Selecting a Target Using Stelledoppie
Adapted from Kalée Tock and Ryan Caputo
Now that you understand the key terms and concepts in double star astronomy, it's time to select your target for observation and research. This is a critical step that will determine the direction of your entire project.
Search Constraints
When selecting a double star system to study, there are several important constraints to consider:
Search constraint #1: Mag secondary < 13
We need the pair of stars to be bright enough to see with 0.4m telescopes, and both stars of the pair must be able to be resolved on a CCD image. Remember that on the astronomical magnitude scale, larger numbers correspond to fainter stars. The dimmest star that is visible to the naked eye in no light pollution is about magnitude 6, but the dimmest star that I can see from my home in the San Francisco Bay Area is around magnitude 3. Using telescopes, we can go much fainter, even in light pollution, and the LCO telescopes are generally located away from light pollution. Therefore, we can measure stars down to approximately magnitude 13. Remember, the primary star is brighter, so it will have a lower magnitude. That is why we only need to limit the magnitude of the secondary star for the search shown in Figure 3.
Search constraint #2: Delta Mag < 3
The stars also need to be similar in brightness (delta mag < 3). If the stars of a pair are too different in brightness, then they will not be able to be captured on the same image. For example, say that the appropriate exposure times are 5 seconds for the primary star and 20 seconds for the fainter secondary. If you only expose for 5 seconds, then you will not see the secondary star at all. But, if you expose for 20 seconds, the bright star will completely wash out its companion star so that the two stars together look like a single bright blob. (There are ways around this problem that involve filters and/or image stacking, but these tricks make the whole project significantly more complex.)
Search constraint #3: Separation between 5, 15 arcseconds
In general, pairs of stars that are closest together are the most interesting, because they're likely to be orbiting fastest, and will be likely to show the most movement since the last measurement. But although closer, high-delta-mag stars are "sexier", they are also harder to resolve and measure. Therefore, it is a good idea to have a backup system ready to request in case you cannot resolve the stars of your original choice.
Search constraint #4: Physical
Two stars that are close together in the sky, which have similar parallax and proper motions in RA and Dec, are likely to have a physical relationship. They may or may not be gravitationally bound, but they are likely to be related in some way. For example, they might share a common origin. Studying the trajectories of stars that were born together can help astronomers understand the evolution of galactic structure on a larger scale. Therefore, we want to select a target whose stars are as close together in the sky as possible, and which have similar parallax (Plx) and proper motion (pmRA and pmDE).
The Washington Double Star Catalog's data is somewhat out of date, but it is still useful as a rough filter. Therefore, be sure to check the "Physical Double" checkbox to make sure that the returned systems are (or at least at one time were thought to be) a physical system.
Search constraint #5: Currently visible
A great way to figure out which stars are up in the sky is to go outside at night and look up! It is fun to image a star in a constellation you can currently see near the zenith of your night sky, because you can look up at night and know that your star is right there. (Instead of looking for a constellation, though, you could also use Stellarium or simply constrain the RA, as we have learned to do previously using these instructions.)
Using Stelledoppie
Enter your search constraints into Stelledoppie, a search engine for the Washington Double Star Catalog, using this link. Figure 3 shows a sample search for stars in the constellation Gemini.
[FIGURE 3: A sample search for targets in Gemini using Stelledoppie.]
Running the search in Figure 3 returns a lot of records! You can pretty much pick one at random, but do check the spreadsheet to make sure that no other student picked your same system. Before selecting any star as your target, you will want to check that the delta mag is okay for the separation (specifically, if the delta mag is around 2 or greater, then the separation should be more than 8 arcseconds, because otherwise the higher exposure time necessary to resolve the fainter star will spread out the brighter star's light too much).
Once you have chosen your target, record the data from Stelledoppie into the white columns of the spreadsheet. Map the Stelledoppie data to the columns as shown in Figure 4. Make sure to insert a link to the Stelledoppie url from the cell with the discoverer code. Otherwise, it will be easy to lose track of your stars!
[FIGURE 4: Keeping track of double star data from Stelledoppie.]
The latest, most accurate data on star positions was released by the third data release of the European Space Agency's Gaia mission in June of 2022 and should be referenced by any study of a star system. Therefore you will want to look up your star's data in the Gaia catalog to make sure that the stars in your pair have similar parallax and proper motion, and to find other juicy tidbits about them.
Next Steps
After selecting potential targets, your next step will be to verify them using the Gaia database and set up your research framework.