Clusters Nebulae Galaxies

The Great Rift is a dark swath in the Milky Way

Densely starry sky with detailed cloudy band of the Milky Way and 3 extra-bright stars well separated.
The 3 brightest stars in this image make up the asterism of the Summer Triangle, a giant triangle in the sky composed of the bright stars Vega (top left), Altair (lower middle) and Deneb (far left). Also in this image, under a dark sky and on a moonless night, is the Great Rift, that passes right through the Milky Way and the Summer Triangle. Image via NASA/ A. Fujii/ ESA.
  • The Great Rift is the name for a long swath of gaseous clouds, darkening a stretch of the starry band of the Milky Way in our sky.
  • The Milky Way is the edgewise view into our home galaxy. It has an estimated 100 to 400 billion stars. So why is this area dark? It’s a region of vast star-forming clouds.
  • You need a dark sky to see the Great Rift. But if you do see it, know that new stars are being born there, shrouded in their gas-and-dust cocoons.

The Great Rift: How to see it

August – at a time when the moon is gone from the evening sky – is an ideal to look for the Great Rift, or Dark Rift, in the starry band of the Milky Way. Under a dark sky, far from city lights, the Milky Way is easy to see at this time of year, stretching across the sky. The Great Rift appears as dark lanes of dust running the length of the starlit Milky Way band.

You can see the Milky Way most easily in the evening from around June or July through about October. From a Northern Hemisphere location, you’ll see the thickest part of the Milky Way above the southern horizon. From the Southern Hemisphere, the thickest part of the Milky Way appears more overhead.

Notice that the Milky Way band looks milky white: hence, its name. The skies aren’t really black like ink between stars in the Milky Way. You’ll know when you see the Great Rift. That’s because it looks as if someone took a marker and colored parts of the Milky Way darker.

Cloudy band of Milky Way across dark starry sky, with several stars labeled and the Summer Triangle outlined.
View larger. | The Great Rift of the Milky Way passes through the constellation Cassiopeia and the Summer Triangle. Image via Wikimedia (CC BY-SA 3.0).

Constellations along the Great Rift

The Great Rift begins just above the constellation Sagittarius the Archer. Follow the Milky Way up until you see a black area in the Milky Way, just before you get to the constellation Cygnus the Swan. Cygnus is shaped like a cross. Deneb is the brightest star in Cygnus and part of the famous Summer Triangle asterism. You can see the Great Rift inside the Summer Triangle.

Be sure to keep your binoculars handy for any Milky Way viewing session. There are many interesting star-forming regions, star clusters and millions of stars that will capture your attention.

An old abandoned house on the prairie with a large, cloudy band of stars in the night sky above.
View at EarthSky Community Photos. | William Mathe made the 100-mile (160 km) drive to Last Chance, Colorado, for this scene on March 16, 2024. William wrote: “The ranch house is a bit of a fixer-upper. But it has spectacular views of the core of our little Milky Way galaxy.” Thank you, William!

The Great Rift is dark due to dust

Stars are formed from great clouds of gas and dust in our Milky Way galaxy and other galaxies. When we look up at the starry band of the Milky Way and see the Great Rift, we are looking into our galaxy’s star-forming regions. Imagine the vast number of new stars that will emerge, in time, from these clouds of dust.

Oval with orange horizontal stripe, dark across middle, and blue regions above and below.
Here’s the interaction between interstellar dust in the Milky Way and the structure of our galaxy’s magnetic field, as detected by ESA’s Planck satellite over the entire sky. Image via ESA.

Ancient cultures focused on dark areas, not light areas

You know those paintings where if you look at the light areas you see one thing, but in the dark areas you see something else?

The Great Rift is a bit like that. A few ancient cultures in Central and South America saw the dark areas of the Milky Way as constellations. These dark constellations had a variety of myths associated with them. For example, one important dark constellation was Yacana the Llama. It rises above Cuzco, the ancient city of the Incas, every year in November.

By the way, the other famous area of the sky that is obscured by molecular dust is visible from the Southern Hemisphere. It’s the famous Coalsack Nebula near the Southern Cross, also known as the constellation Crux. The Coalsack is another region of star-forming activity in our night sky, much like the Great Rift.

Bottom line: The Great Rift or Dark Rift is a darkened swath of the Milky Way where new stars are forming. It’s best seen from a rural location away from light pollution.

Posted 
August 28, 2024
 in 
Clusters Nebulae Galaxies

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Bruce McClure

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