PostHeaderIcon Diffraction Spikes, or Why Stars Have Points

Stars with Points
Tis the season to draw stars – many different kinds of stars. The classic star drawn by elementary-schoolers is a 5-pointed star created in a single stroke, leaving a pentagon in the center. Other hand-drawn classics include: a “Star of David”-style star: two overlapping triangles; a simple starburst of three or more overlapping lines; and a “Christmas” star: four overlapping lines with a longer tail.

2007_12_02 Stars
Caption: Hand-Drawn Stars

A real star is a giant sphere of burning gas. Sometimes we draw stars as single dots, but we almost always draw our stars with points. Why? I don’t know. I do know that photographs of stars through telescopes also exhibit “points.”

2007_12_2 Diffraction Spikes
Credit: Robert Gendler
Caption: The Pleiades with Diffraction Spikes

Telescopic Spikes:
The spikes on a star in a telescope’s photograph are an artifact from the telescope itself. (An artifact is something in a photograph that looks real, but isn’t actually there).

Most large telescopes have lenses or mirrors inside that are between the main opening of the telescope and the main mirror or lens. These smaller bits have to be held up in the middle of the telescope tube somehow. The easiest way? Struts. Support rods. These small struts then deflect the light from the stars, and cause spikes to show up on the image.

Diffraction spikes show up on point-sources of light (like stars) because the light is coming from one location, not many. You can use this technique to identify stars in a photograph, and differentiate them from nebulae and galaxies. Mostly.

A subject for another AstroInfo. In short though, light acts like a wave pattern, so when it encounters an edge or corner, it bends. When light goes around a strut or rod, it splits into two wave-like patterns, which bend around the two edges (left and right) and recombine on the other side of the barrier. This results in prettiness. Diffraction gratings (special little pieces of iridescent, clear plastic) are a fun way to observe lights of all kinds. You can buy them in the store this time of year as “holiday glasses” or “Christmas light viewing glasses.” They’ll make every point of light you see have a little snowflake or “Ho Ho Ho” around it. (They’re doing even cooler optical tricks.)

Want More?
Do this experiment at home! –
Astronomy Picture of the Day –
A Short Brittanica Article –

Wikipedia (decent article, last I checked) –

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