Practically our only means of observing the vast expanses of space outside the Earth is through light. Our telescopes collect it, our instruments record it, and our astronomers analyze it. Light is far more common than your eyes might lead you to believe. Let's explore this amazing phenomenon and take peek at what sort of astronomical objects it can reveal.
First off, what is light? You'd think that the most common entity in nature would be pretty easy to define, but it actually isn't. At its essence, light is energy moving from one place to another. This transfer happens at the incredible speed of 298,000,000 meters per second. Relativity tells us that nothing in the Universe is faster! Sometimes light is described as a wave and sometimes as a particle (the photon). Which is correct? It's turns out both are valid descriptions. An important, albeit slightly technical, point is that light carries energy and momentum, but it does so without mass. All the light in the cosmos weighs nothing, but the force it carries shapes virtually every aspect of the Universe.
We classify different types of light based on its wavelength or frequency. It doesn't matter which you use, but here we'll talk about it in terms of wavelength. You can think about wavelength as the approximate size of the packet of light.
One last thing - everything in the Universe emits light (well, almost - not the elusive dark matter). If an object has a temperature (and everything does!), it emits light. It's just a question of what part of the spectrum that falls within.
Let's start our tour with the longest wavelengths of light: radio. Believe it or not, this is the exact same radio as the radio waves picked up by your car stereo or television set. It's just another kind of light! We find radio waves useful because of their size. They are so big (one meter to hundreds of kilometers) that even relatively large obstacles like buildings don't disrupt them. Convenient for broadcasting a signal to a whole city!
A couple of important classes of astronomical objects emit radio waves. The most important is the Cosmic Microwave Background. This represents the earliest light that could travel in the Universe - it's been travelling more than 13.5 billion years to reach us!
Infrared light is the light emitted by you and I. At a millionth of a meter and smaller, its packets are far tinier than those of the radio. It's characteristic of things that are around room temperature here on Earth. Out in the cosmos, this includes planets, dust, and gas. Terrestrial applications include night vision goggles and remote controls.
Ah, finally - the light we can see! Like most stars, the Sun emits most of its light in the visible, which has wavelengths from a millionth to a few hundred billionths of a meter. It's not an accident that this happens to be what we can see best - humans evolved to make use of the most abundant light available.
For millennia astronomy focused solely on visible light. It's not a bad place to start - right in the middle of the spectrum, anything with a temperature of a few thousand degrees emits a decent amount of visible light. Stars, galaxies, and nebulae can all be seen this way. In fact, it would take until the 1800s to discover any other kind of light and it wouldn't be until the last fifty or so years that we'd begin to really do astronomy with other parts of the spectrum.
These light waves are even smaller than the visible, ranging down to just a few tens of billionths of a meter. Very hot objects emit in the ultraviolet, such as young stars and forming galaxies.
If you've ever had an x-ray at the doctor before, you were being penetrated by light less than a billionth of a meter in size. Because they are so small, x-rays can't travel very far through most materials before getting stopped. This makes them particularly useful in medical imaging, where they can penetrate tissue but are absorbed by bones.
Perhaps the neatest astronomical source of x-rays are super-massive black holes, like the one found at the center of our galaxy. It's not actually the black hole itself emitting the light, but the super-hot disk of material falling into it. Another nifty source of x-rays are certain kinds of binary stars, where a white dwarf and a neutron star or black hole orbit each other.
Gammy rays are the most powerful form of light. With a wavelength that ranges down to just a trillionth of a meter, gamma rays are easily blocked, yet carry tremendous energy. It's a good thing that the Earth's atmosphere protects us from these waves - otherwise they may do substantial damage to our DNA!
There are a number of different sources for gamma rays in space, but none are more spectacular than supernovae. The giant explosions that mark the death of massive stars, supernovae release a truly tremendous amount of energy, including bursts of gamma rays. In fact, certainly kinds of gamma ray bursts can only be created when a dying star emits as much energy in one minute as our Sun will produce in its entire lifetime!