When scientists refer to a spherical cow, we are poking fun at ourselves.
We are admitting that some of our models or descriptions of things are far
more simple than the actual object, like to say that a cow has a spherical
shape. The phenomena we study are often complex, and including too many
details can hinder, rather than help our understanding. Often it is useful
to study a simplified model which contains only the most important
general characteristics. Such a model can be more easily studied using
numerical or analytical methods and then compared to
As an example of this kind of thinking, say we were aliens trying to
understand "humans", a strange race of beings recently discovered on a
small planet orbiting a medium-sized star. We might divide them into two
groups, one which grows facial hair (men), one which does not (women).
Within each group there is a lot of variety - each human in the first
group group can
have facial hair in a wide range of colors and textures, for example. However,
we think that there is some underlying reason for the gross
characteristic of having or not having facial hair. We might then make
more observations to try and understand why this is so. These further
observations might uncover more similarities (humans in the first group
have both an X and a Y chromosome while humans in the second group have two X
shaped chromosomes) that are more fundamental. In astronomy we
try to do the same thing.
Often what we do is look at a limited number of characteristics,
trying to understand each to put into our picture of the whole.
Almost any astrophysical object is too complicated to try and understand
everything at once. It makes more sense to look at only a few characteristics
at one time and to try and understand how each one affects how the
object appears or evolves.
"Flavors" of SNR - Different Viewpoints
Supernova Remnants (http://heasarc.gsfc.nasa.gov/docs/objects/snrs/snrs.html) (SNR) are one
such complex astrophysical object.
You can say that SNRs come in many different flavors, depending on who
you talk to. Each flavor or "ideal" SNR isolates important features, giving
scientists a piece of the
puzzle in helping to figure out the entire SNR picture.
First there is
the theorist's SNR, which is something of a spherical cow. In this
a couple of
thousand years after the star blew up in a supernova explosion, the
material has swept up a considerable amount of the ambient gas, leaving
behind a big, hot hole. There is a little bit of gas inside the hole
that gas emits some thermal X-ray light. There is a lot more gas on the
edges (because the swept-up gas was swept to the edge of the hole).
gas is also very hot and should emit thermal X-ray light.
Another flavor of SNR is the observer's
SNR. The observer generally has many pictures of the SNR, each taken with a
different observing instrument, such as X-ray,
radio, and even visible
light detectors. Generally the SNR looks different "in each of these
different wavelengths", just like you and I look different to another human
being (who looks at the visible light) then we do to a bee or a snake (who
are able to detect ultraviolet and infrared light,
respectively). Also, even when using the same observing instrument, each SNR
looks different from all of the other SNRs and often observers try to
figure out what is causing the differences.
Cartoon Model of a SNR (http://heasarc.gsfc.nasa.gov/docs/objects/snrs/cartoon.html)
reasonable, so where does the spherical cow business come in", you ask.
The answer is that we often pretend that the SNR has a spherical shape, in
spite of the fact that most of them are a bit out-of-round. We also
pretend that all SNRs should look the same even though they don't. But, we
do this for some good reasons -- we want to learn about what happens to
SNRs in general as they mature and what SNRs as a whole
class will do to the galaxy. Concentrating on
their common features helps us to do that. Also, simplifying their shapes
makes it easier for us to perform the mathematical computations that we do.
Scientists are indeed guilty of calling cows (or SNR) spherical. But
there is good reason to do so. It is important to understand not only
what makes a cow more or less spherical as well as to
understand what it is that makes a cow, well, a cow.
Thank you to Robin Shelton for contributing to this article.