[Big Ear Masthead]

Beginner's Guide to Radio Astronomy and SETI

by Cindy Brooman

I am continually amazed by the number of people I run into who have total misconceptions about the work done at the Ohio State University Radio Observatory. I have talked to people who (not knowing that I'm a staff member) have made comments relating both radio astronomy and SETI (the Search for ExtraTerrestrial Intelligence) to some kind of fringe or cult activity involving UFOs and talking to Martians. Therefore, I have decided to put this page together to explain what it is that we do. Just in case there is someone who is a total novice at this stuff, I'll start at the very beginning. (Those of you who already know this, just bear with me.)



The science of astronomy is the study of the universe beyond the Earth, which includes objects such as the planets, asteroids, comets, stars and galaxies. This study is done by analyzing the energy (photons) in the electromagnetic spectrum that is emitted from these objects.

The Electromagnetic Spectrum

Rainbow of Colors

To help visualize the electromagnetic spectrum, think of a rainbow of colors. When you see a rainbow, you are seeing light (photons) spread out into varying wavelengths or frequencies. It starts with deep violet, which is light at 0.4 microns, and extends through red, which is light at 0.8 microns. (A micron is one millionth of a meter, and is used to measure things that are very, very tiny.)

The retina of the human eye can only process photons in the range of 0.4 to 0.8 microns. Therefore, this portion of the electromagnetic spectrum is referred to as visible light. However, the electromagnetic spectrum extends in both directions (shorter than 0.4 microns and longer than 0.8 microns), and is composed of photons that are not visible as light.

[Electromagnetic Spectrum]
The Electromagnetic Spectrum

The portion of the electromagnetic spectrum shorter than 0.4 microns is composed of extremely short-wavelength photons, such as ultraviolet light, x-rays, and gamma rays. The part of the electromagnetic spectrum longer than 0.8 microns contains longer-wavelength photons such as infra-red light (used by the military in night-vision goggles, for example), and radio waves. Radio waves can be anywhere from a fraction of one millimeter long to 300 meters long.

Many objects in the universe emit photons across the entire electromagnetic spectrum, although we can only see those that fall in the visible portion of the spectrum.

Optical Versus Radio Astronomy

[Optical Observatory]
[Optical Telescope]
[Optical Astronomy Photo]
Optical Astronomy

Astronomy is divided into several branches, each involving a different portion of the electromagnetic spectrum. Optical astronomy, which is the most widely-known and popular branch of astronomy, is the study of the universe in the visible light region of the electromagnetic spectrum. When we think of optical astronomy, we think of looking through a telescope. Optical telescopes have mirrors or lenses which collect or refract light.

[Steerable Dish Radio Telescope]
Steerable Dish
Radio Telescope
[Fixed Kraus-Type Radio Telescope]
Fixed Kraus-Type
Radio Telescope

Radio astronomy is the study of the universe in the radio portion of the electromagnetic spectrum, which is from a fraction of one millimeter to 300 meter wavelengths. Radio waves cannot be seen by the human eye; however, a great many celestial objects do emit photons in the radio wave region of the spectrum. To study the universe at radio wavelengths, radio astronomers use radio telescopes, which are entirely different from optical telescopes. Radio telescopes use wires or solid surfaces to focus the radio waves, which are then collected by a receiver similar to the receiver that you use to listen to a radio station (although at a different frequency). Although radio waves can't be seen, they can be heard as a hiss not unlike the static between stations on the radio.

Radio astronomers have made some exciting discoveries. Pulsars (rotating neutron stars) and quasars (dense central cores of extremely distant galaxies) were both discovered by radio astronomers.

There are other branches of astronomy as well (gamma ray, x-ray, ultraviolet, and infra-red to name a handful), but I won't cover those here.

Sky Surveys Done by Big Ear

[Map of the Radio Sky]
What the Sky Would Look Like
If Your Eyes Could See Radio Waves

The Big Ear radio telescope was used in the 1960s to form a giant picture of what the sky looks like in the radio region of the electromagnetic spectrum. This is what we call a Sky Survey. We are currently doing a follow-up survey to compare with the original. The telescope is systematically scanning the entire sky and storing the results in a computer. This data is then used to print images of what the sky might look like if our eyes could see photons in the radio region of the spectrum. These sources are naturally occurring; that is, they are not artificial, but come from celestial objects in the universe.

In addition to surveying the sky for naturally-occurring radio sources, Big Ear is listening for possible signals from non-naturally-occurring sources in the universe. Those signals would be created by intelligent civilizations like ours. This is referred to as the Search for ExtraTerrestrial Intelligence (SETI).

Next, a word of explanation about our universe...


Cosmic Distances

One light year is the distance that a light ray or radio wave would travel in one year going at approximately 186,000 miles per second. 186,000 miles would be the equivalent of circling the Earth about seven and a half times. Imagine that distance spread out in a straight line. That's how far light travels in one second. Now imagine how far the light could travel in 31,536,000 seconds (one year) - almost 5 trillion 866 billion miles! That's 5,866,000,000,000 miles!! It's a distance so vast that it's hard to fathom! And that's only one light year. The nearest star, Alpha Centauri, is 4.3 light years away! If you could travel at the speed of light, it would take you 4.3 years to get there! (In reality, Einstein's General Theory of Relativity predicts that it would be impossible to travel at the speed of light since it would require an infinite amount of energy to accelerate an object with significant mass to light speed.)

Our Home Galaxy, The Milky Way

[You Are Here] You are here. The arrow points to the location of our sun within our home galaxy, the Milky Way.

Our star, the sun, is only one of about 100 billion stars - that's 100,000,000,000 - that make up our own galaxy, the Milky Way, which is approximately 100,000 light years in diameter. (If you could travel at the speed of light, it would take you 100,000 years to go from the outer edge on one side to the outer edge of the opposite side.) The Milky Way galaxy is a relatively flat disk which has a spiral shape with "arms" radiating outward from the center. We are located in the galactic boondocks more or less, toward the outside of one of these arms, about 30,000 light years from the center of the galaxy.

Other Galaxies

[Andromeda Galaxy] The Andromeda Galaxy is the nearest galaxy similar in size and shape to our own Milky Way. The Andromeda Galaxy is 2 million light years away.

Our galaxy is not the only galaxy in the universe. There are about 100 billion other galaxies. The nearest galaxy similar in size and shape to our own is the Andromeda galaxy, which is about 2,000,000 (two million) light years away. Other galaxies are billions of light years away.


Are We Alone in the Universe?

If there are 100 billion stars in 100 billion galaxies in the universe, that means that there are 10,000,000,000,000,000,000,000 (ten sextillion) stars in the universe (1 times 10 to the 22nd power when expressed in scientific notation). With this many stars, it would be hard to believe that there isn't life somewhere else in the universe.

Frank Drake, a famous radio astronomer, came up with a formula for estimating the number of communicating civilizations. This is called Drake's equation. It involves 7 factors:

The rate of star formation per year TIMES
The fraction of those stars that have planets TIMES
The number of those planets that have suitable environments for life to develop TIMES
The number of those planets where life actually does develop TIMES
The fraction of beings on those planets that actually develop intelligence TIMES
The fraction of intelligent civilizations that develop communications TIMES
The number of years that an intelligent civilization can survive

Dr. John Kraus, who built the Big Ear radio observatory, has estimated each of the seven factors as conservatively as possible for our home galaxy, the Milky Way, and has come up with the number 40. As many as forty intelligent, communicating civilizations in our galaxy alone! If the same equation were applied to the 100 billion other galaxies in the universe, we could estimate there to be 4,000,000,000,000 intelligent, communicating civilizations in the universe! That's 4 trillion!

We have little way of knowing whether this estimate is close to being correct, but even if it's off by a factor of 10,000, that would still leave 400 million intelligent, communicating civilizations in the universe!

How Would They Communicate?

Civilizations might communicate in one of two ways.

The first way is by sending signals unintentionally. We do this all the time ourselves. For over fifty years now, our first television and radio signals have been radiating out into space like a giant shock wave, or like waves radiating out from a pebble dropped into a pond. Another intelligent civilization could intercept them and wonder what they say. Imagine an alien race picking up one of our television signals, decoding it, and then sending what they believe to be an intelligent reply: "Lucy, I'm home!" or "So you want to be a wise guy, eh?"

The second way of communicating would be to purposefully send out a beacon with encoded information. The beacon could contain a simple instruction set that periodically repeats, along with a more complex message.

What Would They Say?

Perhaps there are civilizations that are very much more advanced than we are. If so, it's possible that they may have set up beacons to instruct fledgling civilizations such as ours. Maybe they would be broadcasting an "Encyclopedia Galactica" of some sort. Just imagine the wealth of knowledge that would be at our fingertips if we were to discover such a signal and decipher it. Perhaps it would teach us how to build a space ship that travels close to the speed of light. Or maybe it would tell us how to solve our planetary ecological crisis. How about if it told us how to solve our global political problems? The benefits of such a discovery could be beyond our wildest dreams!

Where Would They Broadcast It?

[Sky Noise Diagram] Sky noise diagram showing the radio quiet region and the water hole. An intelligent extraterrestrial civilization might choose this region to broadcast a message.

There is a portion of the radio spectrum that is relatively quiet from naturally occurring noise from stars and the atmosphere. This is called the radio quiet region. Additionally, within this region there is a portion of the radio spectrum known as the "water hole", from 1420 Megahertz (the emission wavelength for neutral hydrogen) to 1638 Megahertz (the emission wavelength for the hydroxyl radical). This region is called the "water hole" because when hydrogen and hydroxyl are combined, they form a molecule of water.

Some people believe that an extraterrestrial civilization might choose this region to broadcast a message, especially if they are a life form based on water like us. They might choose to broadcast in this region, hoping that we would be thinking along similar lines.

An alien message would also most likely be what we call a narrowband signal. This means a signal at a very precise frequency. Radio stations are examples of narrowband signals. Between radio stations you hear a hissing sound. This is broadband noise. The stars (and other celestial objects) also put out broadband noise. An intelligent, communicating civilization would probably use a narrowband signal rather than a broadband one for a beacon, since they wouldn't want their message to be mistaken for regular, ordinary star noise.

Would They Come Here? SETI Versus UFOs

The chances that an extraterrestrial civilization would actually come to the Earth are slim. The amount of time and energy required for the travel would be enormous. The amount of energy required to accelerate a spacecraft weighing several thousand tons to a speed even a moderate fraction of the speed of light would be billions of times more than the energy needed to send out a radio beacon. Therefore, it's more likely that they would communicate instead. For that reason, the Big Ear staff is highly skeptical of reports of UFO sightings. (Translate: we think they're BS.)

[Webmaster's note: No professional astronomer in his right mind would be caught dead stating publicly that he'd seen a UFO or been abducted by aliens. He would be ostracized by his colleagues. Serious educational institutions and research facilities would treat him as though he had suddenly acquired the ebola virus. His career would be finished. Note. The same would be true for female astronomers.]

Search Results

So far the SETI search, at Big Ear and at other radio observatories around the world, has not uncovered any ETI (ExtraTerrestrial Intelligence) signals. The search is being conducted at many different frequencies over many parts of the sky. However, if a signal comes and we're not looking, we would miss it. SETI systems up to this point have been fairly limited in their searching capabilities, however this is now improving with systems such as Big Ear's SERENDIP which can process 4 million channels at once. The problem with only being able to look at one portion of the sky at a time may be solved in the future with Big Ear's "Argus" system. This system would form a picture of the entire radio sky at once, thereby greatly diminishing the chances of a signal being missed.

One thing is for certain when it comes to SETI, if we never look, then we're guaranteed never to find anything!



Copyright © 1996-2005 Ohio State University Radio Observatory and North American AstroPhysical Observatory.

Originally designed by Point & Click Software, Inc.
Last modified: August 15, 2005.