Wednesday, December 30, 2015

Nebulae: What are they and where do they come from?

By Matt Williams via Universe Today, 24 December 2015

A nebula is a truly wondrous thing to behold. Named after the Latin word for “cloud”, nebulae are not only massive clouds of dust, hydrogen and helium gas, and plasma; they are also often “stellar nurseries” – i.e. the place where stars are born. And for centuries, distant galaxies were often mistaken for these massive clouds.
Alas, such descriptions barely scratch the surface of what nebulae are and what there significance is. Between their formation process, their role in stellar and planetary formation, and their diversity, nebulae have provided humanity with endless intrigue and discovery.
For some time now, scientists and astronomers have been aware that outer space is not really a total vacuum. In fact, it is made up of gas and dust particles known collectively as the Interstellar Medium (ISM). Approximately 99% of the ISM is composed of gas, while about 75% of its mass takes the form of hydrogen and the remaining 25% as helium.
The interstellar gas consists partly of neutral atoms and molecules, as well as charged particles (aka. plasma), such as ions and electrons. This gas is extremely dilute, with an average density of about 1 atom per cubic centimeter. In contrast, Earth’s atmosphere has a density of approximately 30 quintillion molecules per cubic centimeter (3.0 x 1019 per cm³) at sea level.
Even though the interstellar gas is very dispersed, the amount of matter adds up over the vast distances between the stars. And eventually, and with enough gravitational attraction between clouds, this matter can coalesce and collapse to forms stars and planetary systems.

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