Why Stars Twinkle

Twinkling stars have inspired songs from the heart, and as the song says, “Twinkle, twinkle little star, how I wonder what you are.” Everyone has wondered why stars twinkle. Like diamonds in the sky, they appear as if twinkling is a hint from God of the gifts beyond the stars. To me, stars represent heaven sent promises, but science may not agree, so let’s see what twinkling stars mean, scientifically.

According to science, a twinkling star is called, ” a stellar scintillation,” and stars twinkle due to turbulence in the Earth’s atmosphere. Since the atmosphere is made up of several layers, each layer has a different temperature and density, and as the light from a star passes through the atmosphere, it bends through each layer and this is what we perceive as twinkling.

This bending of light while passing from one medium to another is like water to air or one layer of air to another. It’s called refraction which is the same thing that makes a straw look bent when you put it in liquid. Refraction makes the stars and the sun near the horizon look higher in the sky than they actually are, but in fact, when the sun is setting, we are actually seeing the sun’s disk below the horizon.

We always see the stars that are closer to the horizon and they seem to twinkle more, but only because there is a lot more atmosphere between us and the star that’s near the horizon, than there is between us and a star in the point directly over our heads. That point is scientifically called, “The zenith.” Stars are so far away that they appear as points of light, as opposed to planets which are much closer and that’s why planets don’t twinkle. The atmosphere isn’t large enough to affect the light coming from the planets.

When we look at the sky we see that some stars are bright and some are not so bright, it could be that they are all bright, but the distance to a given star might be too far. We don’t know the distance of a given star so we can’t tell whether the star appears bright because it is close or because it is far away but just extra bright.

Stars must be bright before we can see them twinkle, so throughout the ages people gazed at the stars wondering what makes them bright and twinkle all night. Even around 150 B.C. the Greek astronomer, Hipparchus assigned numbers to the stars according to how bright they appeared. He also assigned the number 1 to the brightest star of all and called them, ” first magnitude stars.” He called the star in the second brightest category, “the second magnitude star,” and so on. Once he found the dimmest star, he called it, “the sixth magnitude star,” and this is the limit of what human beings can see in dark or clear skies. There are about 6000 stars of magnitude six.

Then in the 18th century, the man who discovered Uranus, Herschel, noticed that we receive 100 times more light from a first magnitude star than from a sixth magnitude star, and in the 19th century astronomers used a new discovery of photography to quantify the amount of light that we receive from a star. They used filters at a specific wave length band and this technique is called, “photometry.” It is still used today to measure the magnitudes of stars, but in 1856 Norman Pogson defined a difference of 5 magnitudes to be exactly a ratio of 100 in the amount of light energy that we receive per second, which scientifically is referred to as energy flux. What this means is that a first magnitude star is 100 times brighter than a sixth magnitude star, and it was then possible to quantify the magnitude scale because this scale is related to the way our eyes perceive light.

By then, they found that two stars of the same magnitude put out the same light and a star brighter than another one by just one magnitude puts out 2.5 times more light. The important thing is the difference in the two magnitudes. Still, Pogson noted that Hipparchus scale wasn’t accurate when Hipparohus assigned Sirius and Vega a magnitude of 1, but although they are two bright stars, Sirius is much brighter than Vega, so today all magnitudes are measured with a photometer mounted on a telescope instead of by human eyes. With this said, it’s absolute magnitude that shows the real value to astronomers so that they know the intrinsic brightness of stars.

To find the absolute magnitude of the stars, astronomers have to know three things. The first thing is to find how bright the stars appear here on Earth and how far away they are. Finally, they need to see how much of the star-light is being absorbed by the interstellar medium, which is the gas and dust, or stuff, between the star and Earth. The process is like trying to decide which light bulb is brighter by placing them at the same distance.

Absolute magnitude is the apparent magnitude that a star would have at a distance of 10 parses from Earth, which is a unit for measuring distance in the sky and equal to about 19 trillion miles. The scientific symbol for absolute magnitude is MV and it’s related to the luminosity of the star, which is the amount of energy the star gives off. The brightest stars are the hottest and have MV that’s equal to 10 magnitude. The faintest stars are the coolest and have MV equal to + 17 mag. The more luminous stars are around 100 billion times brighter than the dimmest. As it turns out, even our sun is an ordinary star that shines at MV equal to 4.8 mag, but the important thing to remember is that the more negative the magnitude, the brighter the star, and the more positive the magnitude, the dimmer the star appears.

As interesting as the scientific facts of astronomy are, a star is still like a diamond in the sky that makes us wonder what they are and ponder the meaning of how the universe works for our behalf. I liked trying to figure it out, but in the end my conclusion is that stars are still a mystery to me and that’s why I’ll always believe they twinkle down on us as God’s way to say, “Some things don’t need to be explained.”

Still, if you simply need to know more about stars, go to the main sources of information for this article: http://www.mira.org/fts0/stars/text/txt001c.htm