Classifying variable phenomena on HST images

It's not always easy to discern the nature of the variations you see between blinked image. Sometimes the variations are not celestial at all; they can be caused by electronic problems or stray particles striking the detector. Using more than 2 epochs sometimes helps to distinguish between real variations and stray events.

The zoomed region of WF4 we've been viewing has no known Cepheids. Let's blink a series of images taken at 6 epochs to try to discern the nature of some of the variations. The images below were taken on April 23, May 4, May 6, May 26, May 31 and June 17 of 1994.

What are some of the possible sources of variation?

  1. Slight shifting of image. The images seem to shift slightly from epoch to epoch. This is simply a measure of how accurately the telescope can be pointed. The pointing may vary by a fraction of a pixel, which causes a noticeable shift on the zoomed scale of these images.
  2. Variable Stars. Most of the small roundish objects in the images are stars. Variable stars are fairly common. Some of the stars in the above images appear to vary. Always remember that Cepheids are a special variety of variable star; not all variable stars are Cepheids. Some of the variable stars may in fact be Cepheids, but the data is not sufficient to discern the Cepheid characteristics. Another point to remember is that it is possible for a constant star to vary a bit in appearance in the sequence of blinked images, owing to slight differences in telescope pointing, exposure conditions, and the images processing used to create these images.
  3. Cosmic rays. Although space is a vacuum by Earth standards, there are still many high-energy particles called cosmic rays whizzing through the universe. Cosmic rays regularly strike astronomical detectors and leave streaks or blotches in the images. The images you are viewing have been cleaned to remove most of the cosmic rays, but the cleaning routines are far from perfect. Blinking "before" and "after" images from May 4, 1994 will demonstrate the difficulty.

    You can see that cosmic rays are a major issue in long exposures. No computer program can perfectly remove all the cosmic rays. How can you tell the difference between a head-on cosmic ray hit and a variable star? Cosmic ray hits are random. It is extremely unlikely that a cosmic ray will hit the exact same location on the chip in two or more of the six epochs you will be blinking. A cosmic ray will appear as a blip in only one image. A variable star will be visible in the same location in at least two of the images.

  4. Nebulosity. The gas and dust in the disk of M100 appears as nebulosity in the zoomed images. Owing to changes in exposure conditions, it is impossible to match the contrast levels across all 6 epochs. Apparent changes in nebulosity intensity are almost always a result of image processing.
  5. Asteroids. It is possible that moving blips may be asteroids. Asteroid orbits may be inclined as much as 30 degrees to the ecliptic. M100 is located less than 20 degrees north of the ecliptic. (See the Messier catalog for coordinates.)
  6. Bad pixels. Some pixels in the detector have electronic problems, and may be unusually bright. The smoothing routine used on the images can make these "bad pixels" appear starlike. Some bad pixels are permanently bad, some are intermittent.
The professional astronomers did not blink images to find Cepheid variables. They used sophisticated computer programs to hunt for them. There was no need to remove cosmic rays for the professional hunt; a cosmic ray or bad pixel does not behave like a Cepheid variable. The computer programs could distinguish subtleties the eye cannot. Checking results visually is still a good idea -- one wants to be sure the program is behaving!
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