Distances to Galaxies | Cepheid Variables | How to Look for Cepheid Variables
 | The Period-Luminosity Relation | Find the Cepheids in M100
3. How to Look for Cepheid Variables
Cepheids are detected by their brightness variability. Variable phenomena can only be detected by comparing two or more images taken at different times, or epochs. Before reading on, think for a minute about how you might compare two images to look for variations.
The left image above represents a portion of the galaxy, M100. The image was taken on May 4, 1994, with one of the cameras on the Hubble Space Telescope. The red box on the left denotes a region 1/64 the area of the full image. On the right is that same region magnified by a factor of 8. On this zoomed scale, individual stars are clearly discernible.

An obvious first method to look for brightness variations would be to place images from different epochs side by side. Let's look at the same zoomed region shown above at two different epochs. The image on the left was taken May 4, 1994. The image on the right was taken two days later, on May 6.
Try to discern some differences between the two images. Some may be easy to note, others more difficult. Sure, you can see changes, but there's a much easier way to do it; we can blink the two images. Below you see the May 4 image being blinked with the May 6 image. Now the changes between the frames seem to jump out at you!
Tip: when looking for variations between blinked images, sometimes it helpful to place the cursor at a place of interest to guide your eye.

Just because something blinks doesn't mean it's a Cepheid variable. It could be a different kind of variable star or even an artifact of the instrument. Cepheid variables change their brightness in a very specific way which can be recognized by taking many images of the same field and comparing the images in a time sequence. In the next part of the demo we will show loops consisting of more than two images.

The images below were taken on April 23, May 4, May 6, May 26, May 31 and June 17 of 1994.

Can You spot the varying star?