By Jim Kaler

It was the monster under the bed. Maybe if we didn't think about it, it would go away. It didn't. "It" was the absorption of starlight by interstellar dust.

As do so many stories, this one goes back to 1784, when William Herschel noted that, as the sky floated past his fixed telescope, the spaces preceding diffuse nebulae were "were generally quite deprived of stars." That is, the dark stuff, later understood to be dust clouds, is related to the bright stuff. Such a conclusion hardly needs a telescope. On a dark clear night far away from artificial lighting (increasingly difficult if not impossible to find) the Milky Way is cut in two by a thick lane of interstellar dust clouds, which is also where we find the glorious diffuse nebulae (Orion Nebula, etc.) that are lit by the ultraviolet light of hot stars and where we also see the blue reflection nebulae like the one that surrounds the Pleiades. Not a smooth layer, the dust is gathered into small to gigantic clouds that give great character to the Milky Way. It was clear almost from the start that these "deprived" areas can't simply be devoid of stars, as stellar motions would rapidly fill them in. Instead, there must be something blocking the light of the background. The dark clouds (like the Coalsack near the Southern Cross) are so prominent that the Incas of South America made constellations out of them. By 1919 E. E. Barnard of the Yerkes Observatory had catalogued 182 individual dark clouds and twice as many now bear his name.

The "interstellar medium" (ISM) holds roughly 15 or so percent of our Galaxy's "illuminated" matter. Though the clouds are dark to the eye, they are bright in the radio spectrum and are quite distinct from "dark matter," which radiates nothing at all. The ISM is an incredibly complex mix of gas and dust. While the gas is mostly transparent, the dust both absorbs and scatters the incoming photons of starlight. On the average, the Galaxy's disk contains about one grain of dust per cubic meter. But nothing is "average." The ISM is pounded and shredded by stellar winds and shock waves from stellar explosions, which together yield a remarkably lumpy mix. By keeping out heating starlight, the dust in the denser clouds acts as a natural refrigerator. Temperatures drop to near absolute zero, which allows a slow, cold chemistry aided by cosmic rays (high-speed nuclei from exploding stars). Made mostly of molecular hydrogen, the clouds also contain a vast number of chemicals that include carbon monoxide, water, ammonia, alcohols, even some that cannot exist under earthly conditions. The dust, which collectively contains about one percent of the ISM's mass, divides into grains made of silicates or carbon (graphite, diamond) that are coated and filled by metals drawn from the gaseous medium. Unlike the dust on your refrigerator, the grains are too small to be invisible to the naked eye.

Within these molecular clouds, blobs of especially dense ISM can collapse under the force of their own gravity. As a blob contracts, it heated internally as would any compressed gas. When the interior (core) temperature hits a few million degrees Kelvin (Celsius degrees above absolute zero), nuclear reactions turn the (now atomic) hydrogen into helium, which provides an outward push of energy and stops the contraction. The result is a new star. If sufficiently massive, above say 10 Suns (class O or hot B), it radiates enough ultraviolet light to ionize the hydrogen within the cloud, and we see a dusty diffuse nebula nested within a surrounding dark molecular cloud, perhaps one like the Trifid or Lagoon Nebulae and hundreds of others.

Stars have finite lifetimes that are determined mostly by birth mass. Lower mass stars (below 10 Suns) run the internal fusion chain up to a mix of carbon and oxygen. While the nuclear engine is going through its various stages, the outer layers expand, yielding giant stars like Arcturus and Aldebaran (made obvious by their orange colors) and yet more advanced giants like Mira, the queen of the "long-period variables" (LPVs), extreme class M stars that can vary by many magnitudes and can take more than a year to complete a pulsation cycle. Luminosities climb accordingly, the expanding, dying stars radiating the light of thousands of Suns. In some stars internal convection can bring the freshly-made carbon to the stellar surfaces, yielding more showpieces, deep red carbon stars like R Leporis (Hind's Crimson Star in Lepus south of Orion). Dust condenses in the outer cool layers that are eventually wafted off to reveal the inner cores, which, while on their way to becoming white dwarfs, are for a brief time surrounded by planetary nebulae like Lyra's Ring Nebula or the Dumbbell in Vulpecula. The outflowing gas and dust (silicates from ordinary oxygen-rich Miras, carbon dust from carbon stars) mixes with the ISM and is ready to do new duty in the creation of another stellar generation. Winds from more massive stars, those above 10 Suns that eventually explode as supernovae, do much the same.

So what is the monster under the bed? Until around 1930 astronomers thought (hoped?) that all the dust was in the discrete clouds and that space was otherwise clear. Woops. In that year, R. J. Trumpler of the Lick Observatory estimated the distances to open clusters based on the apparent magnitudes of their brightest stars whose absolute magnitudes were known from their spectra. He calculated that distant open clusters were much larger than nearby ones no matter in which direction he looked. There must be something dimming the more distant stars. The monster had crawled out.

Except for those nearby, stars in or near the plane of the Galaxy are dimmed by interstellar dust, the effect increasing with distance. Within the Milky Way a magnitude or more is not uncommon. With but one tiny grain per cubic meter, it seems almost impossible that the stuff should have that kind of effect. But there are a LOT of meters along the line of sight, 1015 per light year, giving the dust quite a lot of leverage. To calculate the mass of any single star, we have to know its luminosity, and thus must calculate the degree of interstellar absorption. Fortunately, interstellar dust has a much greater dimming effect at shorter wavelengths than at longer. In long- wave infrared and radio it is insignificant, while violet light is dimmed twice as much as red. The phenomenon gives reflection nebulae their blue colors when the stars are not hot enough to ionize the gas.

A distant star is therefore also reddened. The degree of reddening (in magnitudes) can be found by comparing the observed color (the magnitude determined with a blue filter minus the traditional visual magnitude) with that expected from the spectrum, known as the "selective absorption." Various studies have shown that the ratio (R) of total visual absorption (the visual dimming) is usually about three times the selective (I use 3.2). But sometimes it's as high as 6, depending on location and the kinds and sizes of the grains. Interstellar dimming remains one of the bigger annoyances in stellar astronomy, affecting not just the stars in our own Galaxy but those in others (supernovae, Cepheids) that are used to determine the expansion rate and thus the age of the Universe.

A famous astronomer was once asked if he would do it all over again. He replied that yes, he would, but only if he could be assured that R = 3.

Copyright © James B. Kaler, all rights reserved. These contents are the property of the author and may not be reproduced in whole or in part without the author's consent except in fair use for educational purposes. First published in July 2016 by LYRA, the Lowestoft and Great Yarmouth Regional Astronomers, who are gratefully acknowledged.