So what's this "radiative transfer" thing about?
Radiative
transfer generically means the propagation of radiation (which can
be electromagnetic radiation or particles like neutrons) from its
source of emission. As these particles move through the medium there
is a chance they will be either
absorbed, in which case their
energy is transferred to the medium and the radiation is gone (though
the energy may be re-emitted at a later time), or
scattered,
in which case they change direction.
Apart from the trivial radiation transfer through a transparent
medium, most people are familiar with some natural phenomena where radiation
transfer effects are important, such as:
- Sunset and sunrise. (Hence the name of the code.)
The sun appears red when it is close to the horizon because the
light path goes through the atmosphere for a long distance and the blue
light is scattered off of particles in the air (or just off of the
molecules in the air itself, see next
page). EXAMPLE
EXAMPLE
- The blue sky. This is the flip side of the
previous effect. Some of the sunlight scatters off of the air
molecules and reaches us from random directions. Because this effect
is much stronger in the blue than the red, most of the light
coming to us this way is blue. EXAMPLE
- Clouds. The tiny water drops in clouds strongly scatter
light, which makes clouds appear opaque. However, not much light is
absorbed, so even if the sky is completely filled with clouds, it's
not completely dark down on the ground. Unlike the previous effects,
the water drops scatter light of all colors about the same amount,
so clouds look white. EXAMPLE
- Forest fires. This example might be most familiar to
people from California, but fire smoke contains small carbon
particles which absorb light, predominantly blue light. As a
consequence, if there is fire smoke in the air, the day will have an
eerie red color, and if you see the sun through a thick smoke layer
it will be an amazingly deep red
color. EXAMPLE
- Milk. Liquids like milk and cornstarch water are
colloidal suspensions, where fairly large particles are dispersed in
the liquid. These particles will scatter blue light more strongly
than red light, so the light that passes through is more red. You
can easily try this yourself, just fill a glass with very diluted
milk and shine a light through it.
- Underwater lighting. Underwater pictures taken in
natural light have a strong blue tint. This is because while water
is almost perfectly transparent, it absorbs red light somewhat more
quickly than blue light. As a consequence, the further down you go
in depth the more blue the light becomes. It is interesting that
this effect is opposite of most of the other examples mentioned
here, where blue light is more strongly scattered or absorbed than
red light. This is because unlike in the other cases which mostly
concerns scattering off of particles, water molecules have
vibrational bands that strongly absorb light in the infrared leading
to a very slight excess absorption in the red.
Hopefully this list have given you some feeling for what we are
talking about when we mention "radiation transfer". The forest fire
example above is a quite close analog of the situation that Sunrise
was developed for. It turns out that interstellar space is full of
dust grains, tiny particles of carbon (essentially soot) and
silicates (rock). These dust grains absorb starlight and thus obscure
our vision of what goes on in certain regions of space. For example,
stars form in thick clouds of molecular gas that are full of dust
grains, making them completely opaque to visual light from the
outside. Because the grains absorb more strongly at shorter than at
longer wavelengths, we need to use infrared light to see into these
objects. The idea is very similar to the infrared cameras that
firefighters use to see through smoke.
What happens to the energy in the starlight that's absorbed? It goes
into heating the dust grains, which then emit this energy as heat, ie
long-wavelength infrared radiation. Objects that are enshrouded in
dust and almost invisible in visible light are very bright at
far-infrared wavelengths because of this dust emission.
In this way, the presence of dust grains can profoundly alter the
appearance of objects rich in dust grains like young stars and
gas-rich galaxies.
So what does Sunrise do?
The purpose of Sunrise is to calculate what happens to the starlight as
it passes through the dust in the galaxy (or whatever object being
studied). The mathematical equation describing the behavior of
radiation is called the
equation of radiative transfer, and
is in general very difficult to solve.
A powerful method for calculating a solution is to use
the
Monte
Carlo method. Monte Carlo methods use random sampling to find the
solution to an equation or the integral of a function. In the case of
radiation transfer, the Monte Carlo method means "solving" the
equation in the same way nature does: It emits large numbers of
photons that each take a random path, and then you look at what
happened to them. The following schematic might help:
Simulated "photons" are emitted from sources of radiation, in this
case the stars. These photons then propagate according to the physical
laws governing them, and are absorbed and/or scattered when they pass
through opaque matter. As they pass through matter, they heat the dust
grains, which will subsequently emit this energy as new photons in the
infrared. Just like in real life, some of these photons reach the
"simulated astronomer" on the right. By calculating millions of such
photons, you end up with an image of what the object would look
like if you were to observe it with a real telescope. Something like
this, in the case of a disk around a young star:
Some examples of images of colliding galaxies calculated with Sunrise
are across the top of this webpage. You can find some other examples on the
Wiki.
