Seeing the Night Sky with Our Naked Eyes
With an unaided eye we view our universe through a very small window, from blue light to red light. The “big" window is the electromagnetic spectrum. This band, or range of electromagnetic energy exists from extremely long radio waves, which have a wavelength on the order of miles, to the extremely energetic gamma rays that are some 40 million times smaller than the wavelengths of blue light we can see. Our little window into this vast continuum extends from red light that has wavelengths around 700 nano-meters (nm—a billionth of a meter) to about 400 nm for blue light. The figure to the right will help to understand just how small our visual window is in the electromagnetic spectrum. So when we look up into the night sky and see the stars sparkling above, we are only seeing a very tiny piece of what is out there. How about an example?
To illustrate our visual limitations let’s take a look at a simple example—an electric stove. If you turn the burner on LOW, the heating element has the same blackish-gray color it does when it is off. But hold your hand near it and you can feel the heat—infrared energy—wavelengths longer than the red color we can see. So in this case your eyes are not providing you the essential information needed. You need another sense to tell you the heating element is on; fortunately, the nerves in your hand are sensitive to the longer wavelengths of light and you feel the heat. If you turn the burner up to HIGH, you will see it begin to glow a deep red as it emits more energy that now reaches into the visible portion of the electromagnetic spectrum. You’ve probably experienced the same phenomenon when you placed your hand on the hood of a car that has been sitting in the sun. The metal is very hot, yet you had no visual indication that there was heat there. Some insects and animals have visual ranges that extend into the infrared (IR)—like mosquitoes homing in on our warm blood. Bees are able to see in the ultraviolet (UV) band and their view of the flowers they pollinate is very different from what we see because of how the flower petals reflect the UV wavelengths.
When we look at the night sky, we see the pinpoints of light from the stars, some of which may appear bluish or reddish but most will be brilliant white. If you’re lucky, there may be some planets visible and they too will appear to be white in color, except for Mars, which has a definite red tint to it. When we look at the pictures from the Hubble Space Telescope we see glorious pictures in vivid color. Some of these may be natural and others may have their colors enhanced to bring out details in the object that would otherwise go unnoticed. The use of film or nowadays, CCD cameras (like you have in your digital camera) in astronomy allows astronomers to gather color information by collecting the individual photons of light over long periods of time and building an image of the galaxy or nebula that shows colors and details our eyes cannot see in these dim objects.