The Coal Sack Nebula – Invisible and Everywhere

This image shows part of the Coalsack Nebula, an opaque interstellar dust cloud that obscures the light of the background Milky Way stars; dust grains in the cloud redden the starlight that reaches us by absorbing blue light preferentially. Image credit: ESO.

Is this a giant hole in space?  I show a picture similar to this as I ask this question to students and audiences that I host in my planetarium.  Most people answer that it is a black hole, or dark matter, or dark energy, or something strange like that.  But the amazing thing is that it is actually a thick cloud of dust that is opaque, letting no visible light from the distant stars pass through.

The funny thing is that the cloud is transparent in infrared light, but in the visible spectrum it highlights something interesting about the universe: There are many things we cannot see with our eyes, and many things that we may not see when looking across the entire spectrum of light.

Spiral Galaxies are chock full of these ‘dark nebulae,’ which contain dense knots of cold gas and dust.  They are immensely important as they represent the majority of raw material that goes into forming new stars.  Older Elliptical galaxies can be full of stars but devoid of any cold gas, and cannot form any new stars.

Eventually the Milky Way will run out of gas to form new stars, and along with the rapid star formation that will happen when it collides with the Andromeda Galaxy, the result will be a giant elliptical galaxy full of aging stars and lacking any new star formation.  By that time the Sun will just be another star in a snow globe of old red stars, and the Earth will be long lost in its plasma.

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