Shedding a New Light on the Universe
X-ray Astronomy
Why are X-ray observations different from optical ones? Well, differences arise because X-ray and optical photons have different energies. We know that light can act as a wave or as a particle, depending on how we detect and measure it. In general, in astromomy we utilize the wave properties of light at lower energies, and the particle properties at higher energies. Single optical photons are more difficult to observe because most optical sources typically emit too many of them to count individually. In contrast, X-ray sources generally emit fewer high-energy photons so that X-ray detectors can detect and measure individual X-ray photons, and over time, accumulate enough photons to make an accurate picture of the total source.
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A good comparison would be to imagine looking at a light bulb and seeing the white light coming from it a photon at a time. First you'd see one red photon, then one blue one, then one yellow one, then perhaps another red, then a green, and so on. After you had seen enough photons, you could combine them and say "Ah, I see, it's white light."
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Photons can be used in three ways to give us information about the sources that are emitting them. We can count the number of photons coming from a certain area of the sky and make an image of it, we can measure the energies of the photons being emitted from the source that we are looking at, which gives us a spectrum, or we can make a graph, called a light curve, that will show us how bright a source is over time. Scientists use all of these tools to help understand objects in our Universe.
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