Right now, those who love to see the wonders of the sky have it better than ever. Every day brings at least a few incredible pictures from dozens of observatories operating on Earth or in space, each offering a new perspective on alien worlds, exploding stars, colliding galaxies, or a variety of other astrophysical phenomena. Most of these images are shouts to cosmic forces and unimaginable scales, creating stunning beauty from epic violence.
But not everything in our galaxy (or beyond) is the result of such a terrible chaos. Some of the most visually appealing celestial objects are silent, stable, even calm, and so dark that they emit no visible light at all. absorb which, producing a blackness so deep, appears to be a notch cut in space.
These dark expanses have many names—dark nebulae, dust clouds, knots—but I prefer to call them Bok globulesThey were named after the Dutch-American astronomer Bart Bo who studied them.
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A Bok globule is a small, compact cluster of cosmic dust; billions are scattered across our galaxy. They are cold and opaque to visible light, so much so that until recently the only way to see them was in silhouette against brighter background material. Although they aren’t as splashy as their star factory cousins, for example Orion NebulaBok globes can still make stars, albeit in a more artful way: they make one or more that are largely hidden from our hidden gaze in the dusty abyss.
Of all the dark globules that we can see with our telescopes, this is without a doubt my favorite Barnard 68colloquially called B68. Located 500 light-years from Earth, it is a coma-shaped, jet-black cloud half a light-year across, about five trillion kilometers across. We see it easily because it is in the constellation Ophiuchus, with the center of our Milky Way galaxy as a backdrop. B68 appears as a negative space, an absence of stars
Why is it so dark? Although it is made mostly of hydrogen gas (like everything else in our galaxy), B68 is rich in carbon. Some of these elements are locked into small molecules, such as carbon monoxide, but the rest are locked into long, complex molecules that astronomers call general. dust. A distinctive (or turn off) characteristic of dust is its ability to block visible light.
And dust clouds can be dark indeed. In the case of B68, any star on the other side of us it will reduce its light by a factor 15 billion. To put this into perspective, darkening the sun in our sky that much would reduce it a star of the fourth magnitude it’s hard to spot even in light-polluted skies. If you were on one side of B68 and the sun on the other, the sun’s light would be so dim during that half light year that it would become invisible to the naked eye.
This extreme obscurity subjects B68—and Bok globules in general—to a constant state of mistaken identity. A few years ago astronomers discovered the existence of large volumes of space without galaxies; these are called cosmic voids and may be many millions of light-years across. Alas, I’ve seen quite a few breathless videos and articles illustrated with a picture of the B68. It’s irritating for an astronomer to see this error because they’re very different objects, but it’s also funny because the gaps they’re discussing are millions of times larger than the friendly Bok globules around us.
B68’s incredible ability to absorb light is a modest amount of dust. Even in its center, where it is densest, B68 has less than a million particles per cubic centimeter of matter. That may seem like a lot, but here on Earth it would be a laboratory-grade vacuum: at sea level our planet’s atmosphere contains about 10.19 molecules per cubic centimeter, making the air you breathe a little bit 10 billion times More compact than the B68 at its best.
Despite its complete obscurity, we can discern B68’s density because, like any cloud, it becomes fainter towards its outer edges. This creates an interesting situation: from our point of view, we can see some background stars through the relatively thinner material at their edges, but the closer you look to the center, the more light is absorbed. The stars appear bright at the cloud’s perimeter, but gradually dim as they approach the center. Because dust tends to absorb bluer light better than red rayswhich can pass more easily, such stars do not disappear; them too blush. And infrared light still passes through B68 more easily, so telescopes tuned to these wavelengths can see even more stars. Astronomers can use this reddening and darkening to measure how much dust is inside the cloud.
Using other techniques, they can also measure the temperature of B68. Bok globules are incredibly cold, and B68 is no exception, registering a bone-chilling -256 degrees Celsius at its edges, which drops to -265 degrees C at its center. This is barely above absolute zero!
However, this whisper of warmth is enough to hold the globule against its own gravity. B68 isn’t terribly massive, only three or four times the mass of the sun, but that’s usually enough to cause a gravitational collapse. The poor amount of internal heat keeps the B68 inflated like a hot air balloon, though (or, more accurately, a cold, near-vacuum balloon).
But this fragile impasse cannot last forever. Closer observations of B68 reveal what appear to be two distinct “cores” of denser material.one near its center and one in the stubby “tail” near its southeastern edge (lower left in photo above this article). Radio wave observations suggest that this tail was a separate, smaller cloud merging with B68, upsetting the delicate balance of gravity within the cloud. As a result, B68 may be collapsing now, which means that this dark cloud may literally have a bright future ahead of it: it will form a star.
As the material collapses in on itself, the central density would increase and with it the temperature. This would last for hundreds of thousands of years until a star is born (perhaps more than one, given that there is enough material in B68 to form a pair of sun-like stars) in the core of the cloud. If this happens, almost all of the remaining matter in the cloud will be blown away by the light of the nascent star or stars, all but perhaps a tiny fraction caught in the star’s gravitational fingers, which may condense and collapse. tour to create a disc of material destined to complete the planets.
And who knows? In a billion more years, maybe life and eventually intelligence could develop on some of these worlds, so one day in the distant future alien astronomers will wonder about the universe they see, a view they probably couldn’t see. B68’s young nebula, devouring starlight. Perhaps Earth and the sun will be long gone by then, and the galaxy will have become a very different place. But still, there is comfort in such an ending, knowing that we once began in the same way; our sun was born in a great nebula obscured by dust, which eventually lit up with thousands of other stars, like its own cosmic children, in a stellar nursery. it has long been dispersed.
Everything in the universe is transitory, and much of it is cyclical. We are privileged to be able to observe what we can now, even if what we see is something very difficult to see.
