In the Beginning

Dark Clouds as Stellar Nurseries

The Horsehead Nebula, a dark cloud in Orion. From the Anglo-Australian Observatory.

A dust cloud and the star cluster NGC 6520. From the Anglo-Australian Observatory.

The Milky Way Galaxy is filled with clouds of gas and dust. A small telescope resolves these clouds as either black regions on the sky where background stars are blocked out or as glowing nebula in the case of those clouds that are illuminated by very bright stars. Interstellar dust clouds have two components: gas (which is mostly hydrogen), and dust, composed of tiny grains the size of smoke particles. The gas fills most of the volume of the cloud but the dust is what makes them opaque.

These clouds are the birth place of stars and the material in them is the raw material from which stars are made. Since the clouds are mostly hydrogen, with some helium and trace amounts of other elements, hydrogen is what newly formed stars are principally composed of. Clouds also contains small amounts of the element lithium which is destroyed very quickly in stars which have begun nuclear fusion. This makes lithium a good indicator of stellar youth. If it is still present the star is not yet old enough to have significant amounts of fusion taking place.


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The Collapse: Protostellar Formation

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