Word for the day: Opacity

2007 08 26, 1700-1928 UT

PCW Memorial Observatory, Zanesville, Ohio

(click image for larger version) Credit / Copyright Erika Rix

According to my heavy, red, weathered Merriam-Webster’s Collegiate Dictionary (tenth edition), opacity is defined as:
“n, …1: the quality or state of a body that makes it impervious to the rays of light; broadly: the relative capacity of matter to obstruct the transmission of radiant energy….2b: the quality or state of being mentally obtuse: Dullness…”

I kind of got a kick out this. It appears that with one word, I can attempt to discuss opacity of the Sun and yet at the same time try not to create opacity while doing it.

Studying the Sun, as well as anything worthwhile, can be very confusing and sometimes overwhelming. It helps to understand the basics such as knowing that the Sun is a giant ball of gas. It has several layers starting at the inner most called the core. The majority of the Sun’s core consists of hydrogen. By nuclear reactions, the hydrogen is converted into helium. The key here is that in doing so, energy is created. Energy equals heat. All in all, when we think of the Sun, we think of radiation, or electromagnetic radiation to be more specific. Radiation is a process that transports energy. Electromagnetic radiation is a radiation that carries energy through empty space by means of waves at the speed of light.

You see, atomic particles (created by the nuclear reactions in the core) speed up and grow from the exchange of varying flows of electrical and magnetic fields, which is where electromagnetic radiation originates. Following me so far? Here’s where I start to get back on topic. Electromagnetic radiation has both wavelength and frequency. When you multiply the two together, you get the velocity of light. If one of the variables increases, the other has to decrease for the velocity of light to stay constant.

Oh, how easily it would be to dive in further with all this. But I need to stay on track with the first definition of opacity. Wavelengths are compiled in what we call a spectrum. And this is when we get into means possible for us to view the Sun.

Imagine the energy being transported through a few more layers of the Sun, each layer quite a bit hotter than the previous as it extends away from the core. We finally reach the layer that most call the “surface” of the Sun, the Photosphere. Does that look Greek to you? Well, not to worry. It is Greek. The Greek word “phot” stands for light and “sphere” of course stands for round ball.

In the photosphere, the gas is heated so much that it burns bright giving off most of its energy close to the middle of the spectrum, creating visible light. And it doesn’t end there. Reaching out from that thin layer of burning gas is the chromosphere, meaning round ball of color. After a brief pass through the transition region, the energy enters the corona and then outwards as solar wind. Each layer is visible through specialized means. Each layer involves our word for the day, opacity.

One evening, quite a few years back, my brother in law and I were cooking supper together. I was in charge of the chip pan and cutting up the potatoes. I could see him very clearly across the room and the air was transparent and had a zero optical thickness.

As we were talking to each other from different ends of the kitchen, we soon noticed that we were getting harder for the other to see. In other words, the optical thickness was getting thicker. By the time we became alarmed to this fact, the smoke was nearly opaque with an optical thickness of close to 9. I could hardly see him anymore. As he walked toward me, I could see him more clearly and by the time he reached me the optical thickness was perhaps a 3.

We removed the smoking chip pan that caused the smoke from the stove, opened the kitchen windows, grabbed the dog and a bottle of wine, and sat out on the steps of the flat, watching the smoke roll out of the kitchen window. I don’t recall what we ever did for supper that night, but I suppose that’s beside the point. It was a perfect example of opacity and how I measured it. The same is done when viewing the Sun.

The further into the Sun we look, the higher the opacity. We can only see up to approximately an optical thickness of between 0.5 and 2. The photosphere is said to have an optical thickness range of close to 3/4, and it includes all the light that we can muster from the Sun, meaning white light. If I wanted to view through a narrowband filter such as a hydrogen alpha filter, the optimal optical thickness would be reached before I even gazed into the Sun as far as the photosphere. I would in fact start at the Chromosphere. This is wonderful news for us in that by using special filters, it changes the opacity from a zero to us being able to actually see the color of the light in this layer of gas, blocking out all the other colors that would have hidden this color otherwise.

Well now, I’ve come full circle with opacity! And what does this have to do with my observations today? Well everything to be honest. Opacity is what strives us to find new filters for trying to tease out as much detail as we can. And there’s information to be had if we can look at different layers of the sun. In my observations today, I viewed in both the photosphere and the chromosphere. Two different gas layers with a temperature difference of over 4000 degrees Kelvin (chromosphere at 10,000 K and photosphere at 5780 K). Each will allow us to see slightly different details on the Sun and each are important to consider while studying it.

(click image for larger version) Credit / Copyright Erika Rix

NOAA 10969’s plage intertwined and reached out with crooked fingers.

The next observation was using a white light filter where over 99.999% of the Sun’s light is blocked out, making it possible for me to view the photosphere. This is called white light. You can see NOAA 10969 in the cooler layer. The chromosphere becomes invisible to me again. The two dark sections of umbrae within the penumbra of this action region were very prominent. I could see a darkened outline of the penumbra and it had an almost rectangular shape with curved corners. Of particular interest was the very faint darkened area to the right of the sunspot. This happens to me fairly often, seeing little bonus features like this. I’m still not sure what causes it. Normally I would think it was contrast from faculae that I was unable to discern. Normally we can only see faculae closer to the darker limb regions. But often I can see an outline of contrast suggesting faculae present when the active region is toward the center of the disk.

This time it is a little different. If I didn’t know any better, it looked like a thick triangular cooler region next to the sunspot. By this I mean cooler than the photosphere, hotter than the umbra, and only just slightly hotter than the penumbrae.

With so much to learn concerning the sun, at least we learned one new word. It’s a start in the right direction anyway.

Erika Rix is a Freelance Observer for Scientific Frontline®

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