We Peer Through Dust Into The Milky Way’s Heart

Clouds are silhoutted across a starry western sky last night. The orange glow at bottom is distant light pollution. The sky is slightly illuminated every night by a combination of starlight and airglow, a permanent, weak glow similar to aurora emitted by air molecules. Photo: Bob King

What a night. You plan a nice evening out under a dark sky, and minutes after you point the telescope at that first galaxy, an unexpected flotilla of clouds sails in from the west. This happened last night. Because I was in a very dark location, the clouds looked like big black gaps or holes in the sky. From nearer the city, clouds are ordinarily pale white or orange against a dark sky, illuminated as they are by electric lighting.

American astronomer E.E. Barnard

American astronomer Edward Emerson “E.E.” Barnard was said to have been inspired by a similar sight of clouds silhouetted against the night sky, when he was trying to determine the nature of dark patches in the Milky Way. Were empty regions as many supposed or clouds of starlight-sucking interstellar dust?

Through his pioneering work in astrophotography of the Milky Way, E.E. discovered that the ’empty regions’ were indeed clouds of gas and dust that obscured the background stars. We’ve known them ever since as dark nebulae as opposed to bright, starlit masses like the Orion Nebula. Last night’s clouds resembled a whole series of starless billows.

Our galaxy is filled with these dust clouds, one of which is shaped like a horse’s head, another like an elephant trunk. There’s even a pipe nebula.

Unlike Earth’s clouds, dark nebulae are typically light-years wide and deep. While their dust and gas is thinly spread, the depth of the clouds is such that they easily block starlight coming from behind them.

Barnard 68 from "E.E's" catalog of dark nebulae is silhouetted against a rich field of background stars. Credit: ESO

Visual starlight that is.  Telescopes built to observe in infrared light, the portion of the spectrum beyond the red end of the rainbow, can see through the scree. When they’re pointed at dark nebulae, astronomers often spot newborn stars forming in their dark dusty cores through the attraction power of gravity.

There’s poetry here. Dark clouds as birthplaces for the stars that light our nights. There’s mystery, too. Peering toward the center of the Milky Way, our line of vision intersects cloud after cloud of light-choking interstellar dust, blocking the view of what’s going in the core some 25-30,000 light years away. Until we learned how to built instruments to ‘see’ the sky in infrared and other ‘flavors’ of light like radio waves and X-rays.

The region around the center of our Milky Way galaxy glows colorfully in this new version of an image taken in the infrared by NASA's Spitzer Space Telescope. The picture covers 2,400 light years across and 1,360 light years top to bottom. Credit: NASA/JPL-Caltech

The photo above, recently released by NASA and taken by the infrared-sensitive Spitzer Space Telescope, penetrates the dust and sheds new light on happenings at ‘ground zero’ in our Milky Way galaxy. According to NASA astronomers, millions of stars crowding near the center create the blue haze, while the green features are from carbon-rich dust molecules, called polycyclic aromatic hydrocarbons, which are illuminated by the surrounding starlight as they swirl around the galaxy’s core. The yellow-red patches are the thermal glow from warm dust. The polycyclic aromatic hydrocarbons and dust are associated with bustling hubs of young stars.

Numerous dark clouds - dark nebulae - appear to split the Milky Way in two as seen in this time exposure taken last summer. Galactic center, at lower right, is hidden by dust and gas. Photo: Bob King

The bright white blotch at the very center is a rich cluster of stars in the center of the galaxy some 26,000 light years away. That’s so distant that the thousands of individual stars blur together into a single bright patch. Studying the motion of the cluster, astronomers have determined that a massive black hole resides at the heart of the Milky Way.

The sun and planets also originated in a gas and dust cloud now gone from galactic memory. Dust is in our veins. You can almost feel it while standing under a dark sky dotted with clouds.