Chapter 12 Notes Jessica Brandon

Chapter 12 Star Stuff

****12.1 Star Birth****

•Stars form in dark clouds of dusty gas in interstellar space.

•The gas between the stars is called the interstellar medium.

•Gravity can create stars only if it can overcome the force of thermal pressure in a cloud.

•Gravity within a contracting gas cloud becomes stronger as the gas becomes denser.

•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.

•The cloud can prevent a pressure buildup by converting thermal energy into infrared and radio photons that escape the cloud.

This simulation begins with a turbulent cloud containing 50 solar masses of gas.

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.

As stars begin to form, dust grains that absorb visible light heat up and emit infrared light.

Long-wavelength infrared light is brightest from regions where many stars are currently forming.

Rotation of a contracting cloud speeds up for the same reason a skater speeds up as she pulls in her arms.

Collisions between gas particles in a cloud gradually reduce random motions.

Collisions between gas particles also reduce up and down motions.

Jets are observed coming from the centers of disks around protostars.

•A protostar contracts and heats until the core temperature is sufficient for hydrogen fusion.

•Contraction ends when energy released by hydrogen fusion balances energy radiated from the surface.

•It takes 30 million years for a star like the Sun (less time for more massive stars).

•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.

•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.

•Is there another form of pressure that can stop contraction?

•A brown dwarf emits infrared light because of heat left over from contraction.

•Its luminosity gradually declines with time as it loses thermal energy.

•Infrared observations can reveal recently formed brown dwarfs because they are still relatively warm and luminous.

****12.2 Life as a Low-Mass Star ****

A star remains on the main sequence as long as it can fuse hydrogen into helium in its core.

•Observations of star clusters show that a star becomes larger, redder, and more luminous after its time on the main sequence is over.

•As the core contracts, H begins fusing to He in a shell around the core.

•Luminosity increases because the core thermostat is broken—the increasing fusion rate in the shell does not stop the core from contracting.

•The thermostat is broken in a low-mass red giant because degeneracy pressure supports the core.

•The core temperature rises rapidly when helium fusion begins.

•The helium fusion rate skyrockets until thermal pressure takes over and expands the core again.

•Models show that a red giant should shrink and become less luminous after helium fusion begins in the core.

Combining models of stars of similar age but different mass helps us to age-date star clusters.

•After core helium fusion stops, He fuses into carbon in a shell around the carbon core, and H fuses to He in a shell around the helium layer.

•This double shell–burning stage never reaches equilibrium—the fusion rate periodically spikes upward in a series of thermal pulses.

•With each spike, convection dredges carbon up from the core and transports it to the surface.

****12.3 Life as a High-Mass Star ****

•High-mass main- sequence stars fuse H to He at a higher rate using carbon, nitrogen, and oxygen as catalysts.

•A greater core temperature enables H nuclei to overcome greater repulsion.

•Advanced nuclear burning proceeds in a series of nested shells.

Iron is a dead end for fusion because nuclear reactions involving iron do not release energy. (Fe has lowest mass per nuclear particle.)

Evidence for helium capture: Higher abundances of elements with even numbers of protons

Iron builds up in the core until degeneracy pressure can no longer resist gravity.

The core then suddenly collapses, creating a supernova explosion.

•Core degeneracy pressure goes away because electrons combine with protons, making neutrons and neutrinos.

•Neutrons collapse to the center, forming a neutron star.

****12.4 Summary of Stellar Lives****

•A star’s mass determines its entire life story because it determines its core temperature.

•High-mass stars have short lives, eventually becoming hot enough to make iron, and end in supernova explosions.

•Low-mass stars have long lives, never become hot enough to fuse carbon nuclei, and end as white dwarfs.

•Core shrinks and heats until it’s hot enough for fusion.

•Nuclei with larger charge require higher temperature for fusion.

•Core thermostat is broken while core is not hot enough for fusion (shell burning).

•Core fusion can’t happen if degeneracy pressure keeps core from shrinking.