Hunting Cepheids

How do you search for Cepheids? Cepheids are variable stars, and in the last section you learned how the blink technique can be used to spot variable phenomena. Here you will blink images of M100 taken at 6 different epochs, looking for variations in stellar-type objects. The images used in this lab were taken in 1994 on April 23, May 4, May 6, May 26, May 31 and June 17.

Of course, you cannot possibly discern by eye the specific intensity variations which characterize Cepheids. Sophisticated computer programs were required to do that. You will simply look for variable stars, and click on a suspected Cepheid. If the star is a Cepheid, you will be rewarded with information on the star. If not, the hunt continues!

The hunt will take place only in chip 4 (WF4) of the M100 images. WF4 has been divided into an 8x8 numbered grid for the purpose:

In the real Cepheid hunt, clicking on a grid section with a number will begin a hunt in that section. (Clicking on the above images will have no effect.) The WF4 image without the grid is mapped in the same way as the image with the grid; clicking on the same location in either image will produce the same result. The best way to see how all this works is to walk through the process step by step the first time. Some grid sections contain Cepheids, while others do not. You will be told how many known Cepheids are in each grid section.

Make sure you have a printed copy of the lab sheet before beginning this demonstration.

Let's first look at the case where there are no known Cepheids.

Case 1: Grid section contains no known Cepheids

If the grid section does not contain any known Cepheids, you can explore the images, or return to the main 8x8 grid to choose another grid section. Record the grid section number in Part I, Section A of your lab sheet to prevent you from repeating work unnecessarily.

Just because there are no Cepheids in the grid section certainly doesn't mean there is nothing of interest in the images! If an object intrigues you, you may want to inquire about it. Before you can ask your teaching assistant or professor about the object, you will need some means of accurately conveying the object's position. You will see how to do that in this demo.

In this demonstration, the usual text for the Cepheid hunt will be in normal type. The added demonstration text will be in red type. The blinked images you viewed in the last section (Classifying variable phenomena on HST images) were of grid section 26. Click on grid section 26 in the image below to see a typical Web page for a grid section containing no known Cepheids:


  1. Every time you click on the blinking images to get coordinates, a new Web page is generated. As a result, you may have to click the "Back" button on the toolbar more than once to return to this page. If you want to avoid having to click "Back" several times, use the link "return to the main WF4 8x8 grid" on the grid section page.
  2. The blip at x=178, y=113 is of no known significance, so do not think you should record every such blip. It is probably a cosmic ray.

Case 2: Grid section contains at least one Cepheid

If the grid section contains at least one Cepheid, the procedure is a little different. Look for variable stars in the image; click on a suspected Cepheid. If the object is indeed a Cepheid, a new page will appear which identifies the Cepheid, and displays its light curve. If the object is not a Cepheid, you can continue to hunt, get the coordinates of an object of interest, or go back to the original 8x8 grid and pick a new grid section. Unlike the case where there are no Cepheids in the frame, you must go to a separate page to get the coordinates of an intriguing object.

Grid section 47 contains more Cepheids than any other section, and they are fairly easy to discern. Please follow the demonstration step by step. The usual text for the Cepheid hunt will be in normal type. The added demonstration text will be in red type.

Click on grid section 47 below to begin the demonstration.


  1. Not all Cepheids look like C46. Some are bright, some are dim (sometimes very dim!), and some do not appear to vary much at all in the 6 epochs shown. Some are easy to find, while others are hard to distinguish from other types of stars. It is usually easier to discern Cepheids in regions where there is little nebulosity. You may find it necessary to adjust the contrast and brightness on your screen.
  2. When you do the real Cepheid hunt, try to find the other two Cepheids in grid section 47!

The next step

Are you ready to hunt for Cepheids? Almost... The last thing you need to know is how to determine the physical parameters of each Cepheid.
On to Determining the physical parameters of each Cepheid

Back to Classifying variable phenomena on HST images

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