Why do stars have 8 points in the James Webb images? An astronomer explains

By Pete Lawrence

Published: Thursday, 14 July 2022 at 12:00 am

With the release of the first processed images from the James Webb Space Telescope (JWST), it’s clear that this technological marvel is going to revolutionise our understanding of the Universe. With many discoveries to come, one, perhaps more trivial, aspect of the results has raised a few questions of its own: why do JWST star images have eight spikes emanating from them?

The spikes are not real, they are a consequence of how light interacts with the optical system of the telescope. They are created by the process of diffraction, an effect which occurs within every telescope when light encounters an edge. You may have seen similar ‘diffraction spikes’ on images produced by other telescopes.

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The eight points of light are diffraction spikes from a star located to the top right of Stephan’s Quintet © NASA/ESA/CSA/STScI

Diffraction spikes are typically produced in telescopes which use a secondary mirror held in front of the main mirror; it’s the secondary supports that create them. The JWST has a secondary mirror held in front of the main segmented mirror. There are three supports, one vertical and two angled at 150º to the vertical. Both edges of each support produce a diffraction spike at right angles to the edge. As a result, the three supports produce six diffraction spikes.

Surprisingly, the spikes produced by the JWST’s secondary supports aren’t the most prominent. It’s the edges of the hexagonal mirror sections that produce the most dramatic diffraction spikes, six arranged at 60-degree intervals.

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The eight points, or spikes, of light are a result of diffraction from the hexagonally segmented mirror and the secondary mirror support as they overlap © Pete Lawrence

With the hexagonal mirror sections producing six spikes and the secondary supports a further six, you might expect to see twelve diffraction spikes in total. Clever design means that four of the secondary support spikes overlap four of the mirror section spikes and are hidden as a result. Look at a star in one of the JWST’s images and you’ll see the six large spikes from the hexagonally segmented primary mirror, plus two smaller ones from the vertical secondary support. The effect is easiest to see on brighter stars.

If you look carefully at a typical diffraction spike, you’ll see that it appears like a dashed line. This is caused by interference, the diffracted light waves reinforcing and cancelling one another as the spike is formed.

Read more about the JWST: