September Stargazing: The Universe’s Miracle Substance

Dust is everywhere—and in everything

By Jeremy Miller

September 1, 2021

filename

The dust and gas of the Cone Nebula. | Photo by Jeremy Miller

Last month, the night sky was filled with bright fireballs and pencil-thin streaks of light. The celestial light show marked one of the great astronomical events of the year—the Perseids meteor shower.

In the best years, the Perseids can produce as many as 100 meteors per hour. But where do those meteors come from? The answer may surprise you. The term “meteor” conjures an image of a gigantic space boulder hurtling at tremendous speed through the cosmos. But it is not large boulders or even rocks that create the magic of the Perseids and other annual meteor showers. It’s dust.

Turns out, most of the flashes and streaks we see during a meteor shower are the result of tiny fragments, no larger than a grain of sand, passing through the upper atmosphere. The reason we can predict the Perseids at all is because of the nature of Earth’s orbit. Every year, Earth passes through a cloud of dusty debris shed by the comet Swift-Tuttle, last seen in the mid-1800s. Combine these tiny grains with gravity and atmospheric friction and fireworks ensue.

Here on Earth, we tend to think of dust as an annoyance—a substance that represents age, neglect, decay. Dust loiters on windowsills and clings to mirrors. It tickles the sinuses and triggers sneezes. In the fire-scorched western US, dust’s cousin, ash, has rained down on everything, a reminder of the major climatic changes afoot across the planet. But in the cosmos, astronomers are learning, dust is not merely the aftermath of cataclysmic events but also a fundamental material, a building block of almost everything—from planets and stars to nebulae and galaxies. Some new hypotheses suggest that without this most humble of materials, life itself may have never arisen.

*

Saturn’s rings—another beautiful sight in the night sky—are also the product of dust. Astronomers believe that the rings are composed of the remnants of pulverized planets, moons, asteroids, and comets drawn into Saturn’s orbit by its strong gravitational pull. This jumble of rock, ice, and dust is organized into discrete bands, each orbiting the planet at different speeds. If we could look at them from Saturn’s surface, the rings would appear to us as little more than a formless, fuzzy smear. Like a pointillistic painting, the pattern reveals itself only when you take a step back—in this case, a big step back. From Earth, 800 million miles away, the rings appear solid and symmetrical, almost perfectly composed, as if drawn by hand.  

A third dust-derived sight beloved of stargazers is the zodiacal light, a strange cone of luminescence illuminating the horizon. Best viewed in dark sky locations in late summer or early fall—and depending on which hemisphere you live in, before dawn or dusk—the zodiacal light is thought to result from sunlight striking dust in the ecliptic, the narrow band of our solar system through which the planets pass.

In dust lies the birth of planets and stars, which are the products of the accretion of trillions of tiny particles. In gaseous nebulas, clumps of gas and dust churn and mix. Once they have reached a sufficient mass, these agglomerations collapse inward under their own gravity, forming a hot, glowing mass known as a protostar. Over the eons, these primordial suns continue to draw gas and dust into their midst until they become full-fledged stars. Planets form in a similar process, though on a much smaller scale, as dust clumps into small stones, smashing together into increasingly large masses that eventually become small planet-like objects called planetesimals. Given enough time, these planetesimals may pack on enough material to become moons or planets.

Dust is a key component of nebulas, imbuing them with their beautiful and surreal contours. The famed Orion Nebula, a mainstay of the winter sky, is an emission nebula whose brew of hot gases give off light. But there is another, far fainter class of space cloud known as reflection nebulas. As their name suggests, they reflect starlight. Take for example the Witch Head Nebula, a reflection nebula also located in constellation Orion that resembles the profile of a witch in a storybook. The Witch Head is believed to be the remnant of an ancient supernova that, in the intervening eons, has “gone cold.” The reason we can still see traces of it is because of its shimmering dust particles, including flecks of nickel, iron, and even diamond, which reflect light from the massive star Rigel like shards of a shattered mirror.

For ages, dust has been associated with mortality and death. In Hamlet, Shakespeare wrote of “dusty death.” Robert Frost mused on a “dust of snow,” shaken down onto him by a crow perched in the branches of a hemlock tree. But without dust, there would be no life at all. Lightweight and easily thrown up into the atmosphere and beyond, dust seems to be the ideal mode of transport for would-be microscopic interplanetary hitchhikers. One recent hypothesis proposes that life itself may have been seeded by dust-riding microorganisms, swept across the universe behind comets or cosmic winds.

But from where did dust originate? As it happens, dust is a later addition to the universe. The early cosmos was comprised almost entirely of clouds of hydrogen and helium, which over hundreds of millions of years condensed into stars. Within the cores of these first stars, hydrogen and helium atoms merged to form heavier elements. When those stars died and exploded in massive supernovae, they threw their dusty debris into the void. Our physical world, every animate and inanimate thing, is thus composed of atoms thrown outward from the interiors of stars.

In our strange and complex universe, a simple rule applies: From small beginnings come great things. 

 

WHAT TO LOOK FOR IN SEPTEMBER

September is a great month to see the zodiacal light, particularly if you are close to the tropics and in a location with dark skies. Look toward the eastern horizon at the point where the sun rises an hour or so before sun-up in the weeks leading up to the autumnal equinox, September 22. If your views are unobstructed and your skies are dark enough, you should be able to see a faint wedge of misty light cutting through the heavens. The phenomenon is caused by the scattering of sunlight through dust and ice shed by comets as they race around the sun. Asteroid collisions are also believed to add to the dusty mix. If you are an early riser, or don’t mind setting the alarm to rise well before the sun, be sure not to miss this spectacular and eerie celestial phenomenon. For those living in places impacted by mild light pollution, a camera is a good option. (Sadly, the zodiacal light is too faint to be seen from cities.) Place the camera on a tripod and shoot 30-second to one-minute exposures in the direction of the rising sun. Here’s a good tutorial for photographing the zodiacal light.

September is also a great month to observe Saturn and its exquisite rings. To find the ringed planet, look to the southeast just after sundown. At the beginning of the month, Saturn will rest about 25 degrees above the horizon, not far from the much brighter Jupiter. Keep an eye on Saturn, in particular, on September 16, when the waxing moon will pass within 5 degrees of it. 

If you have access to a telescope, the wonders will increase dramatically as the rings become visible. Most consumer-grade telescopes will reveal the rings as a single, unbroken band—like a gold ring encircling a tiny sphere—but more powerful telescopes can begin to tease out some of the individual bands as well as the so-called Encke Gap, a large black void in the middle of the rings.  

On September 20, the Harvest Moon will appear in night skies. This full moon rises around the time of year when corn, squash, and beans ripen, a celestial cue for Indigenous farmers to bring in their crops. According to the Old Farmer's Almanac, at the peak of harvest, Native farmers would often work late into the night using the moon for illumination.