Q&A: Supernova Remnants and Neutron Stars

Q:
In the Crab Nebula Movie, why do some of the background stars appear to blink? Is this an artifact of the encoding or are they variables?

A:
The blinking of the background stars in the Crab Nebular movie is an artifact; they are not variable stars: The movie is created by showing a sequence of images. These images are from a series of observations using the Hubble Space Telescope. For each observation, the light entering the telescope was subject to various unavoidable instrumental effects. One of the most easily recognized effects is the cross-like shape that bright stars often have in images taken with optical telescopes (including Hubble). The following image is an example of this effect:
http://www.star.ucl.ac.uk/~apod/apod/ap021201.html
This effect (among others) caused the image of each star to not be exactly the same, from one observation to the next. For a point source such as a star, even a very slight difference due to this effect will be quite noticeable (since the effect renders the star no longer truly point-like). So when the images are shown in sequence, this difference becomes noticeable as blinking.