A View From Planet Earth

Our home in the universe is a planet called Earth. It is a rocky body that is nearly three-quarters covered by water and is enveloped in a gaseous atmosphere. The Earth has been likened to a spacecraft with the oceans and atmosphere as part of our life support system. Space ship Earth is hurtling us though space at nearly 1200 miles per minute on yearly 600 million mile trip around our star, the sun. While orbiting the sun, the Earth also spins on its axis completing a rotation once each day. As we turn toward and away from the sun, we experience day and night. During the day, our view of the universe is hidden by the sun’s glare. Instead, we see a brilliant sun against the blue sky. Why is the sky blue? Because that is the color of air. Light from the sun consists of all the colors of the rainbow, but as it passes through the atmosphere, the molecules of air are just the right size to scatter the blue light in all directions making the sky look blue. The remaining colors of light come through the atmosphere giving a yellowish color to the sun when high in the sky, and reddish color when the light must pass through even more atmosphere as the sun nears the horizon.

As your particular locale rotates away from the sun and enters the Earth's shadow, darkness descends and you see hundreds of other suns. Each twinkling star is a brilliant sun, many even larger and brighter than our own. All are reduced to specks of light in the night sky because of their enormous distances. When first observing the night sky from a dark area, it may appear as a rather confusing panorama of stars, fixed and unchanging. However, as with most things in nature, a more careful observation shows intricate differences and subtle changes that become more and more intriguing as you make keener observations and learn to understand more of what you are seeing. For example, even the most non-critical observer notices that the stars are not all the same brightness. The brightest stars are, in fact, more than 100 times brighter than the faintest ones the eye can see.

In about 130 BC the Greek astronomer, Hipparcus, developed a five step brightness or as he called it magnitude scale. It begins with what is called 6th magnitude for the faintest stars. Then each successive magnitude step is 2.5 times brighter than the previous step. The brightest 1st magnitude stars are, therefore, 100 times more brilliant than those just visible to the naked eye--the 6th magnitude stars. Modern astronomers have had to extend the magnitude scale because more precise measurements show that many of the really bright stars are actually brighter than 1st magnitude. Some approach zero magnitude, and four require that the scale be extended toward negative numbers. The brightest star in the sky, Sirius, for example, has a magnitude of -1.42. Although 6th magnitude is still considered the limiting visual magnitude, in reality it is more like 3 or 4 in many urban areas because of light pollution.

Why are some stars brighter than others? The obvious explanation is that the stars are not all equally distant. The fainter stars, of course, being the farthest away. However, this is not the only reason. Astronomers have found that stars differ in their real brightness or luminosity because of their different sizes and temperatures. Two stars may appear of equal brightness even though their distances are widely different only because the more distant star is larger. Take, for example, the brightest star in the summer sky, Vega, and the nearby star, Deneb. They appear of nearly equal brightness, yet Deneb is more than 60 times as far away as Vega. To appear as bright as Vega, Deneb’s true luminosity is some 3000 times that of Vega. How far away are the stars? The nearest star, of course, is the sun. It appears larger and brighter only because it is relatively near to us--only 93 million miles away.

Because stellar distances are so incredibly large, astronomers find it more convenient to describe distances in terms of how long it takes light to reach us from the star. Light travels at the speed of 186,000 miles a second--a distance more than seven times around the Earth. It takes light 1.25 seconds to reach us from the moon, about 8 minutes from the sun, and 4.3 years from the next nearest star, Alpha Centuri. Astronomers would say that Alpha Centuri is 4.3 light years distant. Some have trouble with this idea of expressing distance in units of time. But we frequently do the same thing in casual conversations: " I live only 10 minutes from the mall." What we mean is that traveling by car at some reasonable speed it takes only 10 minutes to reach the mall.

How great are the distances between stars? To get an impression of these distances and the immense emptiness of space, we can scale down the distances to ones more familiar. Let the sun be represented by the size of a green pea. The Earth, a hundred times smaller, then becomes a tiny grain of sand about 2.5 feet away. The most distant planet, Pluto, is an infinitesimal speck almost 90 ft. distant, and the nearest star is represented by another pea at a distance of 112 miles!

As you look up at the starry sky, another more subtle aspect of the stars that may go unnoticed. It is that stars are not all the same color. Pastel hues from red-orange, yellow, through white and blue can be seen. Why these color differences? Because the stars have different temperatures--an iron bar heated in a forge that glows red is cooler than one that glows white hot. The stars are huge globes of hot gases that are heated by nuclear furnaces in their cores. The temperature and, therefore, the star’s color depends upon its size and the amount of material it contains. Our sun is a yellow star with a surface temperature of about 6000 degrees. The famous constellation of Orion, seen in Winter sky, includes stars of three colors. There is the super giant red star, Betelguese, the white star Rigel, and the blue star marking the top end of Orion’s belt. These stars have temperatures ranging from 3000 to 40,000 degrees.

*The above was part of a Monthly Observatory Nights talk entitled Astronomy From the Backyard that I gave at the Harvard-Smithsonian Center for Astrophysics, Cambridge, MA on August 15, 1996.