Why are stars star-shaped?

Electricity and Magnetism Level 2

In astronomical photos of stars, there are sharp lines that emanate from the center of the stars that are point sources of light.

What is the physical effect that causes these spikes?

Diffraction at opening and apertures in the telescope Light scattering on particles in the air Refraction of light at atmospheric layers Aberrations of lenses and mirrors in the telescope

1 solution

Markus Michelmann
May 11, 2018

The spikes are also observed in particular on photos of the Hubble telescope , which is located in outer space. Therefore, the atmosphere can be excluded as a cause, so that the spikes must be created by the imaging in the telescope itself. In the case of lenses and mirrors, aberrations are referred to as deviations of the ideal spherical shape, which cause a blurred or distorted image. These effects can be understood in the context of geometric optics. But stars are point light sources and a distorted point would still have to be imaged as a point, so that aberrations can be excluded as a cause.

The cause of the spikes is the wave nature of the light, which leads to light diffraction at the opening of the telescope ( diffraction spikes ). Diffraction refers to the deflection of waves on an obstacle. A simple circular aperture on the lens causes a point light source to be imaged as a circle surrounded by diffraction rings (airy disk). However, the spikes in the photos of reflecting telescopes are caused by the suspension of the secondary mirror inside the tube, which acts as an aperture for the incident light rays. Depending on the number and arrangement of the supports, you can observe a different number of spikes, so that a star photographed with different telescopes can have two, four or six beams. Even when looking into the night sky with the naked eye, you can perceive a pointy star-like-shape because of diffraction. This is ultimately the reason why we represent stars in symbols and illustrations since the beginning of human history also with points.

This problem was inspired by this minutephysics video .

Some kinds of telescopes, such as Schmidt-Cassengrain or Matzukov, doesn't produce such spiky diffraction patterns, and only produce circular diffraction patterns not too different from Airy diffraction patterns. This is because the secondary mirror is fixed to a clear pre-correction lens in front of the primary mirror. Both models are popular for small portable telescopes, but not practical for very large telescopes.

There are many kinds of telescopes that eliminate the spiky diffraction patterns, and they are usually catadioptric telescopes which combine lens and mirrors in their designs, generally making it possible to avoid internal supports that can cause unwanted non-circular diffraction patterns.

A "perfect" human eye shouldn't see spiky stars on a clear night. However, imperfections in the cornea and lens can produce irregular glare spikes. In illustrations long before telescopes, stars were often depicted as having 4 or 5 or even 6 spikes, giving the reader the general idea of how most people see stars.

Michael Mendrin - 3 years, 1 month ago

You're absolutely right, not every telescope produces diffraction spikes. Especially for refracting telescopes we do not expect this effect in this form, since it has a circular aperture. I did not want to provide too much information in the problem itself, because otherwise you can exclude most of the answer options immediately. In my solution, I have not gone too much into detail, as you can basically read everything on Wikipedia.

But I'm glad, that there are still people like you who also read through what I post here.

Markus Michelmann - 3 years ago

So I just now learned about the suture lines in the lens of an eye, and how that's responsible for the spiky stars that we see. And, yes, the shape of segmented reflecting telescope primary mirrors (like the used for the James Webb Telescope) can cause diffraction patterns.

No need to change the wording of your question or the answer. I enjoy reading your problems and answers.

Michael Mendrin - 3 years ago

Why are stars star-shaped?

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