Friday, November 23, 2007

Blackbody Radiation Exercises

No, not talking about a cosmic Jack Lalane (OK, too old of a reference). At my first alma mater, the University of Tennessee, Knoxville, the physics department has a java applet online that demonstrates why hot stars look blue and cool ones look red using Planck's Law, Wien Law, and Steffan-Boltzmann Law. It calculates the blue-visual and ultraviolet-blue color index, as well as showing where the normalized intensity lies in the ultraviolet - visible - infrared spectrum. This site is found at: <>.

I found it interesting that as a star gets hotter, its normalized intensity peak lies increasingly in the UV (ultraviolet) range of the spectrum. As stars get cooler, the peak moves toward the infrared, but doesn't get into the infrared except for very cool stars.

This would mean that for hot stars, UV would be the more important wavelength to use, while for cool stars, infrared could have some importance. Our eyes are centered around the normalized peak for the sun, with it's 5,780 K surface temperature, which makes logical sense. For beings on a planet around a blue star, their eyes could be centered in the UV band. They may not even have much use at all for the "visible" part of the spectrum ("visible" to us).

For plants, I wonder if this would mean they would be white, since they'd be using the UV photons for photosynthesis instead of our visible light range they'd reflect back all of the visible (to us) light - thus they would appear white. However, it seems unlikely that UV light photons could conceivably be used for photosynthesis. UV light is very energetic, and thus, for the most part, very damaging to life: UV inhibits photosynthesis in present day Earth plants and it can cause destruction to DNA as well as many proteins and lipids. The shorter the wavelength, the more energetic and the more damaging. However, it is thought that UV was an important source of energy in the creation of life and some viruses, bacterium, and fungal spores are rather highly resistant to UV radiation. However, often times creatures that are highly resistant to UV radiation are resistant because they have developed a mechanism for repairing the damage done by the UV radiation, and not because they can just ignore it.

Thus, there are limits to what carbon-based plant life can tolerate with regards to UV radiation. And for a planet around a particularly hot blue star, life would definitely have to find a way to deal with the UV radiation (here's a thought: if UV is thought to be important to the origin of life, would that mean that life is more likely to originate around blue stars than for other stars?). There are two articles, "Limits of photosynthesis in extrasolar planetary systems for earth-like planets," by S. Frank et al. and "Ultraviolet radiation constraints around the circumstellar habitable zones," by Andrea P. Buccino, et al. which I hope to get my hands on soon. Part of the Habitable Zone equation may not only include the zone where water can be in a liquid state, but also where UV radiation isn't extreme.

Anyway, there are some creatures on Earth that, while using the visible spectrum, also use part of the UV spectrum for vision. Various insects, for instance, can see into the UV spectrum - and some flowers have patterns on their petals that can only be seen in UV light to help attract the insects (to aid in pollination).

What would this mean for alien life? That they may be blind to most, if not all, of our "visible" spectrum, and we to theirs. This is not to say they would be blind, rather they would just find an inside room lit with light bulbs to be rather dim if not completely dark: most fluorescent lamps are designed to give off minimal amount of UV radiation - much less than what could be found out of doors (the mercury vapor in a fluorescent lamp does give off UV radiation, but it then collides with the phosphorus coating on the inside of the bulb, and thus is "converted" into visible light). Some lamps, like LEDs, do not give off any UV radiation. LEDs would be invisible to a being who sees only in the UV range.

This isn't an either/or proposition, of course. Some beings could have eyes sensitive to a large range, with part of their visible spectrum in the UV and part in our visible spectrum. They would be essentially color-blind in that they would see some colors, but not all (for instance, can only see blue and green). Many animals on Earth have limited color range, seeing mainly black and white. Even some humans only see in black and white: more than 5% of the natives of the island Ingelap, in Micronesia, have the rare condition of Monochromasy - total color blindness (Oliver Sacks wrote a book about them, titled The Island of the Colorblind).

Not sure how this would affect the alien's culture or theology, though they would see the night sky differently than we do, seeing patterns that we do not see, which could affect their theology to some degree. It would definitely affect their visual art - painting, drawing, and video. They would work on UV cameras first, and create UV TVs - rather different technology than what we came up with (I wonder how a UV TV would work?). The biggest affect, I believe, would be in the contact between us and them.

Of course, some Sci-Fi movies do have aliens that can only see in infrared or UV, though usually just in infrared, as it makes for more exciting drama to have an alien that can see our body heat - making us stand out like glaring targets, while the alien is concealed to our vision. That would indeed play on our visceral fears, if we knew that our new alien friends can readily see in infrared.

What other problems or difficulties can you think of?


"Blackbody Radiation Exercises." Physics Dept., University of Tennessee, Knoxville. 23 November 2007. <>.

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