DUST TO DUST
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.