SIGHTINGS
Seeing Stars
Felice Frankel
Felice Frankel
A few years ago, an image of stars being birthed in the Eagle
Nebula appeared on the front pages of some of the most important
newspapers in the world. People were stunned by its eerily
exotic colors and forms. The "representational color"
applied by the scientists processing Hubble Space Telescope data
helped communicate the rich information captured by the
telescope’s cameras and should, in my opinion, be included
whenever the image appears. Jeff Hester, professor in the
Department of Physics and Astronomy at Arizona State University,
tells us how he and colleague Paul Scowen of Arizona State
created this memorable look into the universe.
F. F. Can you first describe to us how the image
was constructed?
J. H. The image is constructed out of 32 separate
images made using four different filters. Each of the four quadrants
of the image is a physically separate camera. Making the final image
involved: (1) calibrating each frame and removing various
"instrumental signatures" such as bias levels,
pixel–to–pixel sensitivity variations, errors in
digitization and dark currents, etc.; (2) identifying and rejecting
various artifacts such as the streaks left by cosmic rays as they
passed through the detectors; (3) combining all of the images taken
through a given filter into a single frame (which entailed
alignment, correction of geometrical distortions and stitching
individual images together into a four–chip mosaic); and (4)
combination of the different single–filter mosaics into a
color image.
In the final image, blue represents light in an emission line (a
distinct emission wavelength) from doubly ionized oxygen. Green
represents light in the distinct wavelength emitted when a proton
and electron combine to make a hydrogen atom. Red represents light
in a line emitted by singly ionized sulfur atoms. So the colors show
variations in the level of ionization and excitation from place to
place. It is a map of the physical properties of the gas. It turns
out that it is also closer to what you might see through a telescope
with your eye than is a picture taken with color film.
Start to finish, I spent the better part of a week turning the raw
data into the final frame—time that included working on the
computer code used to carry out several of the steps mentioned above.
F. F. How long did it take to capture the data?
J. H. Let’s see.… As I recall, there
were six HST orbits of exposure used to make the final image. So
that’s about 9 hours of telescope time, during about 4 hours
of which the shutter was actually open.
F. F. Did you and Paul Scowen work together on all
aspects of the final image? Did you disagree on various decisions?
If so, how did you resolve your differences?
J. H. Paul was actually the first to see the data
and made a rough version of the image. The image that was released
to the public was a version of the image that I produced. There was
not much in the way of discussion of the image colors and the like.
There was a lot of discussion about the science present in
the image.
The real issue when you look at an image like this is that there is
one hell of a lot of information present. Interpreting that
information is not like interpreting a simple plot or graph.
Instead, you have to look at the relations between different aspects
of the image, as well as the variations in physical properties of
the gas from place to place.
Interestingly, the beauty of the image is not happenstance. When
people talk about "beauty," they are talking about the
presence of pattern in the midst of complexity. Pattern recognition
is so important to humans that perceiving patterns in the midst of
complexity is pleasurable to us. To a great degree, the same
patterns present in the image that make it aesthetically pleasing
also make it scientifically interesting. Our reason for combining
the data in this fashion was not to make a pretty picture just for
the sake of making a pretty picture. Rather, it is by combining the
data into a "pretty picture" that we present the
information in a form that the human brain can more readily
perceive. In short, we use color in the image in much the same way
that an artist uses color. We use it as an interpretive tool to
present our observations of the world in a way that other humans can
more readily understand.