About Light

The light we receive from distant sources is generated on the tiniest of scales. To explore the largest objects, such as galaxies, we have to first understand the smallest of objects, atoms and the particles making up atoms. The photons that we detect with our eyes and catch with our telescopes were generated in many different ways: sometimes by electrons hopping between different orbital levels in an atom, or other times by the energetic collisions of atomic nuclei. We now explore the ways in which photons of light arise, how they get from there to here, and what they can tell us about the objects that we observe.
We have concentrated thus far on optical photons (the ones that we can see with our eyes). As it turns out, our eyes respond to “visible” wavelengths because that is where the peak of the emission from the sun is located in the electromagnetic spectrum. If our eyes were most sensitive to infrared radiation, for example, we would see some things we can’t now see (body heat), but would miss a lot of other useful stuff. In this chapter, we’re going to talk more about visible light and the electromagnetic spectrum, of which visible light is a tiny subset. Think of it this way: If the electromagnetic spectrum is represented by a piano keyboard, then the visible part of the spectrum is but a single key or note. In the cosmic symphony, there are many notes, and we want to be able to hear them all. If you’re concerned that this sounds more like physics than astronomy, you’re right. But don’t be intimidated. Most of astronomy involves applications of physics principles, and we are convinced that understanding what you are seeing when you look at a star greatly enhances the experience of looking. Remember this astounding fact : When you look at the light from our sun or a distant star, you are witnessing the product of nuclear fusion reactions that are, every second, releasing more energy than any atomic explosion Earth has ever witnessed. Yet it is not just brute energy, but also information from the sky. Let’s take a closer look.