Pick a target and make a plan

Today's lab exercise should be carried out in pairs, ideally, but it's okay to do it by yourself, too. In the exercise below, you will see a number of items written in red letters , like the following:

This is an example of an item you must write in your report.

In order to gain credit for this exercise, you must create a PDF document which provides the answers to all the questions and items written in red. Submit the PDF to the instructor via the "Assignments" tab in myCourses.

Today's assignment is meant to give you practice performing all the steps necessary to choose a good target for your project. It's not necessary to have chosen an object for your project yet; you'll have several more weeks to think it over. Consider this exercise as a practice run, just to give you some familiarity with the procedure.
In this practice exercise, I'll pretend that your target is going to be a variable star. In real life, you might choose a variable star, but you might instead select an asteroid, or a star cluster, or something else.

The goal is to choose a target for your observational project -- one that you and your teammate(s) will measure with our telescope later this semester. In order to pick a good star, you will make use of a number of tools:

Table of contents:

Constraints on your target
Searching through catalogs
Can you see your target all night long?
Build a bigger team

Constraints on your target

The big goal is for you to pick a variable star which you should be able to measure accurately with our equipment over the next few months. Let's go over some of the items to keep in mind as you try to choose from the tens of thousands of stars in the sky.

Your star ought to satisfy all of the following criteria:

visible (nearly) all night long. In particular, for dates between Mar 1 and Apr 10,
- at least 15 degrees above the horizon when the Sun is 10 degrees below the horizon after sunset
- at least 15 degrees above the horizon when the Sun is 10 degrees below the horizon before sunrise
pass close to overhead, so we see it as clearly as possible. That means the Declination should fall into the range +35 ≤ Dec ≤ +50
be bright enough to yield a good measurement in a short exposure time. That means it ought to have a V-band magnitude in the range 9 ≤ V ≤ 13
have a relatively short period, so that you can measure its brightness over one complete cycle with just one or two nights of work. A good rule is Period ≤ 0.5 days

In addition, it will be a great help if we can compare our results to those of other astronomers, and make use of additional measurements they might have made. So there's a bonus for targets which

have been the subject of 2 or 3 (or more) publications in the past
(for eclipsing systems) have radial velocity measurements
(for pulsing stars) have records of "times of minimum" published since the 1980s or 1990s

Searching through catalogs

There are two main types of stars from which you can choose:

eclipsing binary systems
pulsing stars

So, first, pick one or the other. You can change your mind later if things don't work out for your original choice.

The next step is to look through a big catalog of variable stars to select a group of candidates. Eventually, you'll winnow this group down to just one or two possibilities.

Go to Vizier and ask for the "General Catalog of Variable Stars".
Select the entry with code name "B/gcvs"
Choose the first item on the list, called "B/gcvs/gcvs_cat". (If you are adventurous, you could pick "B/gcvs/nsv_cat" instead)

You should see a screen which looks like this:

Now, if you leave all the boxes empty, and just click on the "Submit" button, Vizier will interpret that as the request "show me every star in the catalog". It will simply print out a list of all the stars, but it will stop printing the list after a certain number of entries, just to save space.


  Q:  What is the default number of entries returned?

However, we don't want to sift through ALL the variable stars in this catalog. Instead, we want to add some CONSTRAINTS, which will select on those entries which satisfy some conditions.

For example, suppose that you wish to select only stars which are eclipsing binaries. One of the rows in the database is called "VarType". If you click on the item labelled "see file vartype.txt", a new window will pop up, showing the various codes for different types of variable stars. By typing one of those codes into the Constraint box next to the word "VarType", you can choose ONLY stars which belong to a certain type.

In the example below, I am choosing only stars which are of the AM Her type, for which the code is "AM".

How many AM Her stars are in the GCVS catalog?

Okay, now it's your turn. If you wish to choose an eclipsing binary star, type "E*" into the "VarType" box. If you wish to choose a pulsing star, type any of the following: "RR*", "DSCT". Then click on "Submit" in order to get a first idea of the number of possibilities.

Of course, most of these stars won't be visible from Rochester during the night-time sky in March and April. You need to add additional constraints to the query. If you have questions about the manner in which you can add a condition (such as "Dec is between 35 and 50 degrees"), click on the blue question mark next to the word "Constraints" in order to be shown some examples.

Choose a range of Declination values between +35 and +50 degrees. How many candidates of your type are there?

Choose a period which is less than 0.5 days. Now how many candidates of your type are there?

Select only stars with "magMax" less than 13. Now how many candidates of your type are there?

Can you see your target all night long?

At this point, you ought to have a list of candidates; if the list contains only 1 or 2 items, please talk to your instructor. All of the objects on your list will be visible from Rochester at SOME time of year ... but will they be visible during the time when you can make your observations? In order to make your nights at the Observatory most efficient, your star should (ideally) be visible all night long In other words, it should be

at least 15 degrees above the horizon when the Sun is 10 degrees below the horizon after sunset
at least 15 degrees above the horizon when the Sun is 10 degrees below the horizon before sunrise

In addition, it must lie in a region of the sky to which our telescope can point and track.

The mount has a limited range of motion in the East-West direction. In order for a target to be visible, its Hour Angle (abbreviation "HA") must lie within the range 00 hours to 05 hours, or within the range 19 hours to 24 hours. You can see the "HA" value listed by Stellarium in the list of information for an object. See the example below.

In this example, the Hour Angle of Sirius is 21 hours 40 minutes and 43 seconds, or or 21:40:43. That lies in the valid range from 19 hours to 24 hours, so our telescope could indeed point to Sirius at this time.

A good way to check is to use Stellarium to make a map of the night sky. First, use Stellarium to fill in the following table with the local times for "just after sunset" (= Sun is 10 degrees below horizon after sunset), and "just before sunrise" (= Sun is 10 degrees below horizon before dawn).



                  just after sunset                just before sunrise
  Date
                    local time                         local time     
--------------------------------------------------------------------------

  Mar 1



  Mar 20


 
  Apr 10

--------------------------------------------------------------------------

Okay, now you can use Stellarium again to figure out if any of your targets will be ready for you to observe. Set Stellarium to each of the above dates and times, and then locate your candidates. Is the candidate at least 15 degrees above the horizon at those times? As the candidate moves through the sky, does it stay within the range of Hour Angle that lies within our telescope's range of motion?

(Hint: the answer to these questions depends in large part on the Right Ascension (RA) coordinate of the target. You will quickly discover that only a relatively narrow range in RA values will put a star in the "right" portion of the sky).

Make a list of all the candidates which satisfy all these conditions. Make a second list of candidates which satisfy MOST of these conditions, too.

Build a bigger team

At this point, you have a list of good possibilities. In order to increase the efficiency of our observing, we ought to form groups of 3-5 students who will work together on the same target.

So, walk around the room and talk with other people who have reached this point in the exercise. If you all agree on a particular target, then you can create a "team". Each team should make a list of three candidates: the "top" choice, and two "runners-up".

Create a list of all team members.

Make a neat table showing the name, RA, Dec, and type of your three targets. Clearly label the "top" choice.

For more information

You can find a more detailed and step-by-step discussion of eclipsing binary stars in another course I teach:
- Analysis of eclipsing binary systems (PHYS 370)
If you want to know more about the way astronomers measure "color", see
- a quantitative way to measure stellar "color"
For more information about pulsing stars, and how we astronomers use them to measure distances, see
- Using pulsing stars to measure distances (not too technical)
- Lecture on variable stars from a technical course on the distance ladder
The examples in today's notes were taken from Stellar Properties of KIC 8736245: An Eclipsing Binary with a Solar-type Star Leaving the Main Sequence by Fetherolf et al, AJ 158, 198 (2019).
Some of the software developed by astronomers to analyze observations of eclipsing binary stars are