Cosmic dust is a general term for dust in space,
which includes intergalactic dust, interstellar dust,
and interplanetary dust.
Dust leads to extinction, making observation of
stars and other astronomical bodies more difficult,
but is also observed both for itself and for the
data that its reradiation and scattering provide
regarding other sources of electromagnetic radiation (EMR) of interest.
Infrared can be used to see through dust,
and extinction, scattering, absorption, and polarization
reveal information about the dust itself.
Though termed dust, it can be orders-of-magnitude smaller than
everyday dust: in the interstellar medium what are termed dust particles
(or referred to as particles, or dust grains or grains)
are a few molecules, though in the interplanetary medium,
presumably all sizes particles exist, from micrometeoroids down.
Essentially, once two molecules stick together, dust is the term
used. They are presumed to have the opportunity to grow larger in
molecular clouds, and clearly grow further in protoplanetary disks,
a necessary stage in planet formation. In such disks, the term
dust is used for grains gathering from the cloud, on the order
of a micron in diameter. The term pebble is often used
at some point if they grow to a few millimeters or a centimeter in diameter.
Clues to dust particle size include the effects of
the scattering of EMR via diffraction as well as the
particles' emission and absorption, and the
wavelength-dependence of dust-caused extinction.
Interstellar dust (a component of the ISM)
is presumed to include the elements that make up the
general abundances apparent in star systems,
but that appear to be absent from the ISM's gas component,
under the assumption that these star systems formed from this dust and gas.
This includes magnesium, silicon, calcium,
aluminum, iron, nickel, plus about half the carbon.
Oxygen is something of a mystery because there are reasons to doubt
it is a constituent of dust.
Also of interest is whether the dust is generally amorphous or crystalline.
Much dust is thought to be formed by stars toward
the end of their main sequence, such as red giants.
It is also formed or scattered by novae
and supernovae. The term nebula has come to be
most commonly used for clouds that include
sufficient interstellar dust to be opaque
to visible light.
Dust maps of the Milky Way are of interest to aid in
interpretation of photometry of Milky Way stars as well as
basically every other galactic or extragalactic observation. A
means of creating them is through analysis of stellar distance and
photometric survey data.
Grains of interplanetary dust within the solar system
(the interplanetary dust cloud) have larger diameters, on the
order of 10-100 μm. They are presumed to come from comets
and asteroids, e.g., from collisions, within the last 10 million
years or so, since there are processes that eliminate them, e.g.,
radiation pressure pushing away the smallest grains. There are
higher-density regions positioned in such a way as to suggest a past
association with a comet or an asteroid family.