The Life and Times of a Dark Nebula Named 'Coalsack'

In Fred Hoyle's sci-fi classic The Black Cloud, alien contact arrives as a smudge in the night sky—stars that had been visible only weeks before suddenly vanish, consumed by a black emptiness. As befuddled astronomers watch and record, the emptiness grows and grows until it becomes clear that it's not a void at all, but a dark thing traveling through space toward the Earth.

While they may not be sentient cloud beings, dark astronomical objects do populate the universe—and not just in the form of dark matter. Dark matter is dark because it is indifferent to light, while other astronomical entities known as dark nebulae are more akin to Hoyle's cloud. They block light, soaking it up like cosmic sponges, leaving us to observe them as inky voids.

One of these voids is known as the Coalsack Nebula, a new image of which is being released by the European Southern Observatory (ESO) this week (above). The snapshot comes courtesy of the MPG/ESO.2.2-meter telescope at Chile's La Silla Observatory.

As you can see, it's not completely dark after all, allowing some small amount of light to pass through, albeit in a redder form. This is because the nebula, consisting of dust particles coated with frozen water and other organic molecules, acts to filter out blue light, leaving the filtered wavelengths that do make it through several shades more red.

The existence of the Coalsack Nebula was first reported in 1499 by the Spanish explorer Vicente Yáñez Pinzón. In the southern hemisphere, the nebula is rather obvious, a spray of black paint across the bright starry band of the Milky Way. It later acquired the nickname the Black Magellanic Cloud for its contrast with the Milky Way satellite galaxies known as the Magellanic Clouds (though Coalsack is just some dust and not a galaxy). A 1970 study by the Finnish astronomer Kalevi Mattila found that the nebula emits about 90 percent less light than the surrounding Milky Way.

Coalsack's days are numbered. Eventually its constituent dust particles will begin clumping together, coalescing under the attractive force of gravity. These clumps may eventually become protostars and, as their mass builds, more and more material will be attracted faster and faster. The gravitational potential energy of the infalling material is released as kinetic energy, or heat.

This is how a dark nebula lights up. One by one, new stars blink on, like porch lights at twilight. Coalsack is but the promise of profound luminescence, which is not what Hoyle imagined at all.