Dust to dust: in the darkest regions of the Milky way are vast interstellar clouds harboring the remains of dead stars and the nurseries for new ones

Natural History, May, 2003 by Neil deGrasse Tyson

A casual look at the Milky Way on a dark, clear night reveals a cloudy band of light-and-dark splotches extending from horizon to horizon. With simple binoculars or a backyard telescope, the dark and boring areas of the Milky Way look like, well, dark and boring areas. But the bright areas resolve into countless stars and nebulae.

In a small book titled Sidereus Nuncius (The Starry Messenger), published in Venice in 1610, Galileo gives an account of the heavens as seen through a telescope, including the first-ever scientific explanation of the Milky Way's patches of light. Referring to his yet-to-be-named instrument as a "spyglass," he is so excited he can barely contain himself:

The Milky Way itself, ... with the aid of the spyglass, may be observed so well that all the disputes that for so many generations have vexed philosophers are destroyed by visible certainty, and we are liberated from wordy arguments. For the Galaxy is nothing else than a congeries of innumerable stars distributed in clusters. To whatever region of it you direct your spyglass, an immense number of stars immediately offer themselves to view.

Surely to Galileo and his contemporaries, the "innumerable stars" were where the action was. Why would anyone care about the dark areas, where stars were presumably absent?

Three centuries would pass before anybody figured out that the dark patches are thick, gigantic clouds of gas and dust, which obscure more distant star fields. Among the first astronomers to address the problem was an American, George Cary Comstock, who wondered why faraway stars are much dimmer than their distance alone would indicate. Following up on Comstock's observations, the Dutch astronomer Jacobus Cornelius Kapteyn named the culprit in 1909, when he presented evidence that clouds of "meteoric dust" in the space between the stars not only absorb the overall light of stars, but do so unevenly across the rainbow of colors in a star's spectrum. Specifically, the clouds attenuate blue light more than red, making the Milky Way's faraway stars look dimmer and, on average, redder than the ones nearby.

Ordinary hydrogen and helium, the principal constituents of cosmic gas clouds, don't redden light. But large molecules do--particularly the ones that include atoms of carbon or silicon. And when the aggregations of such atoms and molecules get big enough, we call them dust.

Most people are familiar with dust of the household variety, though few know that, in a closed home, it is made up mostly of dead, sloughed-off human skin cells--plus pet dander, if you have a live-in mammal. Last I checked, nobody's epidermis has gotten into the interstellar dust. But the cosmic clouds do include a remarkable ensemble of complex molecules that emit microwaves, and dust that emits primarily in the infrared part of the spectrum. Not until the last third of the twentieth century, however, did the astrophysicist's tool kit enable us to observe the powerful emissions and chemical richness of the stuff between the stars.

Interstellar clouds are intriguing for yet another reason. Deep within them, through the effects of their internal gravity, the dust and gas become thick enough to condense into clumps of matter. If conditions are just right, those clumps can form larger and larger clumps, and eventually full-fledged stars. In other words, those giant clouds are stellar nurseries.

Gas clouds in the Milky Way are not always capable of starbirth. More often than not, even after a cloud forms, it is confused about what to do next. Actually, we astrophysicists are the confused ones. We know the cloud is trying to collapse under its own weight and make one or more stars. But the cloud's rotation, as well as turbulent motion within it, acts against collapse. So, too, does ordinary gas pressure. Galactic magnetic fields also fight collapse: they penetrate the cloud and, latching onto any charged particles roaming within, restrict how the cloud can respond to its own gravity. What's scary is that if no one knew in advance that stars exist, frontline research could offer plenty of convincing reasons stars should never form.

Like the Milky Way's several hundred billion stars, gas clouds orbit the center of the galaxy. On the galactic scale, stars are minute specks, a few light-seconds across, in a vast ocean of space. In contrast, some gas clouds are huge, spanning hundreds of light-years. Such clouds can be as massive as several million suns. And as they lumber through the galaxy, they often collide with each other, entangling their innards. Sometimes, depending on their relative speeds and their angles of impact, the clouds stick together like hot marshmallows; other times, adding injury to insult, they rip each other apart.

If a gas cloud's temperature drops below about a thousand degrees Kelvin, conditions become favorable for forming complex, molecules and dust. Below a hundred, conditions become ideal. Those chemical transitions have consequences for everybody. Dust grains, which are made up of billions of atoms, absorb visible light--strongly attenuating the brightness of stars behind them. The dust then re-emits the energy as infrared radiation, which freely escapes the cloud.