The first question you might have about high energy astrophysics is perhaps, "What exactly is high energy astrophysics?"e; In order to answer this question, you must first be introduced to photons and the electromagnetic spectrum.
Light can sometimes be treated as a particle - like a ball on a billiard table, but sometimes it is better treated as a wave - like the ripples on a pond when you throw a stone in. Throughout this tutorial, I will refer to "particles" of light, which are called photons. However, if light is a wave but we can sometimes treat it as a particle, how do we define that particle? Well, let's say that the light is a perfect sine wave, like this:
Then, one distinct wave traveling along would be a photon. Different waves..er photons..of light will have different frequencies - different rates at which the bumps come and go. This, in turn, means that the waves have different wavelengths (the distance from one peak to the next) - see the picture below. The wave with the highest frequency has the lowest wavelength.
This may get confusing, because some astrophysicists refer to light by its wavelength, others by its frequency. High energy astrophysicists use still another convention - they refer to light by its energy. All of these things are related, so we are all talking about the same thing, but it can be difficult to switch from one to the other quickly. For this tutorial, I will always describe a photon by its energy.
Here's a quick example: red light has a longer wavelength than blue light, so red has a lower frequency than blue and, thus, a lower energy.
Visible light - the light that we can see with our eyes - is just the tip of the "light iceberg." In fact, the light that we can see is only a tiny portion of the types of light that are out there.
The ultraviolet (UV) radiation from the sun that tans your skin during the summer is another form of light. UV radiation is more energetic than visible light. The radio waves that your radio picks up and turns into sound are also a form of light, but a form that has less energy than visible light.
There is a whole spectrum of the light that exists. Below is a depiction of the electromagnetic spectrum with the different names of the different forms of light. As you can see, the visible portion is small compared to the other types of light.
The higher energy light is on the left hand side of the above image - these are the energies with which high-energy astronomy is concerned. Specifically, high energy astronomy is astronomy done with observations in X-rays and gamma-rays.
A couple notes:
