Light in Astronomy

Almost everything we know about the universe comes from carefully observing light from distant objects. By light we mean all electromagnetic radiation, including radio signals, not just visible light.

Graphic showing radio, microwave, infrared, ultraviolet, x-ray and gamma ray electromagnetic radiation.

This light might have been travelling for millions of years, so it also tells us about the history of the universe. Astronomers have developed very sophisticated tools and techniques for studying the light that arrives from distant objects. Its properties reveal information about the objects that emitted it and the space that it travelled through. By breaking the light up into its component colours, we can determine the temperature of the source, what elements are present in the object, what elements the light has passed through, and even how fast the object is moving relative to us. By looking at how the light waves are oriented, we can learn about the magnetic fields near the object.

Graphic shows light reaching telescopes and how the light is analyzed by temperature, source content and relative motion, cloud content, and magnetic field.

Astronomers dissect the light from distant sources to learn about our universe. CREDIT: ESO ; SOURCE:  https://www.eso.org/public/images/spectroscopy-oli-6/.

Wave Nature of Light

Light can be pictured as a wave of oscillating electric and magnetic fields, which are perpendicular to each other and to the direction of motion. To keep things simple, we use the direction of the electric field to describe the orientation of the wave. In a typical incandescent light source, such as a hot filament or star, the electric fields point in random directions and the light is said to be unpolarized.

Graphic shows that electromagnetic radiation consists of self-propagating electric and magnetic fields.

Polarization

When the electric fields are oriented in the same direction, we say the light is polarized. If the electric fields oscillate along one line, we say the light is linearly polarized and assign a direction to it, such as vertical or horizontal. If the electric fields oscillate so that they sweep out a circle, we say that the light is circularly polarized and assign it a direction, such as left-handed or right-handed.

Graphic shows light that is polarized in different ways.
Light can be unpolarized, linearly polarized, or circularly polarized. SOURCE: https://commons.wikimedia.org/wiki/File:Circular.Polarization.Circularly.Polarized.Light_Circular.Polarizer_Creating.Left.Handed.Helix.View.svg

What Causes Polarization?

Light can be polarized when it is produced at the source or it can become polarized as it travels through space. Determining how much polarization is due to the source and how much is due to interactions while travelling is an important part of the data analysis.

Polarized at the Source

When electrons spiral around magnetic field lines, they emit highly polarized light through a process called synchrotron radiation. The orientation of the polarization is determined by the orientation of the magnetic field, so we can use polarization to map out the magnetic field where the light was produced.

Polarized by Magnetic Fields

Magnetic fields can change the polarization of light in a process called Faraday rotation. As light travels through a magnetic field, the orientation of the linear polarization rotates through an angle. The amount of rotation depends on the strength of the field, so we can use rotation to determine how strong the magnetic field is.

Graphic shows Faraday rotation.
https://en.wikipedia.org/wiki/Faraday_rotator

Polarized by Material

Space is filled with dust, and light travelling through can be polarized if the grains of dust are aligned by a magnetic field. By carefully analyzing the polarization at different wavelengths, astronomers can infer the orientation of the magnetic field that is acting on the dust particles.

Polarized by Reflection

When light reflects off a smooth surface, the reflected light is partially polarized. In our everyday world, this shows up as glare on surfaces such as snow, water, and glass. Polarized sunglasses filter out the excess reflection, reducing the glare. In astronomy, exoplanets can be detected by carefully observing the polarization of light. Starlight that reflects off the exoplanet is slightly more polarized than starlight that travels directly to Earth.

How Do We Visualize Polarization?

Polarization data is usually displayed using tick marks on an image. The length of the tick mark conveys the intensity of linear polarization, its direction represents the direction of the electric field vector, and its colour shows what fraction of the light is linearly polarized.

Superimposing the tick marks on an image connects the invisible polarization with the visible brightness. The swirling lines in the M87* visualization represent these tick marks. We can see that the polarization is strongest where the light is brightest.

Tracking the polarization over days also shows that it changes with time, implying a dynamic process in the accretion disk.

Graphic shows how the polarization of light from M87* changes over several days.

EHT Collaboration, APJL, VOL. 910, L13, 24 March 2021
Polarized light emitted from the accretion disk travels through the magnetic field of the jet on its way to Earth.

What Can We Learn from the Polarization Image?

Polarization allows us to infer things about the source of the light and about the medium that the light has travelled through. In the case of M87*, the polarization signature matches models for light produced by synchrotron radiation in the accretion disk and then scrambled by Faraday rotation in the magnetic field of both the disk and the jet. The scrambling of the polarized light that we observe coming from M87* tells us that the magnetic field generated by the extremely hot plasma in the accretion disk is strong enough to restrict the amount of matter that can fall into the black hole. This magnetic field is probably the source of the energetic jet that can be seen in visible light to extend almost 5000 light years into space.

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