Dan wrote:I have a question about the image. The three large and highly visible stars to the left and lower of the spiral galaxy, each have four reflective rays emanating from them. What causes those rays to be multi-colored? Is that caused by the light of the stars passing thru different gases and do all stars have the same gases surrounding them?
Those are diffraction spikes, caused by the absence of light blocked by anything in the telescope's beam - most often, supporting arms for the secondary mirror. 4-part supports are most common, hence the familiar 4-sided pattern (whose orientation reflects the telescope orientation during the imaging observations); in contrast, images from the Keck telescopes show a hexagonal pattern, and refracting telescopes do not show them at all. Noting the color behavior is more subtle - the diffraction phenomenon is wavelength-dependent and comes in multiple orders, each more spread out than the previous one. Some of these stars are bright enough to show many; the radial location of each color around each star matches, but brighter stars show more orders.
Occasionally observers use this effect deliberately; I've seen pictures of the faint companion of Sirius where diffraction spikes were added by putting a mask in front of the telescope aperture, moving the scattered light away from the faint companion; and also seen images taken with a coarse wire grating that created diffraction spikes broken into short segments, allowing a mean position even for stars whose main images were too saturated for accurate measurements. Sometimes one see composite images whose components were obtained with different telescope orientations, so the spikes in each filter are at different angles (Here
is an example from Hubble data).