In This Issue:
Little kids love to ask the question, why? Most of us grow out of the "why stage," but there's one group of people who never do - scientists! Why is probably a scientist's favorite question (along with how): science is all about discovering why the world is the way it is and how it works. Have you ever wondered why the sky is blue or why stars twinkle? This month's newsletter will explore these two why questions.
Why is the sky blue?
One of the most beautiful things about our world is a blue sky on a clear, sunny day. If you've seen pictures of the Apollo astronauts on the moon, you might have noticed that the sky above them was black as night even in bright sunshine. What makes the difference? Why is Earth's sky blue?
Unlike the moon, the earth is surrounded by an atmosphere. The atmosphere is a mixture of gasses, mostly nitrogen and oxygen. The way the sun's light travels through the atmosphere makes the sky colored.
Why blue? It doesn't look like it, but light is made up of several different colors, like you see in a rainbow. Each of these colors travels in a wave, but the wavelength (distance between the tops of each wave) varies. Red light has a long wavelength, while blue light has a much shorter wavelength. When light from the sun enters our atmosphere, the waves collide with gas molecules. The longer wavelengths, like red and yellow, pass straight through and appear to us as "regular" sunlight. Shorter wavelengths, like blue, bump into the gas molecules and scatter in different directions. Some of it still makes it through directly, but the rest is reflected back to our eyes from all directions, so the whole sky looks blue.
You can see similar light scattering by mixing half a teaspoon of milk with a large (quart-size) jar of water. In a darkened room, shine a flashlight through the jar and look at the water. It should have a bluish tint, because the milk particles are scattering the blue light from the flashlight just like the gas molecules in our atmosphere do.
Maybe you're asking a follow-up question: why are sunsets pink and orange? When the sun is low in the sky, near the horizon, its light has to travel through a lot more atmosphere to reach us. The blue light is scattered so much in the extra atmosphere that none of it reaches our eyes from that direction, leaving us to see the beautiful reds and oranges there instead. Sometimes clouds or air pollution can make a sunset even more red because the particles in the cloud help scatter away the shorter wavelengths.
Add a little more milk to your jar - do the extra milk particles allow you to see an orange tint? Try looking in the side of the jar directly opposite where the flashlight is. This is like looking at the sun on the horizon.
Why do stars twinkle?
If we lived on the moon, we probably wouldn't know the song, "Twinkle, Twinkle, Little Star." Just like blue sky, a star's twinkle is a result of Earth's atmosphere. Stars don't twinkle in space! As the light from a star enters the atmosphere, it hits gas molecules and scatters. Since the star is so far away, we only see a tiny beam of light from it. This beam gets scattered away from our eyes and then back into them almost like it is blinking on and off. It happens so fast that it just looks like it is twinkling. (Planets are closer to us and send more light; if some of the light beams are scattered away, others still get through to us, so planets don't usually twinkle.)
Stars twinkle more when they are close to the horizon, because the light has to travel through more atmosphere before it reaches our eyes. Weather can affect how much stars twinkle, too. Cold air scatters more light than warm air, because molecules are closer together in cold air, making it harder for light to pass through without interference. (Think about how it's harder to walk in a straight line through a crowd of people than through an empty room!)
The temperature variation in the atmosphere affects what astronomers call "seeing." If there is a lot of variation, even a perfectly clear night will be bad for stargazing with a telescope. You might be able to see a twinkling star clearly with your naked eye, but an astronomer will say it's "bad seeing" because it's hard to study a star that just won't stop flickering! Next time you're outside on a clear winter night, look for twinkling stars near the horizon. If you have a telescope, you may even see the stars changing colors because their wavelengths are scattering in different directions.
A spectroscope is a device that separates light into its individual colors in a rainbow-like band called a spectrum. The spectrum of each chemical element looks slightly different, so scientists use spectroscopes to see what elements make up the sun and stars. Try making your own spectroscope and turn any light in your house into a rainbow!
You now have a working spectroscope! Use it to look at several different types of light: a normal incandescent light bulb, fluorescent light, LED light, a glow stick, even sunlight. (But be very careful - do NOT look directly at the sun through your spectroscope!) You can also look at the flame of a match or candle, if you have someone else hold it for you.
Do you see a difference in each light's spectra? Incandescent light bulbs and sunlight will produce a continuous spectrum, where all the colors merge smoothly into each other. (Stars actually emit a dark-line spectrum, which has the colors broken up by dark lines. Only very precise spectroscopes can see the dark lines, however, so the sun looks like a continuous spectrum.) A fluorescent light will produce a bright-line spectrum, which has bright lines separated by dark spaces. Try drawing each spectrum with colored pencils and comparing them. You can also try varying the width of the slit - does that change the appearance of the spectrum?
Each of the colors that light is made of has its own wavelength, which reflects and refracts at its own angle, different from all the other colors. When light hits the diffraction grating, it is reflected back onto the wall of the spectroscope. All the little grooves on the grating separate the colors so they reflect at their different angles. The beam of light hits the diffraction grating at one angle, but since each color bends back at a different angle, they are spread out along the spectroscope wall, allowing you to see them.
Noteworthy Scientist: Pietro Angelo Secchi (1818-1878)
You have learned that the sky is blue because light is made of several different colors, but did you know that the colors of light can help us find out what the stars are made of? Father Angelo Secchi was a Jesuit (a type of Roman Catholic) priest and astrophysicist who spent years studying the light from thousands of stars. He was born in Emilia, Italy and educated by the Jesuits in liberal arts, science, and theology. He had such a gift for science that he was made a professor of physics at the Jesuit College when he was only twenty-three years old.
In 1850, Secchi became the director of the observatory in the Roman College. From this observatory he created the first star classification system that used a star's light spectrum. When light passes through a prism, it is split into its individual colors which show up in a band of colors (like a rainbow) called a spectrum. (The plural form is "spectra.") In the early 1800s, a scientist named Joseph Fraunhofer attached a prism to a telescope and looked at the spectra of the sun and other stars. Later, Gustav Kirchhoff and Robert Bunsen discovered that every chemical element produces a different spectrum when heated. Astronomers realized that by studying the spectra of stars, they could discover what elements stars were made of. Imagine that—they could find out the parts of something millions of miles away!
Secchi used these discoveries to analyze the spectra of over 4,000 stars. As he worked, he became convinced that there were different types of stars and that they could be grouped together in categories defined by what their spectra looked like. Secchi's five categories have since been replaced with seven more precise categories, but stars are still classified today using their spectra. (Click here to see four of the five different types of spectra that Secchi identified.)
The classification of stars was not Secchi's only interest. He studied sunspots and solar prominences, photographed a total solar eclipse, and cataloged thousands of double stars. He was also interested in meteorology and physics, and invented a tool called a Secchi disk for measuring water transparency, which is still used today.
Beetle Spectroscope. Spectroscopes use plastic diffraction gratings to separate light into colors. Not all diffraction gratings are man-made, though! Certain kinds of beetles have tiny grooves on them that act like a diffraction grating, giving them luminous, changing colors, called iridescence. Check out some pictures of beautiful iridescent beetles at BugGuide.
Blue Moon. The term "blue moon" usually refers to an extra full moon (thirteen in a calendar year, instead of twelve), but sometimes the moon really appears blue! Forest fires or volcanic eruptions can fill the air with particles just the right size to scatter red light, leaving the moon looking blue. In 1950 huge forest fires in western Canada pumped so much smoke into the air that people as far away as England saw a blue moon a few days later!
In August 2008 NASA astronauts will repair the Hubble Space Telescope and install the Cosmic Origins Spectrograph. Read all about the mission on NASA's website.
Learn about the speed of light with this short interactive feature from NOVA.
What makes a laser different from other light sources? Find out at NASA's Space Place.