Why stars look spiky in images from the James Webb Space Telescope

Image: NASA, ESA, CSA, STScI

The stars in the new images look better than they did before. I am talking about the image quality in more than one way. Many of the bright stars in the images have very distinct Christmas-ornament-looking spikes or, as one of my colleagues puts it, it looks like a J.J.Abrams promo poster.

This isn't a case of a lot of flare. If you look closely, you will see that all the bright objects in the images have the same pattern. The feature is more prominent when the light is bright. Hubble objects don't see as much distortion as other objects.

The pattern of spikes is different to the rest of the world. If you compare images taken by the new telescope to those taken by the old one, you will see that Hubble only has four spikes to the eight taken by the old one. It can sometimes appear as though there are six spikes.

From this moment on you will always be able to tell the difference between a Hubble image and a JWST image:



Hubble stars have four spikes in a cross. JWST stars have six in a snowflake. Thank you for your time. pic.twitter.com/BWsv2WqCqD

— Hank Green (@hankgreen) July 12, 2022

The shape of the spikes is determined by the telescope's hardware. Hubble and JWST use mirrors to collect light from the universe. A reflecting telescope has a large primary mirror that gathers the light and a smaller secondary mirror. The secondary mirror on space telescopes helps guide the light towards the science instruments.

The primary and secondary mirrors have different roles to play in the spikes. Light diffracts around objects. As light interacts with the edges of the mirror, it creates spikes of light. The mirror didn't add to the spikiness because it was round. There are hexagonal mirrors that result in an image.

Image: NASA

The secondary mirror is there as well. The primary mirror is held in place some distance away from the secondary mirror. The struts are 25 feet in length. There are more spikes due to diffracted light passing by these struts.

You can see the different spikes in Hubble pictures. There are three struts holding up the mirror.

Image: NASA

That is a large amount of distortion. Four of the spikes caused by the struts will overlap with four of the spikes caused by the mirror. There are eight soon-to-be-iconic spikes.

Depending on which instrument is processing the light, some of the spikes can be seen. This is visible in the images of the Southern Ring Nebula.

Image: NASA, ESA, CSA, and STScI

The image on the left was taken by the NIR cam. The one on the right was picked up by the telescope's MIRI instrument. According to an explanation posted by the Space Telescope Science Institute, stars have more prominent spikes in the light's wavelength. Diffraction spikes can be seen around stars, but they are fainter and smaller than you'd expect.

NASA and the Space Telescope Science Institute have put together an Infographic about the work of the telescope.

Image: NASA, ESA, CSA, Leah Hustak (STScI), Joseph DePasquale (STScI)

Related: