Chapter 16: Star Birth

stars form in dark clouds of dusty gas in interstellar space

the clouds of gas between the stars

we can determine the composition of interstellar gas from its absorption lines in the spectra of stars

70% H, 28% He, 2% heavier elements (metals) in our region of the Milky Way

• most of the matter in star-forming clouds is in the form of molecules
• they have a temperature of 10-30 K and a density of about 300 molecules per cubic cm
• most of what we know about molecular clouds comes from observing the emission lines of carbon monoxide

• tiny solid particles of interstellar dust block our view of stars on the other side of a cloud
• particles are <1 micrometer in size and are made of elements like C, O, Si, and Fe

• stars viewed through the edges of the cloud look redder because dust blocks (shorter-wavelength) blue light more effectively than (longer-wavelength) red light
• long-wavelength infrared light passes through a cloud more easily than visible light
• observations of infrared light reveal stars on the other side of the cloud

• visible light from a newborn star is often trapped within the dark, dusty gas clouds where the star formed
• observing the infrared light from a cloud can reveal the newborn star embedded inside it

• dust grains that absorb visible light heat up and emit infrared light of even longer wavelength
• long-wavelength infrared light is brightest from regions where many stars are currently forming

• gravity can create stars only if it can overcome the force of thermal pressure in a cloud
• emission lines from molecules in a cloud can prevent a pressure buildup by converting thermal energy into infrared and radio photons

• a typical molecular cloud (T ~ 30 K, n ~ 300 particles/cm³) must contain at least a few hundred solar masses for gravity to overcome pressure
• emission lines from molecules in a cloud can prevent a pressure buildup by converting thermal energy into infrared and radio photons that escape the cloud

• a cloud must have even mass to begin contracting if there are additional forces opposing gravity
• both magnetic fields and turbulent gas motions increase resistance to gravity

• gravity within a contracting gas cloud becomes stronger as the gas becomes denser
• gravity can therefore overcome pressure in smaller pieces of the cloud, causing it to break apart into multiple fragments, each of which may go on to form a star
• this simulation begins with a turbulent cloud containing 50 solar masses of gas
• the random motions of different sections of the cloud cause it to become lumpy
• each lump of the cloud in which gravity can overcome pressure can go on to become a star
• a large cloud can make a whole cluster of stars

Gravity can overcome pressure in a relatively small cloud if the cloud is unusually dense

• Elements like carbon and oxygen had not yet been made when the first stars formed
• without CO molecules to provide cooling, the clouds that formed the first stars had to be considerably warmer than today's molecular clouds
• the first stars must therefore have been more massive than most of today's stars, for gravity to overcome pressure

simulations of early star formation suggest the first molecular clouds never cooled bellow 100 K, making stars of ~ 100M_sun

• as contraction packs the molecules and dust particles of a cloud fragment closer together, it becomes harder for infrared and radio photons to escape
• thermal energy then begins to build up inside, increasing the internal pressure
• contraction slows down, and the center of the cloud fragment becomes a protostar

matter from the cloud continues to fall onto the protostar until either the protostar or a neighbouring star blows the surrounding gas away

• Evidence from the Solar System: the nebular theory of solar system formation illustrates the importance of rotation
• Conservation of Angular Momentum: the rotation speed of the cloud from which a star forms increases as the cloud contracts. rotation of a contracting cloud speeds up for the same reason a skater speeds up as she pulls in her arms
• Flattening: collisions between particles in the cloud cause it to flatten into a disk. collisions between gas particles in cloud gradually reduce random motions. collisions between gas and particles also reduce up and down motions. spinning clouds flatten as it shrinks.
• Formation of Jets: rotation also causes jets of matter to shoot out along the rotation axis. jets are observed coming from the centers of disks around protostars.

• protostar looks star-like after the surrounding gas is blown away, but its thermal energy comes from gravitational contraction, not fusion
• contraction must continue until the comes becomes hot enough for nuclear fusion
• contraction stops when the energy released by the core fusion balances energy radiated from the surface- the star is now a main-sequence star

life track illustrates star's surface temperature and luminosity at different moments in time

luminosity and temperature grow as matter collects into a protostar

surface temperature remains near 3 000 K while convection is main energy transport mechanism

luminosity remains nearly constant during late stages of contraction while radiation is transporting energy through star

core temperature continues to rise until star arrives on the main sequence

• models show that the sun required ~ 30 million years to go from a protostar to the main sequence
• higher-mass stars form faster than lower-mass stars

• fusion will not begin in a contracting cloud if some sort of force stops contraction before the core temperature rises above 10⁷ K 
• thermal pressure cannot stop contraction because the star is constantly losing thermal energy from its surface through radiation

• particles (electrons) can't be in the same state in the same place. laws of quantum mechanics prohibit it
• doesn't depend on heat content

• depends on heat content
• the main form of pressure in most stars

• star-like objects not massive enough to start fusion
• emits infrared light because of heat left over from contraction
• loses thermal energy -> luminosity gradually declines with time
• degeneracy pressure halts the contraction of objects with < 0.08M_sun before core temperature becomes hot enough for fusion

In Orion, infrared observations can reveal recently formed brown dwarfs because they are still relatively warm and luminous

• photons exert a slight amount of pressure when they strike matter
• very massive stars are so luminous that the collective pressure of photons drives their matter into space

• models of stars suggest that radiation pressure limits how massive a star can be without blowing itself apart
• observations have not found stars more massive than about 150M_sun. 

• stars >150M_sun would blow apart
• stars < 0.08M_sun can't sustain fusion

observations of star clusters show that star formation makes many more low-mass stars than high-mass stars