[Stellar Mapping] How to make your own stellar database!

Looks like my new Stellar Mapping page has been well received so far – thanks to everyone who has shown an interest in it, I hope you’re finding it useful!

In this article I’m going to show you how to make your own stellar database, with the same tools I used to construct the ones I presented on my mapping page. For this exercise we’ll be relying on something called VizieR, which is a huge online database of thousands of star catalogues. You’ll need to have a basic understanding astronomy to make the most out of this, but it’s not that tricky.

Let’s say you want to make a database of stars in a corridor between Sol and the famous Pleiades star cluster (if you’re familiar with the 2300AD RPG, this is essentially the path the Bayern took to the Pleiades). We’ll be using the Hipparcos star catalogue, since it has the most accurate parallax measurements (from which we can derive distances).

Searching for a single star

0) First, we need to find the location of the Pleiades in RA/Dec. We know they’re in the constellation of Taurus, but we’ll need to be more precise than that! There are a number of named stars in the Pleiades (e.g. Maia, Alcyone, Pleione) so we’ll pick one of those – Alcyone is roughly in the middle of the cluster. It’s also in the Hipparcos catalogue – its wiki page (and other sources) tell us that its HIP number (Hipparcos catalogue number) is HIP 17702. Now we can look it up on VizieR!!

1) Go to http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=I/311/hip2. This is the search form for the New Reduction of the Hipparcos database (the latest version of the Hipparcos data), and it looks like this:

2) Type “17702” into the box that says “HIP”, and press Enter. This will generate a one-row table containing that star’s data.

3) Click the little black arrow in the sidebar on the left where it says “Compute”. This brings down a new menu with some checkboxes. You can use this to specify extra terms that the interface will calculate for you. In this case, tick the “J2000” box. Also, select the “decimal °” button to get the J2000 RA/Dec in decimal degrees rather than in h/m/s format. This tells us that Alcyone is at RA 056.87115°, and Dec +24.10514° (these are alpha and delta respectively in the spreadsheets on my Stellar Mapping page), and has a parallax of 8.09 milliarcseconds (0.00809 arcseconds).

Now we know where Alcyone is, we can use this to generate a range of distances and coordinates in order to find the locations of all the stars in a corridor between Sol and Alcyone. The Pleiades spans about 2 degrees of sky in total, but we’ll actually need to make our “corridor” much wider than that if we want a decent number of stars between there and Sol – going about 10 degrees on either side of Alcyone would give us a good corridor.

Creating the corridor

4) Hit the back button on your browser to return to the search page, and remove “17702” fom the HIP box.

5) Untick all the boxes in the “Show” column on the main page (left of the text boxes) except for the one next to “HIP”, and “Plx”. Make sure the sidebar settings are the same as they were before (see step 3) and change the “max” dropdown box there to “unlimited” and the box below that to “ascii table”. Now the results will be presented as an ASCII table, with just the computed J2000 decimal RA/Dec, the HIP number of the star, and its parallax.

6) In the “RArad” box, type “51.87 .. 61.87”. This tells VizieR to search between an RA of 51.87° and 61.87° (5 degrees either side of Alcyone in the sky).

7) In the “DErad” box, type “19.1 .. 29.1”. This tells VizieR to search between a Dec of 19.1° and 29.1° (5 degrees below and above Alcyone in the sky).

8‌) In the “Plx” box, type “6.5 .. 143.8”. This tells VizieR to show all the stars that have parallaxes between 6.5 and 143.8 milliarcseconds, which corresponds to a distance range of 22.7 to 501 lightyears from Sol (distance in lightyears = 3.2616/Parallax in arcseconds). We want the corridor to extend beyond Alcyone (whose parallax corresponds to a distance of 403 ly from Sol) so we can maximise our chances of getting the whole cluster.

The form should now look like this (click to expand image):

9) Press Submit to generate the list – you should get a list of 146 stars in ascii table format that looks like this:

Now we need to get this list of stars into the bulk_converter spreadsheet, so we can determine the Galactic XYZ co-ordinates for each star and (optionally) import it into Astrosynthesis.

Importing the list into Excel

10) Use your mouse to select and highlight the entire table in your browser and copy it into a text file. Save the text file as “input.txt” or something.

11) Import this file into Excel as a delimited, space-separated text file. You should end up with an Excel spreadsheet with 5 columns. The first is the index number of the star (which we don’t need), the second and third columns are the J2000 RA and Dec decimal degree coordinates (alpha and delta) of the star, the fourth column is the star’s HIP number, and the fifth column is the star’s parallax in arcseconds. All we need from this is the RA/Dec, parallax, and HIP number – you can delete the other columns.

12) We need to convert the parallax into milliarcseconds before we can import the parallax values. In an empty column, multiply the Parallax values by 0.001 for all the values in the parallax column (if your plx values are in column D, then in column E you would enter “=D1*0.0001” (without the quotes) to calculate this). The table should then look like this:

13) Open up the bulk_converter.xlsx file from the Stellar Mapping page. It should have a sample star in it already:

14) Copy the data in the alpha and delta columns from the table you imported into Excel and paste them into Columns H and I in the bulk_converter file. Copy the HIP numbers from the imported file into column A of the bulk_converter file, and copy the recalculated parallax (in milliarcsecons) from the imported file into Column L of the builk_converter file (you’ll have to make sure you Paste Values for the parallax!). The first row of the pasted data should replace the sample entry already in the bulk_converter file, so you should end up with something that looks like this:

15) In the bulk_converter.xlsx file, copy the formulae in Columns J and K and Columns M to U down to the bottom of the sheet. You should see the empty rows fill up with the calculated galactic coordinates, distances, and X/Y/Z coordinates. The end result should look like this:

(Note: Columns B to G are empty since we’re importing the data as alpha and delta and not as sexagesimal RA/Dec, so you can just delete those columns).

Now we have all the information we need! If all you need are the Galactic Lat/Lon and/or Galactic XYZ co-ordinates then you can just delete columns S, T and U, save the file, and use it for whatever nefarious purposes you desire! However, if you want to import that into Astrosynthesis, then there’s one more sequence of steps remaining…

Importing the data into Astrosynthesis

Next, we need to create a CSV file to import the data in to Astrosynthesis – this has to be in a specific format:

16) Create a new Excel file (keep the edited bulk_converter file open in another window).

17) Fill out the new file as follows:
In column A, type “Star” into the first row. Astrosynthesis needs this to know what it’s reading.
In column B, type a big number like 10000 in the first row. This is the index number for the first star that we’ll import into Astrosynthesis – it needs to be unique.
In column C, paste the HIP numbers (column A of the edited bulk_converter file).
In Column D, E and F, paste the AS X, AS Y and AS Z values (Columns S, T and U of the edited bulk_converter file). Be sure to Paste Values here!
In Column G, type “0.5” into the first row. This is a generic placeholder for the mass of the star (which we don’t know, but Astrosynthesis requires).
Column H and I should be blank.
In Column J, type “M0 V” into the first row. This is a generic placeholder for the spectral type and size of the star (which we don’t know, but Astrosynthesis requires).
Colummn K should be blank.
In Column L, paste the Distance/ly (column M of the edited bulk_converter file). Be sure to Paste Values here!

The file should look like this:

18) Each row needs its own unique number in Column B. In the second row, type “=B1+1” (without the quotes) to increment the number you typed in the first row by 1. Copy this formula down to the end of the data. You should now have a list of increasing numbers in Column B.

19) Copy/paste the contents of Column A, G and J to the bottom of the data. The file should now look like this:

20) Save the file as a CSV (Comma delimited) file wherever you store your astrosynthesis data files (call it “AS3_pleiades.csv” or something). Press “OK” on the first window that pops up when you save (“save the sheet only?”, and “Yes” on the second window (“do you want to keep the workbook in this format”). When you close Excel, it will ask again if you want to save the file – say “no” since you’ve just saved it and haven’t made any changes.

21) Import the CSV into Astrosynthesis (see instructions in Section 3 of my Stellar mapping page).

22) Congratulations! You’ve now created your own Pleiades corridor, containing the Pleiades themselves and all the stars from the New Reduction Hipparcos catalogue that are between 22.8 and 500 lightyears from Sol! You can import this along with the RECONS data to fill in the stars up to 22.8 ly from Sol. If you like, you can replace the HIP numbers of the named stars in the Pleiades with their proper names too!.

Hopefully there’s enough info here for you to figure out how to make other databases from these instructions too (e.g. if you want all the stars within 500ly of Sol, you can just enter the appropriate parallax range into the VizieR search form without entering any RA/Dec)!