Star Formation

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Star Formation

• Processes in Stellar Formation
• Sequence of Events
• Role of Mass in Stellar Formation
• Observational Evidence
• New Theories

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Stellar Formation
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Stages of Stellar Evolution
There are 7 distinct stages of stellar
development from interstellar cloud to main
sequence star These stages are characterized by
differing core and surface temperatures and radii
of the prestellar object Gravitational attraction
drives the evolutionary tract, leading ultimately
to nuclear fusion, signaling the birth of a star
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Stage 1Interstellar Cloud
Dense, dark, and cold interstellar
cloud Large10-100 parsecs across (1014 1015
km) 1000X mass of our Sun Mainly atomic and
molecular gas Gravitational instability in
cloud-caused by some external event-triggers
cloud collapse
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Interstellar Cloud Collapse-Stage 2
Stars form inside relatively dense concentrations
of interstellar gas known as molecular clouds.
These regions are extremely cold, causing the
gas to clump to high densities. Star formation
begins when the denser parts of the cloud core
collapse under gravity. These cores typically
have masses around 104 solar masses. As the cores
collapse they fragment into clumps around 0.1
parsecs in size and 10 to 50 solar masses in
mass. These clumps then form into protostars and
the whole process takes about 10 million years.
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Stage 3 to 5
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Protostar H-R Diagram-Stage 4
Evolutionary tract followed by contracting
interstellar cloud fragment High luminosity
results from large size of gas
cloud Evolutionary track known as the
Kelvin-Helmholtz contraction phase Internal heat
gradually diffuses out and is radiated away
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Evolutionary Time Scale
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Route to Main Sequence
The track from stage 4 to stage 6 is known as the
Hayashi track Stars on this track are called T
Tauri stars Luminosity drops dramatically as
contraction occurscore temperature rises to 5
million K Heat and gravity compete between stages
6 and 7 until core reaches about 10 million K
nuclear fusion begins.
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Stars of Different Masses
Features of the Hayashi Track similar for each
mass star However, the time required to arrive on
the main sequence differs considerably,
decreasing rapidly as the mass increases Stars do
not evolve along the main sequence they arrive
at some point on it depending on their mass and
composition
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Relative Sizes of Different Mass Stars
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Conditions for Stellar Stability
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Conditions for Stellar Stability
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Star Cluster Formation
When stars are born they develop from large
clouds of molecular gas. After the remnant gas is
heated and blow away, the stars collect together
by gravity. During the exchange of energy between
the stars, some stars reach escape velocity from
the protocluster and become runaway stars. The
rest become gravitationally bound, meaning they
will exist as collection orbiting each other
forever.
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Star Clusters
Jewel Box-Young Cluster
M80-Old Cluster
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Brown DwarfsFailed Stars
If a protostar forms with less than 0.08 solar
masses, nuclear fusion never begins This failed
star is called a brown dwarf, a planet sized
object Brown dwarfs still emit energy, due to
gravitational collapse Brown dwarfs are
important to astronomy since they may be the most
common type of star out there and solve the
missing mass problem Brown dwarfs eventual fade
and cool to become black dwarfs.
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Evidence of Stellar Formation
The region surrounding the nebula M20 shows
evidence of contraction A huge, dark molecular
cloud surrounds the visible nebula Density and
temperature are low The glowing region of ionized
gas results directly from a massive O-type star
at stage 6 or 7 on its evolutionary track.
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Evidence of Protostars
Star forming regions known as "EGGs" are
uncovered at the end of this giant pillar of gas
and dust in the Eagle Nebula (M16) EGGs, short
for evaporating gaseous globules, are dense
regions of mostly molecular hydrogen gas that
fragment and gravitationally collapse to form
stars.
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Shock Waves and Star Formation
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Carbon Star