Expert: Courtney Seligman - 9/20/2008

Question
Most photos of stars have four rays at 90 degrees to each other. Obviously they are an artifact of the telescope/camera.How do they form?
Most drawings of stars have 5 or 6 points. Do some telescopes give give 5 or 6 points to a star?

Answer
(Your question was sent to the Question Pool, because the expert you sent it to was on vacation, had already been asked their maximum number of questions for that day, or declined to answer it. I just found it in the Pool, a few moments ago.)

The effect you refer to is caused by diffraction, which is a result of the physical structure of the telescope interacting with the light entering the telescope. All telescopes create circular diffraction patterns surrounding the central stellar images, because the opening of the telescope is a round 'hole', but not all telescopes produce diffraction 'spikes', which are caused by wires or vanes used to hold a mirror in the center of the telescope opening. Such 'secondary' mirrors are used only in reflecting telescopes, and not even in all of them (e.g., Schmidt telescopes support the secondary mirror on a thin glass plate, so no wires or vanes are required). The number and geometry of the spikes is determined by the number and placement of the supports, and simplicity of construction usually results in the four rays you mentioned.

It would be possible to create 6-rayed images with the proper support geometry (though I doubt it has ever been done, as the rays are considered a drawback, despite their attractiveness), but the number of spikes is always an even number, so 5-rayed images are not possible.

The use of 5- or 6-pointed drawings to represent stars is not related to their telescopic appearance, but more likely to the twinkling bright stars exhibit when low in the sky. Six-pointed stars are easy to draw, and are an ancient mythic symbol, going back thousands of years, and five-pointed stars go back at least several hundreds of years. But I have seen atlases in which some of the brighter stars are drawn with more than a dozen points, so there is no specific 'tradition' involved.

Courtney Seligman
Professor of Astronomy
Long Beach City College