Go to the SIMBAD Query by identifier page, query for "alf Cas"
This brings up the entry for the star.
Near the bottom of the page there is a section called "External archives". This lists the star's entry in various catalogues.
Click the link titled Gaia DR2 418551920279473536.
Take a look at the parallax listed there (and its error). This might give you some idea of what's going on.
Note that the distances in stars.dat are not simple 1/parallax. For Gaia stars, the relevant catalogue is the Gaia Geometric Distance Catalogue, which can be queried on the Gaia Archive website.
Click Search, then select "Advanced (ADQL)".
Enter the following query
Code: Select all
WHERE source_id = 418551920279473536
You will see a result with 1 row. There is a small icon which looks like a table, which shows a tooltip "Display top 2000 results". Click it.
Take a look at the value in the r_est column, which contains the estimated distance in parsecs.
- The Gaia DR2 parallax for this star is really bad, probably because it's a bit too bright for Gaia to measure reliably, at least using the Gaia DR2 pipeline.