In the Beginning
Dark Clouds as Stellar Nurseries
The Horsehead Nebula, a dark cloud in Orion. From the
A dust cloud and the star cluster NGC 6520. From the
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
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.