Tracking Matter Around a Black Hole
How are you going to answer the questions of what is falling into the
black hole and how fast does it fall? It would be great if you could
see the accretion disk directly; then, you could track the accretion by
watching matter fall into the black hole. However, the black hole is too
far away, and the accretion disk too small, to see it directly.
Instead, you are going to look for emission or absorption lines in the
Emission and absorption lines
What is an emission or absorption line?
When atoms are excited, they emit and/or absorb light at certain
fixed energies. These "excitations" show up in spectra as a spike or
dip at a very specific energy based on the element that emitted or
absorbed the light. By precisely measuring the energy of these features
in a spectrum, astronomers can determine what elements are present.
The features in the spectrum are called "lines", either "emission line"
if the feature comes from atoms emitting the light, or "absorption
line", if the feature comes from atoms absorbing the light.
See the examples below:
Two different spectra showing emission and absorption lines. The
one on the left shows emission lines (labeled silicon, sulfur,
argon, calcium and iron) in the supernova remnant called Cassiopeia
A. The spectrum on the right shows absorption lines (labeled Ca K
and Ca H) in the optical spectrum of galaxy M 31.
(Click images for larger versions.)
Have you ever listened to a police car as it drives by? The pitch of
its siren appears to change as it passes by. The siren itself doesn't
actually change pitch it stays constant. The thing that changes
is that the police car first is first moving toward you, then is moving
away. This change in pitch is due to the Doppler effect. Essentially,
the peaks in the sound waves "bunch up" as the car is approaching you,
so the frequency is higher. Similarly, the peaks in the sound waves
spread out as the car is moving away from you, so the frequency of the
siren is lower. A similar thing happens with light.
When atoms that produce an emission line are moving toward or away
from you, the energy at which you see the line changes. Hopefully you
remember that light is made of waves, just as sound (though different
kinds of waves). Just as the pitch of the sound you hear is shifted
depending on whether the source of the sound is moving or stationary,
the energy of the light will also change if the light source is moving.
If the source is moving away from you, the light you see will have
lower energy than the emitted light. If the source is moving toward
you, the light you see will have higher energy than the emitted light.
Astronomers refer to these as redshift (source moving away) or blueshift
(source moving toward).
The figure below shows redshift and blueshift for an optical
The three spectra above show how spectral lines shift if the
source is moving relative to the observer. The top spectrum is
produced by a source that is stationary with respect to the
In the middle spectrum, the absorption lines (the dark lines) are
shifted to the right (toward the red), and are thus redshifted and
produced by a source that is moving away from the observer.
In the bottom spectrum, the absorption lines are shifted to the
left of where they lie in the stationary spectrum. In this cast the
source is moving toward the observer.