Why stars look pointed in this image from the James Webb Space Telescope

The stars in new images from the James Webb Space Telescope look sharper than ever. And I’m not just talking about image quality, which is amazing. I’m talking about the fact that many of the bright stars in photos have very distinct nails that look like Christmas ornaments or, as one of my colleagues put it, “look like JJ Abrams promotional posters, and I love that. ”

But this is not the case Lens flare too much. These are diffraction spikes, and if you look closely, you will see that all the bright objects in the JWST image have the same eight-dot pattern. The brighter the light, the clearer the gain. Faint objects such as nebulae or galaxies don’t tend to see much of this distortion.

This yaw spike pattern is a unique pattern in JWST. If you compare the photos taken with the new telescope to the images taken by its predecessors, you will see that Hubble has only four diffraction heights to eight at JWST. (Two of the JWST spikes can be very dim, so sometimes there seems to be six.)

From now on, you’ll always be able to tell the difference between a Hubble image and a JWST image:

The Hubble star has four spines that cross each other. The JWST star has six in the snowflake. Thank you for your time. pic.twitter.com/BWsv2WqCqD

– Hank Green (@hankgreen) 12 July 2022

The shape of the diffraction spikes is determined by the telescope, so let’s start with a quick refresher of the important parts. Both the Hubble and JWST reflecting telescopes, meaning they collect light from the universe using mirrors. Reflecting telescopes have a large primary mirror that collects light and reflects it back onto a smaller secondary mirror. Secondary mirror In space telescopes, this light helps direct this light to scientific instruments which turn it into all the beautiful images and data we see today.

Both primary and secondary mirrors contribute to diffraction spikes but in slightly different ways. Light is reflected or bent around objects such as the edges of a mirror. So the mirror shape itself can produce spikes of light as the light interacts with the mirror’s edge. In Hubble’s case, the mirror is round, so it doesn’t add any forks. But the JWST has a hexagonal mirror that produces an image with six diffraction spikes.

Photo: NASA

There is also a secondary mirror. The secondary mirror is smaller than the primary mirror and is held in place away from the primary mirror using brackets. In the case of JWST, the struts are 25 feet long. Light passing through these struts is deflected, creating more ripples, each perpendicular to the strut itself.

In the case of Hubble, the four supports produce the four different elevations you see in the Hubble image. The JWST has three supports that hold its secondary mirror in place, resulting in six more rivets.

JWST struts during cryogenic tests in the field.

Photo: NASA

This is a lot of distortion. In order to reduce the number of diffraction spikes, the JWST is designed such that the four strut-induced protrusions overlap the four mirror-induced protrusions. This leaves eight diffraction spikes from the soon-to-be iconic JWST image.

Some nails will appear more or less visible depending on the tool that also treats the light. This is most evident in the JWST images of the Southern Ring Nebula, released this week.

Two JWST square views of the Southern Ring Nebula.

Image: NASA, ESA, CSA, and STScI

The image on the left was taken by JWST’s NIRCam, which collects near-infrared light. The image on the right was captured by the telescope’s MIRI instrument, which captures medium infrared light instead. “In near-infrared light, stars have more remarkable diffraction spikes because they are very bright at this wavelength,” said Explanation Posted by the Space Telescope Science Institute. “In mid-infrared light, diffraction heights also appear around stars, but they are lighter and smaller (zoom in to find them).”

If you want a visual look at how diffraction altitude works at JWST, check out this helpful infographic from NASA and the Space Telescope Science Institute:

This graphic includes a lot of text. For text descriptions, Please click here.

Photos: NASA, ESA, CSA, Leah Hostak (STScI), Joseph DePasquale (STScI)

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