What is HISA?

We have adopted the acronym HISA to stand for H I self-absorption, where ``H I'' is astronomer shorthand for neutral atomic hydrogen gas, and ``self-absorption'' is defined below. H I is composed of atoms containing one proton in the nucleus and one electron moving in orbit around the nucleus; ionized hydrogen, in which the electron has become separated from the proton, is called ``H II''. Hydrogen is not the only element in space, but it is the most abundant, comprising about 70% of the mass and 90% of the atoms in interstellar gas.

Interstellar H I is quite easy to detect, because it radiates energy with a characteristic wavelength of 21 centimeters, which radio telescopes can receive. By measuring the brightness of the radiation at different positions in the sky, a radio telescope can be used to make images of the clouds of gas producing the radio waves, much as an optical camera collects light through its lens and produces a picture. Interstellar clouds can be just as detailed, varied, and beautiful as terrestrial clouds (see our image gallery). Their appearance is quite different from Earthly clouds in many respects, which should not be surprising given the vastly different environment in which they form -- the cold, dark, near-vacuum of interstellar space. But the physics of how light propagates through diffuse objects remains the same, so terrestrial cloudscapes are convenient for illustrating some of these concepts of radiative transfer:

• emission - the radiation of internally-generated light (or radio waves) from an object; the sun emits light

• scattering - the redirection (reflection) of light shining upon an object; all visible light which appears to be emitted by Earthly clouds is really scattered sunlight

• absorption - the ``swallowing'' of light by an object on which it shines, making the object appear relatively dark; a dark cloud absorbs most of the light incident upon it

• self-absorption - the absorption of light shining on an object by a process similar to that which produced the light originally; if a cloud emits less light than it absorbs from other clouds behind it, this net loss of light, termed self-absorption, causes the foreground cloud to appear darker than the background field (the term is from spectroscopy, where small absorption dips sometimes occur in larger emission lines, making the latter look as if it is absorbing part of itself)

These processes can occur anywhere with a cloudy medium -- in space, or in a glass of beer. The main difference between terrestrial and interstellar clouds is that interstellar H I is often self-luminous and does not scatter radio waves, while terrestrial clouds shine by scattered sunlight. Because of this, terrestrial clouds do not self-absorb in the strict sense; instead they attenuate light by a mixture of absorption and re-scattering (self-scattering?). But the effect is much the same. One often sees dark clouds in front of lighter ones. So while the parallel isn't perfect, it remains useful for illustrating interstellar cloud self-absorption.

Below are several snapshots taken out of an office window. Views like this are quite common, if you keep your eyes open. Each thumbnail image links to a larger version.

A mass of clouds at different heights and distances. Because the sun, the major source of light, is shining down from above, those clouds at the lowest level of the cloud deck receive the least light, and appear dark in comparison with others behind them. The effect is most obvious near the horizon.

A similar view, showing some nearer dark clouds more prominently on the left. In the lower center, a cloud is light on the top and dark on the bottom, a clear scattering effect which highlights the 3-D shape of the cloud. Interstellar hydrogen does not scatter 21 centimeter radio waves, but only absorbs and emits them. However, dust particles in the same clouds do scatter optical starlight, which can produce 3-D shape effects in photographs.

Steam rising from a chimney behind the peaked roof at right appears light against the dark trees, and dark against the lighter background clouds. The brightness of the steam does not actually change, but its contrast against the background reverses. Some interstellar H I clouds also appear to change from light to dark, or from emission to absorption, across varying background fields.

A nicely conspicuous small dark cloud at lower left, with some faintly distinguishable light and dark foreground features in the center of the picture. Our survey finds a number of compact and conspicuous HISA features, along with many fluffy, extended objects that are barely visible.

A new element is added: background blue sky, with light clouds against it and dark clouds against them. All of the H I clouds in our survey lie against an additional background of smoother radiation from H II gas in the Milky Way.

A complex case -- light clouds in front of dark, dark in front of light, and each in front of background ``blue'' sky in places (unfortunately the blue sky came out fairly similar in color to the dark clouds, so you have to look carefully to distinguish it). As in this picture, the interstellar sky in our survey can be difficult to disentangle at times.

A short summary of our scientific interest is given in the science overview, with more details in our online publications.