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Chapter 17
Formation of Stars
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Chapter 17
Star formation is an ongoing process in
the Universe. Star-forming regions are
seen in our galaxy as well as others
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Star formation happens when part of a dust cloud
begins to contract under its own gravitational force;
as it collapses, the center becomes hotter and
hotter until nuclear fusion begins in the core.
When looking at just a few atoms, the gravitational
force is nowhere near strong enough to overcome
the random thermal motion.
Even a massive cloud of gas and dust will remain
just a cloud until some shock wave or pressure
wave arrives to initiate its gravitational collapse
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The collapse
process from
nebula to star is
similar for all stars
and can be
followed by
observing the
temperature
produced by the
compression
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Star Formation
The collapse process from nebula to star is
similar for all stars and can be followed by
observing the temperature produced by the
compression
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Stage 1: An interstellar cloud starts to
contract, probably triggered by a shock
or pressure wave. As it contracts, the
cloud fragments into smaller, irregular
size pieces.
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Stage 2:
Individual cloud fragments begin to collapse.
Once the density is high enough, there is no
further fragmentation.
Stage 3:
The interior of the fragment has begun
heating, and is about 10,000 K.
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Stage 4:
The core of the cloud
is now a protostar,
and the surface
temperature is high
enough to make its
first appearance on
the H–R diagram
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By stage 5 Planetary
formation around the
star has likely
begun, but the
protostar itself is still
not in equilibrium –
all heating that
effects the system
comes from the
gravitational
collapse.
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The last stages
can be followed on
the H–R diagram.
The protostar’s
luminosity
decreases even as
its temperature
rises because it is
becoming more
compact.
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Review of Astronomy
UVU
Chapter 17
Star Formation
At stage 6, the core
reaches 10 million K,
and nuclear fusion
begins. The protostar
has become a star but
it not yet on the main
sequence. This stage is
often called the T Tauri
stage and it is a period
of adjustment.
T Tauri stars are mostly:
•between 105 and 108 years in
age;
•of low mass (0.5 to 3.0 M¤);
•surrounded by hot, dense
envelopes; and
•losing mass via stellar winds with
typical v = ~100 km/s.
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Review of Astronomy
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Star Formation
The star continues
to contract and
increase in temperature, until it is
in equilibrium. This
is stage 7: the star
has reached the
main sequence
and will remain
there as long as it
has hydrogen to
fuse.
This H–R diagram shows
the evolution of stars
somewhat more and
somewhat less massive
than the Sun.
The shape of the paths
is similar, but they wind
up in different places on
the main sequence.
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The main sequence is a band, rather
than a line, because stars of the same
mass can have different compositions.
Most important: Stars do not move
along the main sequence! Once they
reach it, they are in equilibrium, and do
not move until their fuel begins to run
out.
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End of Chapter 17
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Chapter 17
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Chapter 17
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