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STELLAR NURSERIES
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Molecular Clouds
• Large, dense molecular clouds are very special environments in space.
Composed mainly of molecular hydrogen and helium, with small amounts
of heavier gases, they are the birth place of new stars and planets.
Molecular clouds that exceed the mass of 100,000 suns are called giant
molecular clouds. Giant molecular clouds are the largest inhabitants of
galaxies, reaching up to 300 light years in diameter. They contain enough
dense gas and dust to form hundreds of thousands of Sun-like stars. These
stars are formed in the densest parts of the clouds. Molecular clouds are
very cold, having temperatures ranging from about -440 to -370 degrees
Fahrenheit (-263 to -223 degrees Celcius or 10 to 50 degrees Kelvin). They
usually do not radiate their own visible light and appear dark when viewed
with an optical telescope. In these cold, dense environments, many atoms
can combine into molecules. Giant molecular clouds can last for 10 to 100
million years before they dissipate, due to the heat and stellar winds from
newly formed stars within them. An average spiral galaxy, like our own
Milky Way, contains about 1,000 to 2,000 Giant Molecular Clouds in
addition to numerous smaller clouds.
The Hidden Universe
What is a Stellar Nursery
• A “stellar nursery” is romantic way of referring to
a molecular cloud in the process of forming new
stars. A molecular cloud is a region of space
dense enough with hydrogen atoms that
molecules, most commonly H2, or diatomic
hydrogen, can form. Molecular clouds may be
giant, with 1000 to 100,000 times the mass of the
Sun, or smaller, less than a few hundred times the
mass of the Sun. These are called giant molecular
clouds and small molecular clouds respectively.
• As far as we know, star formation occurs exclusively
within these molecular clouds, hence the moniker
“stellar nursery.” For a molecular cloud to be a stellar
nursery, several conditions must be meant. First, the
molecular cloud must have enough pockets of
sufficient density (“molecular cores”) to provide the
raw material to produce stars. Second, the molecular
cloud must be subject to agitating forces, such as
nearby large stars or supernovae. When a portion of a
molecular cloud is lit and ionized by the radiation of a
nearby massive star, it is called an HII region.
• Because HII regions are the portions of molecular
clouds being most vigorously agitated by outside
sources, they are the most likely place to be a
stellar nursery. Outside influences are necessary
to create a star, because otherwise, a critical
density is rarely achieved in a molecular cloud. If
density is not sufficient, then gas particles in the
cloud just keep orbiting each other forever. Due
to an outside influence, such as a supernova
shock wave, molecular clouds can condense in
localized regions, becoming what is called Bok
globules.
• Bok globules are very dense cores found in stellar
nurseries. Typically, they contain about 10-50 solar
masses worth of material in an area about a light year
across. Bok globules are notable in astronomy because
they contain a variety of molecules not usually found in
typical sparse interstellar space: molecular hydrogen,
carbon oxides, helium, and silicate dust. Sooner or
later, it is thought that many Bok globules collapse to
form stars, or, more frequently, binary star systems or
star clusters. Our Sun is actually thought to be an
anomaly in that it has no binary pair.
• Stellar nurseries are eventually destroyed by
the stars which create them. The new stars
either suck up much of the local material, or
blow it away via solar wind. Eventually, these
newborn stars might explode in a supernova,
triggering the formation of other stars in
nearby stellar nurseries.
Orion: Closest Stellar Nebula
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