This is an image of the sun in H-alpha -- i.e., an image taken
through a filter centered on a specific spectral line of Hydrogen,
with a wavelength of 6563 A. A high resolution JPG image
can be found here.
Active regions in this image show up as
brighter than their surroundings. The dark, string-like features
The image was digitized
from a photograph provided by the
Space Environment Laboratory,
National Aeronautic and Oceanic Administration (Boulder, CO).
For more information on this image, see
the HAO slideset
"The Sun: a Pictorial Introduction", by
P. Charbonneau and O.R. White.