Stars, galaxies, planets, moons, supernovae, nebulae...although we think of the night sky as dark, it is of course full of bright objects, and many fainter ones too. Their light is the key to our understanding of them – by analysing the light emitted by an object astronomers can incredible amounts, such as the object's size, distance, and age, and even what it is made of.
But actually some parts of the sky appear totally dark. For example, you may have noticed dark areas in parts of the Milky Way. And in this image of another part of the sky, it looks as if a huge chunk of stars has simply been deleted!
In fact, there is a simple explanation – these dark areas are huge clouds of 'cosmic dust', blocking out any visible and ultraviolet light from objects behind them.
So what is this dust exactly? It's tiny grains of matter, some as small as just a few molecules, formed by various processes in the universe, such as dying stars, collisions between asteroids, impacts on planets, and even volcanic eruptions (eg on Io, a moon of Jupiter), and plumes (eg on Enceladus, a moon of Saturn). It is everywhere in space but collects in certain areas, often in massive clouds between stars, and eventually becomes the part of raw material for new stars and planets.
Astronomers are particularly interested in collecting samples of dust as particles from distant parts of the universe can then be analysed directly, with microscopes instead of telescopes. For this reason, many space missions carry dust detectors. Most of the dust that they detect and collect originates from within the solar system, but in August 2014 NASA announced the capture by its 'Stardust' spacecraft of a handful of particles that may have come from much further away, from the vast region of space between stars.
Interesting though the origins of cosmic dust may be, the way that it blocks out light can be a serious problem for astronomers.But although it blocks visible and UV light, it actually absorbs these wavelengths and re-emits it in infra-red wavelengths, so in recent years space telescopes such as Herschel and Spitzer have been developed specifically to observe infra-red light. These telescopes can see the dust as glowing clouds, providing some stunning images.
And observations of dust can even be used analyse properties of objects that it obscures. For example, Circinus X-1, a neutron star in the Milky Way that is obscured by thick clouds of dust, recently emitted some brief X-ray bursts that could be observed. A team from the University of Wisconsin-Madison found that the X-rays appeared in a ring pattern, as you can see in this image, the dark rings being caused by deflection of the X-rays by dust. They announced last month that, by analysing the geometry and timing of the deflected X-rays, compared to the undeflected ones, they were able to estimate the distance to the star, a new method of making a calculation that was previously impossible because the star is invisible to optical telescopes.
http://www.irs.uni-stuttgart.de/cosmicdust/
http://herschel.cf.ac.uk/science/infrared/dust
http://lasp.colorado.edu/home/sdc/studying-dust/about-space-dust/
http://www.space.com/26833-nasa-stardust-spacecraft-captured-interstellar-dust.html
http://www.news.wisc.edu/23858 - 'Neutron star’s echoes give astronomers a new measuring stick '
1. ESO http://www.eso.org/public/news/eso0102/
2. Creative Commons https://en.wikipedia.org/wiki/Cosmic_dust#/media/File:Porous_chondriteIDP.jpg%20creative%20commons
3. NASA/CXC/U. Wisconsin/S. Heinz