NATS 1740 Chapter Notes - Chapter 12: Triple-Alpha Process, Binary Star, Protostar

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Published on 13 Apr 2013
York University
Natural Science
NATS 1740
Chapter 12 Star Stuff
12.1 Star Birth
How do stars form?
- A star is born when gravity causes a cloud of interstellar gas to contract to the point at
which the central object becomes hot enough to sustain nuclear fusion in its core
- Stars are born in cold dense clouds of gas whose pressure cannot resist gravitational
-Molecular clouds: star forming clouds that are cold and dense
Allow hydrogen atoms to pair with hydrogen molecules (10-30K temperature range)
From clouds to protostar
- A molecular cloud fragment heats up and spins faster as gravity makes it contract,
producing rapidly rotating protostar at its centre
Disks and Jets
- Conservation of angular momentum ensures that protostars rotate rapidly and are
surrounded by spinning disks of gas
Single star or binary?
- Neighbouring protostars sometimes end up orbiting each other in binary star systems
-Close binary: 2 stars end up quite close to each other (0.1 AU apart, orbit every few days
around each other)
From protostar to main sequence
- A protostar becomes a main-sequence star when it achieves energy balance between
hydrogen fusion in its core and radiation from its surface
How massive are newborn stars?
Limits on stellar masses
- Stars more massive than 300M sun blow themselves apart while protostars smaller than
0.08M sun become brown dwarfs that never get hot enough for efficient hydrogen
fusion (failed star)
Pressure in brown dwarfs
- Brown dwarfs are supported against gravity by degeneracy pressure which does nto
weaken with decreasing temperature
Thermal pressure: ordinary gas pressure
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