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Colors of the Stars

The Moon plays the magician for the third time in less than five weeks. As noted in last month's article, the Moon eclipsed the Sun on February 26th and eclipsed (occulted) the bright star Aldebaran on March 4th. It performs the same trick with the Jupiter system on March 26th. Presto-Chango! Now you see it and now you don't. The Moon/Jupiter grand finale is a five for the price of one event. Jupiter's moons will take part in the ceremonies as under cards to the main event. Our Moon will be a thin crescent. The biggest problem for this event is that Jupiter and our Moon will be inconveniently placed in the dawn twilight. You need to get up early and you'll need a telescope for this event. At 4:51 AM (all times calculated for Westerly) Callisto and a few seconds later Europa will be occulted.. At 4:54 AM, Io will be occulted. At 4:55 AM Jupiter will take about a minute to under go occultation. Finally at 5:01 AM Ganymede finishes the cycle. They will reappear some 56 minutes after they disappear, but in the mean time at 5:40 AM the Sun rises. It will take a good telescope with a clock drive to catch the reappearance in the daylight.

Stars shine in every color of the rainbow. In fact, their colors spill over into the infrared and ultraviolet. If this is true why do all stars appear white with just a few of the brightest ones faintly tinted red, yellow or blue? Most of the reason lies in the human eye. Light receptors consist of color sensitive cones located primarily in the fovea (the center of our eyes) and dim white light sensitive rods which generally surround the fovea. Very few stars are bright enough to activate the cones, so most stars are only seen by the rods, making them appear white. This has some interesting implications for observing stars.

The first implication is that to see more stars in color, you need a fairly substantial telescope. The large objective lens or mirror collects many hundreds of times as much light as our unaided eye. The extra illumination activates the cones displaying stars in all colors of the spectrum. You will be impressed by the hues you can see through a sizable telescope.

The second implication is that you can train your eye to be more sensitive by using something called averted vision. The dim light sensitive rods are peripheral. You can see dim light better out of the corner of your eyes (averted vision). I've trained my eyes so that I can frequently see dim nebula and faint stars that most people miss even though my eyes are no better than theirs. It certainly helps when I am trying to point my telescope to some faint object.

Most of the information we have about the Universe outside the Solar system comes as electromagnetic radiation. Electromagnetic radiation includes radio, microwaves, infrared, light (the visual colors), ultraviolet, X-rays and gamma rays. While we do get some information from other sources including cosmic rays and neutrinos, astronomers are in the midst of debating confusing results from these sources. While interesting and promising, relatively little has been learned from them yet.

Electromagnetic radiation, particularly light, has been utilized to determine the distance, chemistry, the age, the size, the speed and even the history of a star. We can tell if the star is stable, or suffers pulsation's, or emits giant flares, or periodically erupts in huge explosions. Recently, astronomers have even been able to find planets around ten nearby normal stars, by measuring subtle changes in the stars' color. Not bad to learn all that from something that seems so commonplace and ordinary as light.

The wavelength at which a star radiates light most strongly determines the temperature of the star rather precisely. Very red stars have temperatures as low as 2500°K (on the Kelvin scale) and the blue-violet stars may have surface temperature of 40,000°K or more. Initially, astronomers assumed that the red stars were small stars and that the blue-violet stars were the giants, but this was discovered to be a much too simple concept. Betelgeuse is very red and it isn't close to us at all. If a star is radiating feebly in the red end of the spectrum and yet is brilliant over long distances there is only one possibility, the star must be truly huge. Betelgeuse is a red super giant. At the other end of the scale, Sirius has a companion (Sirius B) which radiates very intensely in the blue-white part of the spectrum. However, we can't even see Sirius B without a fairly stout telescope because it is so small. Sirius B is the burned out core of a once great star. Sometime in the distant past most of this star was blasted away into space leaving a white dwarf.

Our Sun is a middle of the road star. Like all stars it radiates in a wide spectrum of colors, but it strongest radiation is yellow. Since it is yellow, we know its surface is 6100°K. Take a prism (a triangular bar of glass) and hold it in Sun light. You will see the Sun's bright white light form a rainbow. If you put a thermometer just outside the red end of the spectrum, it will rise as it is warmed by infrared rays. If you expose photographic film just past the violet end of the spectrum, it will turn black from the ultraviolet light. Some animals actually see in the infrared and ultraviolet. Rattlesnakes see very nicely in the infrared. In the dark, warm objects (like mice) appear to be brightly lit objects to them. Bees see well in the ultraviolet. This allows them to forage on cloudy days.

Leslie Coleman
Author:
Leslie Coleman
Entry Date:
Mar 1, 1998
Published Under:
Leslie Coleman's Columns
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