Multiwavelength Astronomy
Background
We’re used to thinking of light of having colors like red, orange, yellow, green, blue, indigo, and violet (ROYGBIV). But really, the same property that separates those colors (wavelength) creates many more ‘colors’ of light that we simply can’t see.
If we can’t see these colors with the naked eye, we’ll need to use detectors to collect and interpret the light for us. We can then use computers to create a visible picture of something that was previously invisible.
False Color
Look at the two images on the slide show of a dog that were taken in the infrared. These are two versions of the same image, with the same scale of temperature, but the picture on the right has a larger range of colors to show the same range in temperature. When there are more colors in the same temperature range, you can easily detect more detail about small temperature differences. Of course, the dog isn’t actually purple. The people who make images using invisible wavelengths, such as infrared, choose the visible colors to represent those wavelengths. They try to make the information useful at a glance, like in these pictures.
1. Which areas on the dog are hottest? ______
2. Which are coolest? ______
3. When you look at a dog in visible light, can you tell which areas are hottest and coolest just by looking?
X-Rays
The wavelength of X-rays is so small that they have the ability to penetrate all but the densest materials (like bone). When you look at picture taken with X-rays, the black regions are actually where the X-rays got through undisturbed and exposed the film and the white regions are where the film was left untouched by X-rays but something dense enough to absorb them.
4. What do you think this object is?
5. What are the bright areas highlighting?
Wien’s Law
Every thing that has a temperature which is not Absolute Zero (which is everything) is *always* emitting electromagnetic radiation. Some things emit all forms of EM radiation, but they emit the largest percentage in a region of the spectrum associated with their temperature. Our Sun happens to emit most of its EM radiation in the visible region of the spectrum. Since you are considerably cooler than the Sun, you emit most of your EM radiation in the infrared portion of the spectrum.
The law that describes the wavelength of peak emission as a function of temperature is called Wien’s Law.
Graphs of intensity vs. wavelength are referred to as blackbody curves. Look closely at where the peak of each curve is.
6. Do hotter objects give off their maximum radiation at shorter or longer wavelengths?
The graph at the right shows examples of actual blackbody curves.
7. Where do you think extremely hot young stars (100,000°K) emit most of their radiation? (circle one)
Ultraviolet Visible Infrared Microwave Radio
Have you ever noticed your doctor taking your temperature by pointing an instrument in your ear for a split second? Well, that instrument is an infrared detector that finds your wavelength of maximum intensity and then converts it into your body’s temperature. It is a perfect example of utilizing Wien’s Law.
How is this used in astronomy?
When astronomers first started looking at galaxies, they could only see them in the visible wavelengths. From what they saw, they were only able to describe galaxies in the most basic way: large collections of stars, sometimes with a spiral structure. They couldn’t understand much more than that. Now that we have the technology to detect other wavelengths of light, we are finding out all sorts of fascinating information about what galaxies are made of, how they evolve, which parts are the most active, and much more. As we just learned, each wavelength can show us objects that represent specific temperatures. The table below puts it all together:
Type of Radiation / Characteristic Temperature / Emitting ObjectsGamma Rays / greater than 100,000,000 K / · Pulsars
· Areas around black holes
X-Rays / one to one-hundred million K / · Neutron stars
· Supernova remnants
· Solar atmosphere
Ultraviolet / 10,000 to 1,000,000 K / · Quasars
· Hot, young stars
Visible / 1,000 to 10,000 K / · Stars
· Galaxies
· Nebulae
Infrared / 10 to 1,000 K / · Small stars
· Interstellar dust
· Planets
· Comets & Asteroids
Radio / less than 10 K / · Empty space
· Cold, dense regions
Galaxy Matching Activity
Directions: In this activity, you will have three sets of cards with images of galaxies; a set of visible light images, ultraviolet images, and radio images (the same eight galaxies are represented in each wavelength).
First, match the visible to the radio images and describe why you chose to pair each match in the way you did.
Note: To save ink, the card images are negatives of the real pictures (black and white are reversed). The darkest spots on the cards represent the brightest parts of the galaxies.
Visible / Radio / Reasoning90 / Z / spiral arms extend clockwise, matched features on upper left, same orientation
(A sample answer has been shown in the chart)
Now match the VISIBLE to the ULTRAVIOLET images and record your answers below.
Visible / Ultraviolet / Reasoning
Now match the RADIO to the VISIBLE / ULTRAVIOLET pairs so that you have a set of 3, which each set representing the same galaxy.