Several of the radio telescopes used for
listening to the messages from the cosmos.
WHEN WAS the last time you received a message from someone? How did it arrive? Was it in the form of a telephone call, a letter, a fax, or an e-mail message? We humans are aware of the cosmos and are able to learn about it because it sends us all kinds of messages day and night-- messages in the form of electromagnetic radiation or light. We see the universe because our eyes have adapted to visible light. But there are other wavelengths of "light" along the electromagnetic spectrum that our eyes can not see. We have built many kinds of telescopes to help us "see" these other wavelenghts. You can learn more about the spectrum of "light" available to us by reading the following:
Waves and Particle--A Light Motif.
Thermal Radiation--Nailing
the Planck Curve.
Basics of Radio Astronomy - Excellent stuff!
Radio Astronomy
Casual observers and beginning stargazers notice immediately that stars differ in brightness or MAGNITUDE. The human eye and mind are very good at judging the difference in apparent magnitude between two celestial objects. And with a little practice your eyes can see a 0.5 magnitude difference. Advanced observers can detect changes as little as 0.1 magnitude. Astronomers use Photometry to help them determine the magnitudes of celestial objects. The sky at each person's observing site is unique. Depending on the current weather and sky conditions and the amount of light pollution, each observer has what is called his/her limiting magnitude. Your limiting magnitude is the magnitude of the faintest star you can see with your unaided eyes under the given conditions. Check your own limiting magnitude by counting the number of stars you can see in some of the suggested areas of the sky.