Background: The Hertzsprung-Russell Diagram shows the relationship between stellar luminosity and temperature (spectral class, color index) and is discussed in virtually every introductory astronomy class. Can we construct an HR diagram from VO-accessible resources?
Software needed: VOPlot or TOPCAT for plotting VOTables.
1) We need to find a stellar catalog that includes, or for which we can compute, absolute magnitudes, or where the stars are all members of a cluster (and thus at the same distance). Most tables of stellar properties in clusters originate from studies of HR diagrams, so that seems a bit of a cheat! So instead, we look for star catalogs that include a parallax measurement, from which we can determine distance and convert an apparent magnitude to an absolute magnitude.
Use the NVO Registry and enter the search keyword "parallax". http://nvo.stsci.edu/VORegistry/index.aspx Both the Bright Star Catalog and Gliese Catalog of Nearby Stars would work for our purpose, though the BSC has parallaxes for only a fraction of its entries. We'll use the Gliese Catalog.
2) We need to get the Gliese Catalog downloaded in VOTable format. From the Registry entry we can follow the ReferenceURL and see that this is a HEASARC copy of a catalog V/70A at CDS. There are two ways to get the table.
First, let's use the NVO Cone Search service for the copy of the catalog at HEASARC. Click the View button on the registry search results page to see the metadata describing the catalog. Select and copy the text in the box called ServiceURL and paste this text into the browser Address. To this we must append the arguments for the cone search that will provide us with the complete catalog. The easiest way to do this is to just specify a position on the sky and a radius of 180 degrees (the whole sky). Also, we want to make sure we get all columns in the table, so we have to add the VERB (verbose) argument. The URL should look like this:
http://heasarc.gsfc.nasa.gov/cgi-bin/tam/conetest.pl?table=cns3&RA=0.0&DEC=0.0&SR=180.0&VERB=2
Then hit return. You should get a File Download dialog box. Click Save, and save the file as gliese-heasarc.xml.
Another way to get the VOTable is from the
VizieR service at CDS: http://vizier.u-strasbg.fr/viz-bin/VizieR. Enter "gliese" in the first text box and select Find Catalog. On the next form, set the Maximum Entries per table to 9999, the Output layout to XML-VOTable (XSL), and then click Submit Query. The next screen will show you the catalog in VOTable format. On the browser File menu, select Save As... and create a local copy of the table e.g., gliese-vizier.xml.
3) Look at the table using VOPlot or TOPCAT. First let's try VOPlot.
Start VOPlot. On the File menu select Open, locate the gliese-vizier.xml file, and click Open. You should see a plot with right ascension on the x-axis and declination on the y-axis. Change the axes to Vmag on the y-axis and B-V on the x-axis. Then click the box labeled Rev on the y-axis (reverse order) and you have a sort of HR diagram, but with apparent magnitudes rather than absolute magnitudes. This is why there is one point off-scale, at apparent magnitude -27 (the Sun).
4) Compute a new column contain the absolute V magnitude.
Select the Functions menu, and choose Create New Columns. Absolute magnitude (apparent magnitude if the star were at a distance of 10 parsecs) is computed from the following formula:
where p is the parallax angle in arcsec. The Gliese catalog gives parallax in the field called plx in milliarcseconds, so the expression we have to enter into VOPlot is as follows:Vabs = Vmag + 5 + 5 log p
Enter column name: Vabs
Enter expression: $8 + 5 + 5 * log(0.001*$10)
Enter unit: mag
Then click Add, and the new column will be created.
5) Display the HR diagram.
Select Vabs for the y-axis and click Plot. Voila. The main sequence is the broad diagonal band. Below and to the left are white dwarfs, and above and to the right are red giants. You can verify these spectral classifications by selecting regions of the plot and viewing the spectral type column entries.
Drag out a box around some of the white dwarfs. Small blue squares are drawn around the points. Then select the View menu and pick Data in Table Format. Scroll over to the column labeled Sp (spectral type), and all are type D…dwarfs.
6) Let's try the same thing with TOPCAT.
Start TOPCAT. On the File menu select Load Table, then Browse Files, and locate the gliese-vizier.xml file, and click Open Table. Then click the Plot icon. TOPCAT shows a plot of proper motion position angle vs. proper motion because these are the first two floating point columns in the table. (VOPlot recognizes RA and Dec and converts the sexagesimal format into decimal form automatically.) Change the axes to Vmag on the y-axis and B-V on the x-axis. (In TOPCAT you give x then y, the opposite of VOPlot. Also, TOPCAT has no option to reverse the axis. )
7) Compute a new column contain the absolute V magnitude.
Select the Display column metadata icon (6th from left on the main TOPCAT window). In this new window, click the green plus sign. We enter the same basic formula as above, but of course the interface is a bit different, and we have to convert the log because TOPCAT’s log function is base-e, not base-10:
Name: Vabs
Units: mag
Description: absolute magnitude
Expression: Vmag + 5 + 5 * 0.4343 * log (0.001*plx)
Enter unit: mag
UCD: [browse UCDs if you wish]
Index: [leave as is]
Then click OK, and the new column will be created.
8) Display the HR diagram.
Select Vabs for the y-axis and click Plot. Voila. Unfortunately you have to stand on your head and look in a mirror to see things plotted in the traditional way, but you get the idea.
9) Save your work. TOPCAT allows you to save the VOTable with the new column.
Select the main File menu and click Save Table. Click Browse files, and save your file, e.g., as gliese-tc.xml
10) A final cross-check... It turns out the when we asked for all possible columns from the HEASARC cone search, we also got a column with the absolute magnitude. (Didn't tell you that!) You can compare our computed absolute magnitude with the catalog value and see how we did. Try it with VOPlot, TOPCAT, or Mirage.