Astronomy Essentials

Dark nebulae: Obscuring clouds of gas and dust

Dark nebulae: Dense starfield with large, blobby dark areas near bright blue and bright red stars.
Meet the Coalsack, a huge cloud of gas and dust in space. The dust in this and other dark nebulae absorb and scatter the light of background stars. Therefore, they create a region of the sky that looks starless, but it’s really a place where new stars are forming. April and May are some of the best months to see the Coalsack in the constellation Crux in the Southern Hemisphere. Image via ESO.

Plunge into the darker side of the night sky, and get to know dark nebulae. Dark nebulae – or absorption nebulae – are clouds of gas and dust in space that are dense enough to obscure and block light from background stars. These vast clouds are mostly composed of molecular hydrogen. Often, they are locations where material is coalescing to form new stars. American astronomer Edward Emerson Barnard was the first to create a large catalog of 182 dark nebulae in the year 1919. Since then, hundreds more have been found and charted in our Milky Way galaxy.

Below are some of the best-known dark nebulae: favorites of observers and astrophotographers.

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The Great Rift

The Great Rift is frequently seen by stargazers with the eye alone from a dark-sky site. You know how in northern summer you can see the starry band of the Milky Way – the edgewise view into our own galaxy – stretched across the night sky? If you look closely, that starry band appears to be split into two longways by darkened lanes of obscuring dust. This is the Great Rift.

Glowing band arching across dark sky with long dark swaths along it, and lots of thin, bright meteor streaks.
View at EarthSky Community Photos. | Osama Fathi in Egypt took this image on August 6, 2022. Osama wrote: “Panorama of the Milky Way arch over the sand dunes of the Egyptian western desert near El Fayoum at the beginning of Perseid meteor shower. Taken over 3 nights (August 5-6-7, 2022). We captured more than 1,200 photos to get this number of meteors near the desert of El Fayoum.” Thank you, Osama! The Great Rift or Dark Rift is a dark area in the starlit band of the Milky Way. It’s really clouds of dust where new stars are forming.

The Coalsack

The Coalsack is generally considered the easiest dark nebula to see, but you have to be in the Southern Hemisphere to see it. To those on the southern half of Earth’s globe, the Coalsack is visible to the unaided eye as a dark, cloudy patch in the Milky Way. It’s located in the famous constellation Crux the Southern Cross. Unfortunately for the majority of Earth’s population, the Coalsack Nebula is visible only in Southern Hemisphere skies. Watch the video below and note the many dark clouds in the Milky Way as you zoom in to see the Coalsack.

The Horsehead Nebula

The Horsehead Nebula, or Barnard 33, is striking for its horsehead shape. It lies in the direction of the constellation Orion the Hunter. Find the Horsehead Nebula by looking for Orion’s Belt, the constellation’s prominent line of three stars. The easternmost belt star (the star farthest left on the belt when looking at Orion in the Northern Hemisphere) is the star Alnitak. Just south of Alnitak is the Horsehead Nebula. While famous, the Horsehead isn’t easy for amateurs to spot. You’ll need excellent seeing conditions, a large telescope and the help of a filter. It’s a great target in the winter skies.

Large, swirly red cloud of gas with a horsehead-shaped indentation and numerous bright foreground stars.
View at EarthSky Community Photos. | Martin Curran in Cheyenne, Wyoming, captured the Horsehead Nebula in the constellation Orion on October 21, 2023. Martin wrote: “First time hitting this target as a primary … The iconic Horsehead is a cloud of dust blocking light from a much larger HII region. Will definitely be revisiting this target.” Thank you, Martin!

The Snake Nebula

The Snake Nebula, Barnard 72, is an S-shaped dark nebula, appropriately enough in the constellation Ophiuchus the Serpent Bearer. The dark blobs below the Snake Nebula are a type of dark nebulae with the name of Bok globules.

Extremely dense starfield with dark S-shaped path in the middle. Some black blobs below.
The dark squiggle cutting through this starfield is the Snake Nebula, or Barnard 72. The best time to spot the Snake Nebula is around June and July. Image via Wikimedia Commons (CC BY 3.0).

Bok globules

Smaller, isolated dark nebulae get their own designation: Bok globules. Bok globules are cool clouds of gas and dust that look like drops of black paint. Usually, multiple star systems are forming inside. Dutch-American astronomer Bart Bok gets credit for the discovery of Bok globules. But he married fellow astronomer Priscilla Fairfield in 1929, and they collaborated from then on. The Royal Astronomical society said:

It is difficult and pointless to separate his achievements from hers.

An example of a Bok globule is Barnard 68. This dark absorption nebula of molecular gas is within the Milky Way galaxy. It lies in the direction of the constellation Ophiuchus at a distance of 500 light-years.

Stark black blob fanning up and to the right in a dense, colorful starfield.
Barnard 68 is a Bok globule close to the Snake Nebula in the constellation Ophiuchus. It’s also at its best in June and July. Can you spot Barnard 68 in the image of the Snake Nebula above? Image via ESO/ Wikimedia Commons (CC BY 4.0).

Dark nebulae in other galaxies

You can see dark nebulae in distant galaxies where a dark dust lane obscures part of a spiral arm or the bulge, such as in the Black Eye Galaxy. The Black Eye Galaxy – M64 – also has the nickname of the Evil Eye Galaxy. This spiral with huge swaths of absorbing dust lies in the direction of the constellation Coma Berenices. This galaxy lies 17 million light-years away.

Oblique view of giant spiral galaxy showing dark brown obscuring dust along the front half.
The Black Eye Galaxy displays great swaths of dark, obscuring clouds in its spiral form. The Black Eye Galaxy lies toward the constellation Coma Berenices, a good target in May skies. Image via the Hubble Space Telescope/ Wikimedia Commons (CC BY 2.0).

Bottom line: A dark nebula is a cloud of interstellar gas and dust that blocks light from background stars and other luminous objects. The Horsehead Nebula is a famous dark nebula.

November 5, 2023
Astronomy Essentials

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