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Detecting X-rays
Early astronomers used to peer at the skies with their own eyes,
making catalogs of what they saw, tracking the motion of the planets
and so on. Modern astronomers also look at the sky, but they do so
with the help of scientific instruments. These instruments have
helped astronomers study parts of the electromagnetic spectrum, such
as X-rays, that we cannot see with our eyes.
Even with the aid of better instruments, however, X-rays are relatively
difficult to detect compared to other types of light. Why? First, X-rays
are so energetic that they pass through many types of material rather
than being reflected or absorbed. In order to detect them, scientists
need to find materials that are good at absorbing X-rays. Second, X-ray
photons are not as plentiful as some other types of photons. For example,
a white light bulb emits many optical photons, so many that lots of them
arrive at your eyes (your detectors) at the same time. Imagine instead
that these photons arrived at your eyes much more slowly: first a red photon,
followed by a blue one, then a yellow one, and so on. After you had seen
enough photons, you could combine them and say "Ah, I see, it's a
white light." This is how astronomical X-ray detectors operate:
They detect the energy level of each individual photon and then, over
time, accumulate enough measurements to make an accurate picture of
the total source.
As a result of these two properties of X-rays (very energetic and not
as plentiful), scientists have had to work hard to build instruments
that are able to detect every X-ray photon that reaches them. This is
why advances in our understanding of objects that emit X-rays are tied
very closely to advances in the instruments that we have built to detect
X-rays that those objects emit.
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