There are many mysterious dark objects and phenomena observed in outer space. The most well-known is dark matter, an invisible substance that makes up about 27% of the universe. Dark matter cannot be directly observed, but we know it exists because of its gravitational effects on visible matter like stars and galaxies. Another mysterious dark phenomenon is dark energy, which makes up about 68% of the universe and drives the accelerating expansion of the cosmos.
In addition to dark matter and dark energy, there are other dark things lurking in the depths of space, from black holes to dark nebulae. Some absorb all light while others simply reflect very little back to our telescopes. Let’s explore some of the most intriguing dark objects and phenomena observed by astronomers across the electromagnetic spectrum.
Black Holes
One of the most mystifying dark objects in space is the black hole. Black holes form when massive stars run out of fuel and collapse under their own gravity. The gravity becomes so strong that not even light can escape, rendering the black hole invisible. However, we can detect black holes through their interaction with nearby stars and gas clouds.
For example, rapidly orbiting stars near the center of our Milky Way galaxy indicate the presence of a supermassive black hole about 4 million times the mass of our Sun. Material pulled from a companion star into an ordinary black hole can heat up and emit powerful X-rays detectable from Earth.
Supermassive black holes with masses millions to billions of times that of our Sun lie at the center of most large galaxies. They gobble up surrounding material, emitting intense radiation. Our galaxy’s central black hole suggests that most large galaxies host these giant black holes at their cores.
Event Horizon
The boundary of a black hole, beyond which no light can escape, is called the event horizon. The immense gravity creates a steep curve in spacetime, causing light rays to bend down and back into the black hole. Anything that passes the event horizon is lost forever to the outside observer.
While the interior of a black hole remains a mystery, physicists theorize that it contains a singularity, a point of infinite density and space-time curvature. At the singularity, Einstein’s general theory of relativity that describes space-time and gravity breaks down. A complete theory of quantum gravity is needed to understand what happens inside a black hole.
Accretion Disk
As black holes draw in surrounding gas and dust, the material spins around the black hole, forming a flattened rotating disk called an accretion disk. Friction heats the disk to millions of degrees, causing it to glow brightly at X-ray wavelengths, especially near the black hole’s event horizon.
By studying the structure and luminosity of accretion disks around black holes, astronomers can learn about the black hole’s mass and rate of consumption. Flares of radiation from accretion disks can reveal when a black hole suddenly consumes more material. Analysis of accretion disk properties is one of the primary ways that black holes are detected across the cosmos.
Dark Nebulae
Dark nebulae are extremely dense, cold clouds of gas and dust that block visible light from passing through them. Also known as absorption nebulae, they appear as dark, snake-like silhouettes against brighter backgrounds. These starless clouds are the nurseries where future stars will form.
Horsehead Nebula
One of the most iconic dark nebulae is the Horsehead Nebula in the constellation Orion. Named for its resemblance to a horse’s head and neck when viewed from Earth, the Horsehead nebula is about 1500 light years from us. It is a small region within a larger nebula called IC 434.
The nebula is cold and dense, with a temperature of about 10 K (-440°F). It contains mostly hydrogen gas and dust that absorb the visible light from the emission nebula IC 434 behind it. The dust grains that give the Horsehead its dark appearance may eventually clump together to form new stars.
Coalsack Nebula
Another prominent dark nebula is the Coalsack Nebula, visible in the southern sky near the Southern Cross constellation. Catalogued as Crux Dark Nebula (DC 268-5), it measures about 7 light years by 5 light years. Like the Horsehead, the Coalsack consists of thick interstellar clouds of gas and dust that block light from passing through.
Deep inside the Coalsack lies the open cluster NGC 4755, whose young stars are illuminating the outskirts of the nebula. Supernovae explosions from cluster stars in the past may have compressed gas into the dense dark knot that forms the Coalsack Nebula today.
Dark Globules
Against the glow of bright emission nebulae, small, opaque concentrations of gas and dust can be seen as dark spots or irregular blobs. Astronomers call these dark globules or Bok globules, after astronomer Bart Bok who first described them in the 1940s.
Bok globules typically have masses equivalent to 50-500 solar masses contained within a region about a light year or less in diameter. They are some of the coldest and densest pockets of gas found in our galaxy, with temperatures around 10 K.
Thackeray’s Globules
Thackeray’s Globules are a famous set of dark globules located in the IC 2944 emission nebula in the southern constellation Centaurus. These opaque, dark knots of gas and dust span from 0.5 to 1 light year in length.
First observed in the 1950s by astronomer A.D. Thackeray, these globules are incubators for new stars. Ultraviolet radiation from young stars surrounding the globules is slowly eroding them, pushing their material into elongated tails. Gravity then causes the densest parts of the globules to collapse into new stars.
Barnard 68
One of the most opaque dark globules known is Barnard 68 (B68). Discovered in 1919 by astronomer E.E. Barnard, B68 lies about 430 light years away in the constellation Ophiuchus. It has about twice the mass of our Sun despite spanning only about 0.4 light years.
B68 is so dense that it casts a distinct shadow against the glowing interstellar material behind it. Its core temperature is around 16 K. Astronomers hypothesize B68 may be on the verge of gravitational collapse, making it a prime target to study star formation.
Dark Molecular Clouds
Related to dark nebulae and globules are dark molecular clouds. While dark nebulae are primarily absorbing visible light, molecular clouds contain such high densities of molecules like hydrogen and carbon monoxide that they emit strong signals detectable with radio telescopes.
Taurus Molecular Cloud
The Taurus Molecular Cloud, named for the Taurus constellation where it lies, is one of the nearest large star-forming molecular clouds to our Solar System at about 450 light years away. It has a mass of over 400,000 solar masses and spans roughly 100 light years.
Within the Taurus Molecular Cloud are the dense, opaque dark nebulae known as the Taurus Dark Clouds. These are the sites of ongoing star formation. Astronomers have catalogued over 300 stellar embryos evolving inside these dark incubators.
Orion Molecular Cloud
The Orion Molecular Cloud Complex spans about 250 light years and lies 1,500 light years distant within the Orion constellation. The Orion nebula and Horsehead nebula photographed against the bright background emission are part of this vast region strewn with cold molecular gas.
Here too stars are born inside contracting cores of molecular hydrogen. Newborn stars then sculpt the surrounding gas into stellar nurseries and picturesque nebulae. Over two thousand potential developing star systems have been identified inside the Orion molecular cloud.
Dark Galaxies
In some instances, what appears to be a dark spot or cloud within a bright galaxy turns out to actually be a “dark galaxy” – an area where stars failed to ignite for unknown reasons during galaxy formation in the early universe. Unlike bright galaxies brimming with glowing stars and nebulae, these mysterious dark galaxies emit little if any detectable radiation.
Virgo Dark Galaxies
In the Virgo galaxy cluster about 50 million light years from Earth, astronomers have detected around 1000 dark galaxy candidates orbiting the bright galaxies we can see. The dark galaxies contain few if any stars – they are up to 10 billion times less luminous than the Milky Way galaxy.
Some may be “failed” galaxies where issues like gas turbulence or supernova shockwaves prevented material from condensing into stars. Others may simply consist of matter too spread out for stars to form. Analyzing them may yield clues to the role dark matter played in early galaxy evolution.
Coma Dark Galaxy
In the Coma galaxy cluster over 300 million light years away, one mysterious dark galaxy known as Dragonfly 44 was discovered via the motion of its surrounding globular clusters. The dark galaxy is ultra-diffuse with mass comparable to the Milky Way but with nearly no discernable stars inside.
If dark galaxies like it exist in Coma, they may likely lurk in other galaxy clusters too. Finding more dark galaxies could reveal what physics and conditions enable galaxies to light up with stars or remain perpetually dark.
| Object Name | Description |
|---|---|
| Dark Matter | Invisible matter comprising 27% of the universe. Detected through gravitational effects. |
| Dark Energy | Mysterious energy making up 68% of the universe and driving expansion. |
| Black Holes | Formed from collapsed stars. So dense even light cannot escape. |
| Dark Nebulae | Cold, dense clouds of gas and dust that block light. |
| Dark Globules | Small pockets of opaque gas that block light from bright nebulae behind. |
| Molecular Clouds | Cold clouds of molecules like hydrogen detectable in radio wavelengths. |
| Dark Galaxies | Galaxies containing few or no stars, only detected through gravity. |
Conclusion
Aside from the dark matter and dark energy that appear to comprise most of the universe, we find a diverse array of intriguing dark objects lurking across the cosmos. Black holes warp space-time to such extremes that even photons cannot escape. Dark nebulae, globules, and molecular clouds collapsing under gravity spawn new generations of stars. And ghostly dark galaxies meander through space with their stellar nurseries somehow snuffed out.
While these dark entities may not emit light of their own, their gravitational influence and silhouettes against bright backgrounds give them away. Analyzing the different shades of darkness across the universe provides insights into star birth, galaxy formation, the behavior of matter under extreme gravity, and the fundamental nature of the cosmos. Astronomers continue to uncover these shadowy, elusive objects, illuminating the dark side of the universe.