Tuesday, July 20, 2021

A cosmic bag full of coal

Part of the Coal Bag Nebula seen up close

Dark spots almost completely block a rich star field.  The dark areas are small parts of a huge dark nebula called the Sac of Charcoal, one of the most prominent objects of its type, visible to the naked eye.  Millions of years from now, pieces of this Sack of Charcoal will ignite, like the fossil fuel of its name, with the glow of many young stars.

 The Coal Bag Nebula lies about 600 light-years away in the Southern Cross constellation. This huge dusty object forms a conspicuous silhouette over the bright starry band of the Milky Way and is why this nebula is known of the peoples of the southern hemisphere since humanity walks on Earth.

 The Spanish explorer Vicente Yáñez Pinzón was the first to point out to Europeans the presence of the Saco de Coal Nebula in 1499. The Saco de Coal was then nicknamed the Black Magellanic Cloud, due to its dark appearance when compared to the intense brightness of the two Magellanic Clouds, which are actually satellite galaxies of the Milky Way.  These two bright galaxies are clearly visible in the southern sky, having caught the attention of Europeans during Ferdinand Magellan's explorations in the 16th century.  However, Saco de Coal is not a galaxy.  Like other dark nebulae, it is an interstellar cloud of dust so thick that it does not allow most of the radiation emitted by background stars to reach observers.

 A significant number of dust particles in the dark nebulae are covered with water ice, nitrogen, carbon monoxide and other simple organic molecules.  These grains prevent visible radiation from passing through the cosmic cloud.  To get an idea of ​​how dark the Coal Bag is, in the 1970s Finnish astronomer Kalevi Mattila published a study that estimated that the Coal Bag Nebula had only about 10% of the brightness of the Milky Way around it.  A small part of the stellar background radiation does however manage to pass through the nebula, as shown in this new ESO image and other observations taken by modern telescopes.

 This small amount of radiation that passes through the nebula does not leave the other side unmodified.  The radiation we see in this image looks redder than it normally would.  This effect is due to the fact that dust in dark nebulae absorbs and scatters more blue radiation from stars than red radiation, “painting” stars various shades redder than they would otherwise be.

 Millions of years from now, the dark days of Saco de Carvão will come to an end.  Thick interstellar clouds like the Bag of Coal contain a lot of gas and dust — the fuel of new stars.  As the material dispersed in the nebula coalesces under the effect of gravity, stars form and begin to glow, causing the “chunks” of coal to “ignite,” almost as if they had been touched by a flame.

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