Background: Hubble's discovery of the expansion of the universe was one of the watershed developments in astronomy in the first half of the 20th century. The discovery was made by plotting Doppler velocity as a function of magnitude for a sample of elliptical galaxies. The basic premise is that, although there is a considerable spread in absolute luminosity, the luminosity of the brightest galaxies is constant for at least modest redshifts. This exercise reproduces Hubble's discovery by locating and harvesting the relevant data through the VO techniques.
1. The first thing needed is to find a catalog of elliptical galaxies that contains apparent magnitudes as well as radial velocities. Go to the VO registry: http://nvo.stsci.edu/VORegistry/index.aspx Search on the keywords "elliptical galaxies" "radial velocities". The most attractive catalog is Muriel 1995 which, according to the reference URL can be found at Vizier. Its identifier is J/AJ/110/1032.
2. Go to Vizier, using a suitable mirror, and ask for catalog J/AJ/110/1032. Whatever columns you specify, make sure you get bJ magnitude and radial velocity. The catalog has 292 rows, according to Vizier; make sure you select a sufficient number of rows to be returned and submit your query.
3. If you selected the HTML output option, you will find at the bottom a link that allows you to plot the data directly in VOPlot. Plot RV on the Y axis, bJmag on the X axis.
What this shows immediately is that the upper envelope displays a correlation between radial velocity and maximum apparent magnitude: the farther away the galaxy, the greater the radial velocity/
4. Additional exercise 1: Retrieve the information in VOTable format and feed it into TOPCAT. This will allow you to convert the magnitudes to luminosities.
5. Additional exercise 2: Having brought the data into TOPCAT also allows transforming the coordinates to galactic coordinates and correcting the radial velocities for solar motion in the Galaxy.
6. Additional exercise 3: Muriel 1995 is convenient because it contains magnitudes and radial velocities for a distinct sample of elliptical galaxies. But in principle one could combine information from multiple catalogs to increase the size of the sample.