Thursday, June 17, 2010

Astro 101: The electromagnetic spectrum


Today, we'll continue through my occasional series on basic astronomy concepts.  Previously, we've discussed the difference between solar systems, star clusters, and galaxies.  We've also discussed telescopes and observatories.  Today we're going to talk about the main way astronomers learn about distant planets, stars, and galaxies: the electromagnetic spectrum.

 Within our Solar System, we can send robots (and maybe, someday, people) to run all kinds of tests.  But even the closest star is still trillions of miles further away than our most distant and speediest robot probes.  To explore other stars and the Universe, we have three choices:
  1.  Electromagnetic radiation ("light").  This is by far the most common and most successful method.
  2.  Gravitational waves.  Gravitational waves are like ripples of gravity propagating through the Universe.  This might be detectable in the not-too-distant future, but for now they have not been definitively detected.
  3.  Cosmic rays.  These are high-energy particles coming from all corners of the Universe.  We can detect these on Earth and in space, but so far it is hard to identify where a particular particle came from (except for the many that come from the sun).

Electromagnetic waves, which I'm just going to call "light" from now on so I don't have to keep typing "electromagnetic", seem to have a range of properties.  Radio waves carry information, microwaves heat our food, infrared light carries heat and allows us to "see in the dark" with night-vision goggles, visible light is at the heart (or eyeball) of one of a human's primary senses, ultraviolet light gives us suntans and skin cancer, X-rays allow us to see inside our bodies, and gamma rays turn us into monstrously strong, large, green humanoids when we get angry (or at least that's what I've been told).

Yet all of these "different" types of light are really the same thing; the differences are all due to the different energies of the light.  Radio waves are very low energy -- an AM radio tower transmitting at a few kilowatts can be heard for hundreds of miles, yet a house lit by a few kilowatts of lightbulbs is difficult to see more than a mile or two away.  X-rays can be hundreds or thousands of times more energetic than visible light, which is one reason why they can be harmful to our health.


Because all forms of light are the same basic thing, physicists and astronomers often refer to all these types of light as the electromagnetic spectrum.  There's no official dividing line between one type of light and another, they all sort of blend like a rainbow (hence the term spectrum).   Therefore, we astronomers tend to use a property of light called wavelength to identify exactly where in the spectrum we are looking. 

Why would astronomers study different parts of the electromagnetic spectrum?  Because the light we see at different parts of the spectrum come from different processes.  Radio waves tend to come from very cold clouds of gas and from electrons caught in magnetic fields.  Infrared light comes from objects we might consider "warm" or "hot".  Humans glow in infrared light.  So do planets and stars.  Visible light comes mostly from this same heat radiation, just from very hot objects like the sun and other stars.  X-rays and gamma rays come from very energetic phenomena, such as gas at temperatures of millions of degrees, collisions of fast-moving objects, and nuclear reactions.  Therefore, when we look at the sky in different types of light, we see different things.

Here is a slideshow I created that shows how the entire sky looks in different parts of the electromagnetic spectrum, starting with the weak radio light and moving up in energy through gamma ray light.  I don't have a pretty map of ultraviolet light, and I also add in a few special wavelengths of light where hydrogen, the most common element in the Universe, likes to reveal itself.  

These pictures are like maps of the sky, centered on the center of our Milky Way galaxy.  In each image, the Milky Way runs from left to right across the center of the image.  Each image is also labeled with the part of the electromagnetic spectrum, the wavelength of the light (shorter wavelengths = more energetic light), and an everyday object that is roughly the size of the wavelength of light.  I also give credit as to where the image came from (and I put links at the end of this post).

Note how different the sky looks in different parts of the spectrum!  I could go on for hundreds of pages talking about all of the neat things you can see in these pictures, and maybe I'll write a few blog posts about it soon, but for now just admire how the sky changes.

The different physics behind how light is created also affect how we can detect light.   Optical light detectors work a lot like our eyes: light hits a pixel (or rod or cone in the case of the eye) and is converted to a signal. This works great for visible light, infrared light, and ultraviolet light.  For radio light, we need to use antennas and dishes just like satellite TV dishes to detect a signal.  And gamma rays are so energetic, they can pass right through normal cameras, so we need all sorts of clever detection equipment.

One last comment.  Visible light is a very tiny part of the whole electromagnetic spectrum, the tiniest part that gets its own name.  Even so, visible light is still the most common flavor of light used in astronomy.  There are several reasons for this.  We understand visible light very well.  Stars emit most of their light in the visible spectrum.  Most atoms have unique fingerprints that we can observe and study in visible light.  The atmosphere is transparent to visible light. Perhaps the biggest reason is tradition.  Visible light astronomy dates back to the first human who noticed the sun, moon, and stars.  Radio wave astronomy, on the other hand, didn't start until the 1930s, and gamma-ray astronomy started in the 1960s. 

All-Sky Map Image Sources (original image sources are given at each site; I list the site from which I downloaded the map):

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