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Mizar and Alcor, each about 83 light-years away (in fact Mizar presents system of four stars and Alcor is binary system. It is not yet known if those 6 stars presents one or two star systems. In further years, data provided by GAIA space telescope will show us if there is one or two systems), have a fainter line-of-sight companion known as Ludwig's star, which was mistaken for a planet in 1722. The Wikipedia page only says it is "roughly five times more distant," citing A New View of Mizar by Leos Ondra. Is a better distance estimate available?

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SIMBAD lists a Gaia DR2 parallax of 10.85±0.03 milli-arcseconds for that star. Without going into the biases and uncertainties in current Gaia parallax data, $$r \approx \frac{1}{\varpi}$$ where $\varpi$ is the parallax in arcseconds and $r$ is the distance in parsecs. This works out to a distance of roughly 92 parsecs or 300 light-years.

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  • $\begingroup$ Where can I read from SIMBAD distance that you quoted? $\endgroup$ – Fil May 20 '19 at 13:29
  • $\begingroup$ Actually, the SIMBAD page does show the distance: scroll down to "Collections of Measurements" and select "display all measurements". It gives the distance as 92.1353±0.2377 pc. $\endgroup$ – Chappo Says SE Dudded Monica May 21 '19 at 23:33
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    $\begingroup$ Your error bar does not take account of the published systematic uncertainty in the Gaia parallax zero-point. An additional 0.1 mas of error is warranted at present. $\endgroup$ – Rob Jeffries May 22 '19 at 10:26
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Ludwig's star, or Sidus Ludoviciana is an apporoximately +8 magnitude star between Mizar and Alcor.

The first Wikipedia article states:

Sidus Ludoviciana /ˈsaɪdəs ˌljuːdoʊˌvɪsiˈeɪnə/ is an 8th-magnitude star in the asterism of the Big Dipper in the constellation Ursa Major, halfway between Mizar and Alcor. It was discovered on 2 December 1722 by Johann Georg Liebknecht, who mistook it for a planet and named it after Louis V, Landgrave of Hesse-Darmstadt. A line-of-sight companion with Mizar and Alcor (with a spectral type similar to the latter), it is roughly five times more distant. It has the spectral type A8/F0 III.

Since Mizar and Alcor are 83 light-years away from the Sun, that puts Sidus Ludoviciana at a little more than 400 light-years from the Sun.

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    $\begingroup$ I've added some supporting links and some formatting to give you a better idea of how a good Stack Exchange answer might look. $\endgroup$ – uhoh May 20 '19 at 6:41

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