2016-10-11T19:32:51.073Z
Please clarify your question and what catalogues or sources you are looking at.
ProfRob
<star>
Why does the Tycho-2 catalog not include Hipparcos stars?
questionhang
# Why does the Tycho-2 catalog not include Hipparcos stars?

(Asked by: username_0 on 2016-09-27)

Both use the same data. But at least, 90k stars are lost in Tycho-2.
Why?

**Tags:** star
> **Comments:**
> **username_0**: @barrycarter There are 90k Hippacos sources in tgas catalog released by Gaia recently.  
> **username_0**: @barrycarter  The question is 93k >> 18374.  
> **username_1**: Please clarify your question and what catalogues or sources you are looking at.  

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## 1 answers

### Answer by username_1 on 2016-09-27. Score: 3

Could you give a source or example: The Tycho-II catalogue paper by [Hog et al. (2000)](http://adsabs.harvard.edu/abs/2000A&A...355L..27H) suggests there are about 19,000 stars in Hipparcos or Tycho-I that are not in Tycho-II. This can be because they are too bright (but that is not very many stars) or for other reasons that are not clearly specified (binarity is an issue I think).

Looking at [this website](http://www.astro.ku.dk/~erik/Tycho-2/faq.html), it suggests 4% of stars did not have proper motions and could not be assigned epoch 2000 positions in Tycho II but I don't think these are *missing*.


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