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The Stuff Between The Stars

The cosmos is laced with tiny specks of dust that decide the fate of young stars and planets. Now, NASA scientists can study the properties of far-flung space dust in the lab.

by Patrick Barry

Space is a vacuum, right?

Well, almost... The space between the stars is about as empty as the best artificial vacuums created by scientists on Earth, but throughout space, the void is faintly sprinkled with gas molecules and dust grains.

These extremely sparse crumbs of matter drifting in lonely spaces between the stars may seem utterly obscure and insignificant, but they turn out to play an important role in the formation of stars and planets and many other astrophysical phenomena.

To better understand how dust grains respond to conditions in space, researchers at NASA's Marshall Space Flight Centre (MSFC) have built an apparatus in the Dusty Plasma Laboratory (DPL) that can suspend individual dust grains in a near vacuum. Once a dust grain is captured, scientists can bombard it with forms of radiation found in space and see what happens.

"What we're doing here is taking one particle and exposing it to these space-like environments and studying what happens to its (electrical) charge and other properties," said Catherine Venturini, who worked on the project for more than four years while pursuing her master's degree in physics at the University of Alabama in Huntsville.

The electrical charge on dust grains in space can, among other things, determine how small particles of dust stick together and grow into larger-sized grains that lead to the formation of stars and planets. Gravity pulls dust and gas in interstellar clouds together, but because the electrostatic force over short distances is so much stronger than gravity, even a small electrostatic repulsion between dust grains can influence (and possibly prevent) a cloud's collapse.

Stars in the Keyhole Nebula began to form about 3 million years ago. Tiny grains of interstellar dust influence how rapidly the nebula can collapse. They also enrich the cloud with heavy elements that are important for the formation of rocky Earth-like planets.

Also, the gas in the cloud tends to heat up as it compresses. If the cloud cannot radiate that heat away, the expansive force of the heat will resist further compression. Dust grains in the cloud are able to radiate this heat as infrared light, cooling the cloud and allowing collapse to continue.

If a nebula does collapse into a stellar system, dust provides many of the elements such as carbon, iron, magnesium and silicon that comprise the planets. (Unlike household dust, which is largely composed of dead skin cells and other organic debris, cosmic dust probably consists mostly of heavier elements.)

The role cosmic dust plays in planet and star formation is only one of many reasons astronomers are interested in better understanding its properties.

Dust also plays a role in crafting some of the most beautiful features of the cosmos: planetary rings (like those around Saturn), the long tails of comets and the spectacular, colourful clouds of nebulae.

Saturn's rings are marked by strange dark radial features called spokes. Since they have been observed on both sides of the ring plane, spokes are thought to be microscopic dust grains that have become charged and are levitating away from the ring plane. Another possibility is that a meteoroid punched through Saturn's rings, lifting dust particles away from the plane of the rings. When the Voyager spacecraft first observed these spokes, their movements seemed to defy gravity and had the scientists very perplexed. Since the spokes rotate at the same rate as Saturn's magnetic field, it is likely that electromagnetic forces are at work. This is still an unsolved puzzle.

Comet tails also contain a large amount of dust expelled by gases released when the comet passes close to the Sun. Because comets are composed mainly of dust and ice, studying the properties of cosmic dust will help scientists understand comets better, says Venturini.

Interstellar nebulae are laced with dust, too. The percentage of dust in nebulae is much less than in comets - less than 1 percent - but still the dust has important effects on the properties of the cloud.

For example, dust influences the way the cloud reflects, absorbs or emits light.

Understanding the optical properties of dust is especially important when an interstellar cloud upstages some other astronomical object that scientists are trying to study.

Dark nebulae completely block light coming from stars behind them, creating a dark patch in the sky. Some nebulae shine with reflected light from nearby stars (like clouds in Earth's atmosphere illuminated by the Sun), while other nebulae emit their own radiation in the form of infrared light.

"Most of the infrared light observed from the sky results from space dust," said Dr. James Spann, a DPL principal investigator. "Many times astronomers wish it was not there, but it is. They have to remove the contribution of the dust so that they can study other objects they are interested in."

This image, from a picture captured by Dave Palmer, shows the Milky Way in the constellation of Sagittarius. The Centre of our galaxy lies near the middle of the image, but we can't see it because dust grains in intervening clouds block starlight coming from the core of the Milky Way. The dark areas in this image are places where the dust concentration is high. (Airplanes caused the lines through this picture.)

The Dusty Plasma Lab allows scientists at MSFC to study the optical characteristics of the dust grains trapped in their apparatus.

Venturini said that other experiments will measure how dust grains of different sizes and materials scatter, absorb and emit light of different frequencies. Experiments with the DPL have already measured the effects of ultraviolet radiation and an electron beam on simulated cosmic dust grains. The electron beam is used to mimic the plasma in which cosmic dust is sometimes immersed - hence the name "Dusty Plasma Laboratory."

To complement these experiments, the MSFC investigators are working on a collaborative arrangement with Auburn University's Space Plasma Laboratory (SPL). The SPL, which is coordinated by Dr. Edward Thomas, performs experiments on large groups of dust particles rather than individual grains.

"The characteristics that we will see from the individual dust particles can be calibrated and correlated and investigated in connection with (Thomas' results)," explained Dr. Mian Abbas, an MSFC scientist and the principal investigator for planned experiments dealing with optical characteristics of dust grains and their condensation processes. "We look at single particles and Dr. Thomas looks at large collections of particles, so the two are sort of the complements of each other."

Laboratory work is only one aspect of better understanding cosmic dust, Venturini continued.

"It's another facet to understanding the whole picture. You have the modeling, you've got the theory, you've got the observation from satellites, and then you need the lab work to help verify the other components."

Satellite missions with instruments for measuring interstellar dust - such as Cassini, Galileo, and STARDUST - have fuelled a surge of interest in such research over the last decade or so, Venturini said.

"Because of (the satellite data) they're realizing, 'Hey, these little dust particles are playing a much more important role than we thought before,'" Venturini said.


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First Science 2014