Interstellar dust: Not just bunnies under the bed

By U.J. Sofia,
William K. and Diana R. Deshler Chair and Professor of Astronomy

Most people have never heard of it and very few have ever thought about it, yet everyone owes their existence partially to it. Without it, the Sun could not have formed, planets would not exist, and the basic molecules of life could not have developed. What is this amazing substance? It’s interstellar dust.

Don’t be turned off by its lowly moniker. This is not the same stuff that lurks under your bed forming bunnies; it’s a whole different animal. But 78 years after its discovery by Robert J. Trumpler, we’re still learning about this material that is so important to the formation of life, and to the structure of the Universe and the field of astronomy.

My main interest in dust is its effects on astronomical observations. Astron-omy differs from other sciences in that we rarely get to touch or affect what we study. That means we have to rely on what our subjects are willing to tell us rather than being able to tease out their secrets through manipulation. The only data that we get from our objects is the light that they emit. From gamma rays to radio waves, these data travel through space faster than anything else (at 670 million miles per hour), thereby allowing us to sample the most distant and oldest objects in the Universe.

The problem is that as the light travels through space toward our telescopes it gets distorted by dust in the interstellar medium. It’s the same effect that results in beautiful red sunsets; filtering light through the Earth’s atmosphere causes the yellow Sun to change color. As with sunsets that can have different colors depending on whether there was a recent dust storm or volcanic eruption, distortions of the light in space will vary with the composition of the interstellar material.

So, if the only astronomical data that we have to work with is light, and that light is distorted by interstellar dust, then how can we ever figure out the nature of distant objects? We have to understand the grains that are causing the distortions.

Dust is an elusive subject of study. First off, dust lives in the interstellar medium, which is the region between stars. In familiar terms, the best way to describe the interstellar medium is to say that it is a complete absence of everything. Obviously that can’t be precisely correct — otherwise I would be out of a job — but it’s close. Stand up and take one step forward through the Earth’s atmosphere. You would have to travel 10,000 light years through the interstellar medium in order to pass through the same amount of material. Ten thousand light years is almost unfathomable. If you were traveling at 1,000 miles per hour, it would take 7 billion years (half the current age of the Universe) to travel that distance. So, the interstellar medium is essentially nothing, and that makes it difficult to study. Dust is also tricky to characterize because it’s basically invisible. We can see dust’s effects on light traveling through it, but the dust itself is undetectable with current technology.

Given these difficulties, it may not be surprising that most of what we do know about dust has come from the most sophisticated astronomical observatory ever created, the Hubble Space Telescope. Before Hubble we knew that the interstellar medium was 99 percent gas and 1 percent dust. The dust was known to be small (on average about one millionth of an inch in diameter or the size of particles in cigarette smoke), solid pieces of material composed of about one million atoms. Beyond that, dust was pretty much a mystery. The educated guess, however, was that its constituents were a combination of silicates (think tiny grains of sand) and amorphous carbon (soot-like material).

Using data from the Hubble, we have been able to learn about the dust by studying what elements are missing from the gas. Hubble has indeed confirmed that the dust population includes silicates and carbon grains. But we have further been able to infer the specific mineralogy of the silicates (not all sand is created equal), and have discovered that iron oxides (rusty metal) are a significant component of the grain population. Knowing this information allows us to quantify the light-distortion properties of the various grain types, which in turn results in a better correction for astronomical observations.

U.J. Sofia teaches astronomy to eighth- and ninth-grade students on campus this summer for the Whitman Institute for Summer Enrichment (WISE) program.

But what about carbon dust? These grains still have us scratching our heads, which is particularly disconcerting because carbon is the most abundant element in dust, yet it remains an elusive element that is difficult to analyze. We knew absolutely nothing about its abundance in dust before Hubble, and as the saying goes, ignorance was bliss. Since nobody could provide limits, we modeled the light distortion with as much carbon dust as we wanted, and we wanted a lot. Once Hubble came onto the scene, it quickly became apparent that our carbon budget was deep in the red. There simply was not enough carbon in dust to account for the light distortions that we were seeing. Within the field, this problem became known as the Carbon Crisis. It’s quite the dramatic name, but in the world of dust it was, and is, a pretty big deal. The crisis means that there is a fundamental flaw in our understanding of the most important dust constituent.

As I noted earlier, carbon is a difficult element to analyze. In fact, even with about 100 orbits of Hubble data dedicated to its study, corresponding to $10 million worth of instrument time, we have carbon measurements toward only 13 stars. These measurements all support the idea that the crisis exists. But what if we’re looking at the data incorrectly?

The carbon measurements to date all make use of the same electronic transition of the atom. I have recently found a way to analyze a second transition, which because of its properties was formerly thought to be useless. We were recently funded by NASA to mine the previously observed, second-transition data in order to triple the sample of interstellar carbon measurements.

While extending the sample is a worthy goal, our initial results have proven to be much more interesting. They indicate that an error may exist in the published atomic constants for the first electronic transition. As a result, our second-transition data suggest that there is more carbon in dust than previously thought. If this result holds, we have solved the Carbon Crisis (no bailout needed), which in turn will allow us to fully quantify the corrections for astronomical observations. Stay tuned.

If you’re still not convinced that dust is interesting and important, I suggest for further reading, Dr. Suess’ “Horton Hears a Who” and Philip Pullman’s “The Golden Compass.”

U.J. Sofia joined the Whitman faculty in 1998. He received a bachelor’s degree at Wesleyan University and a Ph.D. at the University of Wisconsin-Madison.