I came across this "floating star" on New Scientist. It's an illusion that was created by Kaia Nao, and works because of something called 'peripheral drift', much the same way as the famous rotating snakes illusion.
The star and snakes appear to move when you don't look at them directly, i.e., when they lie in the periphery of your vision, because of the colour gradients in the images. The movement is usually from dark to light colours.
There's a bit of a debate over what exactly causes this perception of motion. It was previously thought that slow drifting eye movements interpret signals differently, depending on the luminance (intensity of light), thereby tricking the motion perception system into thinking that the image is moving. That is, it was thought that the eyes interpret dark and light colours in different manners as the eyes slowly drift around an image. Recently, an article in the Journal of Neuroscience suggested that it's not the slow movement of the eyes, but rapid eye movements called 'saccades' that are involved. But it still remains unclear as to why we can so easily trick our brains into perceiving something that doesn't exist.
Following this line of thought, I wondered if illusions could somehow be used to 'trick' the mind in terms of therapy. Google pointed me to the Oxford Journal of Rheumatology, where I found a couple of abstracts on illusions being used to treat chronic pain. I haven't had a chance to go through any of the papers, yet, but I am definitely intrigued. Studying illusions is an interesting insight into how our brains work. And potentially using them for therapy? Pretty darn cool.