ASTR 3 Chapter Notes - Chapter 13: Brown Dwarf, Nucleon, Thermostat
13.1 STAR BIRTH
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
• Gravity doesn't always succeed bc the cloud's internal gas pressure can resist gravity
Gravitational Equilibrium - size remains stable in stars (Sun) because the inward pull of gravity is
balanced by the outward push of gas pressure
• Pressure is weaker in low-density gas of interstellar space, but so is the pull of gravity
Stars are born in cold, dense clouds of gas whose pressure cannot resist gravitational contraction
These molecular clouds are cold and dense enough to allow atoms to combine and form molecules
• Typically 10-30K
• Tend to be quite large --> more total mass helps gravity overcome gas pressure
o Thousands of times as massive as typical star
o Results in many star births --> clusters!
From Cloud to Protostar:
1. Large molecular cloud collapses
a. Gravity pulls the gas towards the cloud's densest regions
2. Fragments into smaller pieces that each form one or more new stars
3. Each fragment heats up as it contracts
a. Source of heat is gravitational potential energy released as gravity pulls into center
b. Contracting begins quickly radiating away much of this energy, preventing temp and
pressure from building enough to resist gravity
i. Temp of cloud remains below 100K, so it glows in long-wavelength infrared light
4. Central region becomes dense enough to trap infrared radiaiton, it can no longer radiate away its
heat
5. Central temperature and pressure rise dramatically
a. Slowing contraction
6. Dense center is now a Protostar
a. Clump of gas that will become a new star
How massive must the cloud be for a specific density and temperature??
Mminimum = 18MSun
Example: cloud has temp 30K and average density of 300 ppc. What mass must this cloud have in
order to form stars?
18*
= 171Msun
A molecular cloud fragment heats up as gravity makes it contract, producing a Protostar at its center
Rotation of the shrinking cloud fragment also produces a spinning disk of gas around the protostar
--> cloud fragments flattens along its rotational axis
--> may later form planets in the disk
Observations show that many young protostars fire high-speed streams of gas, or jets, into interstellar
space
• Two, shooting in opposite directions along rotation axis
• Sometimes lined with glowing blobs of gas, presumably clumps of matter swept up as the jets
plow into the surrounding interstellar material
Strong magnetic field helps protostar generate a strong protostellar wind, an outward flow of particles
similar to the solar wind
Because of angular momentum, when the molecular fragments form protostars that end up quite close
to one another, gravity controls them
• They go into orbit around each other
• Pairs w large amounts of angular momentum have large orbits and those w smaller amounts orbit
closer together
• Two stars close to each other are in a close binary system
o Usually <0.1 AU and orbit each other every few days
A protostar becomes a true star when its core temp reaches 10 million K --> hot enough for hydrogen
fusion to operate efficiently
A. Core temp continues to rise until fusion in core generates enough energy to balance the energy
lost from the surface in the form of radiation
B. Gravitational contraction then stops, because the star has achieved energy balance
C. NOW A MAIN-SEQUENCE STAR
Length of time from formation of Protostar to main-sequence star:
• Dependent on mass of star
o More massive do everything faster
• High-mass star of spectral type O or B may take <1 million years
• Star the Sun takes ~30 million years
• Low-mass star of spectral type M may take 100 million years
Most stars are low mass
Maximum mass of star is 150Msun
• Begin to blow off outer layers at 100Msun
Minimum mass of star is 0.08Msun
• 80 times the mass of Jupiter
• Anything less can't reach 10M K --> brown dwarf
o Between a planet and a star
Pressure in Brown Dwarfs
The source of pressure that stops gravity from squeezing a brown dwarf's core to the point at which it
could sustain fusion is quite diff from the type of pressure we know
• Ordinary: thermal pressure bc its closely linked to temp
• Degeneracy Pressure doesn't depend on temperature at all; instead depends on laws of quantum
mechanics that also give rise to distinct energy levels in atoms
o Based on restrictions on where the particles can go
Document Summary
Stars are born in cold, dense clouds of gas whose pressure cannot resist gravitational contraction. Example: cloud has temp 30k and average density of 300 ppc. A molecular cloud fragment heats up as gravity makes it contract, producing a protostar at its center. Rotation of the shrinking cloud fragment also produces a spinning disk of gas around the protostar. -> cloud fragments flattens along its rotational axis. -> may later form planets in the disk. Observations show that many young protostars fire high-speed streams of gas, or jets, into interstellar space: two, shooting in opposite directions along rotation axis. Sometimes lined with glowing blobs of gas, presumably clumps of matter swept up as the jets plow into the surrounding interstellar material. Strong magnetic field helps protostar generate a strong protostellar wind, an outward flow of particles similar to the solar wind.
