Subject: Происхождение звезд
Date   : 20 Jun 1998 09:17 GMT
From   : Vladimir Sazonov [vladimir] (vladimir@berlin.snafu.de)
To     : All

Вот популярный текст, найденный в Интернете

http://oposite.stsci.edu/pubinfo/pr/97/34/af2.html

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Giant Molecular Clouds: Breeding Grounds for Star Birth
Space between stars in a galaxy is nearly empty, except for a
scattering of hydrogen atoms. The atoms are so far apart that, if an
atom were an average- size person, each person would be separated by
about 465 million miles, which is the distance between our Sun and
Jupiter. These atoms are moving very fast because they are extremely
hot, baked by ultraviolet radiation from stars. This makes it
difficult for atoms to bond to form molecules. Those that do form
don't last for long. If radiation doesn't break these molecules apart,
a chance encounter with another atom will. 

Some parts of space, however, are not wide open frontiers containing a
few atoms. These cosmic spaces comprise dense clouds of dust and gas
left over from galaxy formation. Since these clouds are cooler than
most places, they are perfect breeding grounds for star birth. When
the density is 1,000 times greater than what is found in normal
interstellar space, many atoms combine into molecules, and the gas
cloud becomes a molecular cloud. Like clouds in our sky, these
molecular clouds are puffy and lumpy. Molecular clouds in our Milky
Way Galaxy have diameters ranging from less than 1 light-year to about
300 light-years and contain enough gas to form from about 10 to 10
million stars like our Sun. Molecular clouds that exceed the mass of
100,000 suns are called Giant Molecular Clouds. 

A typical full-grown spiral galaxy contains about 1,000 to 2,000 Giant
Molecular Clouds and many more smaller ones. Such clouds were first
discovered in our Milky Way Galaxy with radio telescopes about 25
years ago. Since the molecules in these clouds do not emit optical
light, but do release light at radio wavelengths, radio telescopes are
necessary to trace the molecular gas and study its physical
properties. Most of this gas is very cold (about -440 degrees
Fahrenheit) because it's shielded from ultraviolet light. Since gas is
more compact in a colder climate, it is easier for gravity to collapse
it to form new stars. 

Ironically, the same climate that is conducive to star formation also
may shut off the star birth process. The problem is heat. Young stars
are very hot and can heat the molecular gas to more than 1,000 degrees
Fahrenheit, which is an unfavorable climate for star birth. When the
temperature exceeds about 3,000 degrees Fahrenheit, the gas molecules
break down into atoms. 

The density of the gas can increase considerably near the centers of
some Giant Molecular Clouds: Gas as dense as 1 billion molecules per
cubic inch has been observed. (Though dense by astronomical standards,
such gas is still 100 billion times thinner than the air we breathe
here on Earth at sea level!) In such dense regions, still denser blobs
of gas can condense and create new stars. Although the star formation
process is not fully understood, there is observational evidence that
most stars are born in the densest parts of molecular clouds. 

What happens when stars begin forming in Giant Molecular Clouds
depends on the environment. Under normal conditions in the Milky Way
and in most other present-day spiral galaxies, star birth will stop
after a relatively small number of stars have been born. That's
because the stellar nursery is blown away by some of the newly formed
stars. The hottest of these heat the surrounding molecular gas, break
up its molecules, and drive the gas away. As the celestial smog of gas
and dust clears, the previously hidden young stars become visible, and
the molecular cloud and its star-birthing capability cease to exist.
Two years ago the Hubble Space Telescope revealed such an emerging
stellar nursery in the three gaseous pillars of the Eagle Nebula. 

Giant Molecular Clouds in colliding galaxies may experience a
different fate. As the collision crunches the interstellar gas and
stars form at an accelerating rate, the gas pressure around the
surviving Giant Molecular Clouds increases one-hundred- to
one-thousand-fold. Calculations suggest that the hot surrounding gas
can trigger rapid star birth throughout the clouds by driving shock
waves into them. The several hundred thousand stars that form from the
cold molecular gas in such clouds use up most of the gas before it has
time to be heated and dispersed. The result of such violent events is
the nearly complete conversion of Giant Molecular Clouds into rich
star clusters, each containing up to 1 million stars. Observations by
the Hubble telescope suggest that many of these newly born star
clusters remain bound by their own gravity and evolve into globular
clusters, like those observed in the halo of our Milky Way.

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