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.
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.
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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