Chapter 12Star Stuff

Protostar to Main Sequence

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

• Contraction ends when energy released byhydrogen fusion balances energy radiated from thesurface.

• It takes 50 million years for a star like the Sun(less time for more massive stars).

Summary of Star Birth

1. Gravity causes gas cloud toshrink and fragment

2. Core of shrinking cloudheats up

3. When core gets hot enough,fusion begins and stops theshrinking

4. New star achieves long-lasting state of balance

A cluster of many stars can form out of a single cloud.

Temperature

Luminosity

Verymassivestars arerare.

Low-massstars arecommon.

Upper Limit on a Star’s Mass• Photons exert a

slight amount ofpressure when theystrike matter.

• Very massive starsare so luminous thatthe collectivepressure of photonsdrives their matterinto space.

Upper Limit on a Star’s Mass• Models of stars

suggest thatradiation pressurelimits how massivea star can be withoutblowing itself apart.

• Observations havenot found stars moremassive than about150MSun.

Lower Limit on a Star’s Mass

• Fusion will not begin in a contracting cloud if somesort of force stops contraction before the coretemperature rises above 107 K.

• Thermal pressure cannot stop contraction because thestar is constantly losing thermal energy from itssurface through radiation.

• Is there another form of pressure that can stopcontraction?

Degeneracy Pressure:

Laws of quantum mechanics prohibit two electronsfrom occupying the same state in the same place.

Thermal Pressure:

Depends on heat content

The main form of pressurein most stars

Degeneracy Pressure:

Particles can’t be in samestate in same place

Doesn’t depend on heatcontent

Brown Dwarfs• Degeneracy pressure

halts the contractionof objects with<0.08MSun beforethe core temperaturebecomes hot enoughfor fusion.

• Starlike objects notmassive enough tostart fusion arebrown dwarfs.

Brown Dwarfs• A brown dwarf

emits infrared lightbecause of heat leftover fromcontraction.

• Its luminositygradually declineswith time as it losesthermal energy.

Temperature

Luminosity

Stars moremassivethan150MSunwould blowapart.

Stars lessmassivethan0.08MSuncan’tsustainfusion.

What are the life stages of a low-mass star?

A starremains onthe mainsequence aslong as it canfuse hydrogeninto helium inits core.

Main-Sequence Lifetimes and Stellar Masses

Thought Question

What happens when a star can no longer fusehydrogen to helium in its core?

A. Core cools offB. Core shrinks and heats upC. Core expands and heats upD. Helium fusion immediately begins

Life Track After Main Sequence• Observations of star

clusters show that astar becomes larger,redder, and moreluminous after itstime on the mainsequence is over.

Broken Thermostat• As the core contracts,

H begins fusing to Hein a shell around thecore.

• Luminosity increasesbecause the corethermostat isbroken—the increasingfusion rate in the shelldoes not stop the corefrom contracting.

Helium fusion does not begin right away because itrequires higher temperatures than hydrogen fusion—largercharge leads to greater repulsion.

The fusion of two helium nuclei doesn’t work, so heliumfusion must combine three He nuclei to make carbon.

Thought Question

What happens in a low-mass star when core temperature risesenough for helium fusion to begin?

A. Helium fusion slowly starts up.B. Hydrogen fusion stops.C. Helium fusion rises very sharply.

(Hint: Degeneracy pressure is the main form of pressurein the inert helium core.)

Helium Flash

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

• The core temperature rises rapidly when heliumfusion begins.

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

Helium burning stars neither shrink nor growbecause the core thermostat is temporarily fixed.

Life Track After Helium Flash• Models show that a

red giant shouldshrink and becomeless luminous afterhelium fusionbegins in the core.

Life Track After Helium Flash• Observations of star

clusters agree withthose models.

• Helium-burningstars are found in ahorizontal branchon the H-R diagram.

Combiningmodels ofstars ofsimilar agebut differentmass helpsus to age-date starclusters.

Using the H-R Diagram to Determine the Age of a Star Cluster

How does a low-mass star die?

Thought Question

What happens when the star’s core runs out of helium?

A. The star explodes.B. Carbon fusion begins.C. The core cools off.D. Helium fuses in a shell around the core.

Double-Shell Burning

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

• This double-shell-burning stage never reachesequilibrium—the fusion rate periodically spikesupward in a series of thermal pulses.

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

Planetary Nebulae• Double-shell

burning ends with apulse that ejects theH and He into spaceas a planetarynebula.

• The core left behindbecomes a whitedwarf.

Planetary Nebulae• Double-shell

burning ends with apulse that ejects theH and He into spaceas a planetarynebula.

• The core left behindbecomes a whitedwarf.

Planetary Nebulae• Double-shell

burning ends with apulse that ejects theH and He into spaceas a planetarynebula.

• The core left behindbecomes a whitedwarf.

Planetary Nebulae• Double-shell

burning ends with apulse that ejects theH and He into spaceas a planetarynebula.

• The core left behindbecomes a whitedwarf.

End of Fusion

• Fusion progresses no further in a low-mass starbecause the core temperature never grows hotenough for fusion of heavier elements (some Hefuses to C to make oxygen).

• Degeneracy pressure supports the white dwarfagainst gravity.

Life stagesof a low-mass starlike the Sun

The Death Sequence of the Sun

Life Track of a Sun-Like Star

What are the life stages of a high-mass star?

CNO Cycle• High-mass main-

sequence stars fuseH to He at a higherrate using carbon,nitrogen, andoxygen as catalysts.

• A greater coretemperature enablesH nuclei toovercome greaterrepulsion.

Life Stages of High-Mass Stars

• Late life stages of high-mass stars are similar tothose of low-mass stars:—Hydrogen core fusion (main sequence)—Hydrogen shell burning (supergiant)—Helium core fusion (supergiant)

How do high-mass stars make theelements necessary for life?

Big Bang made 75% H, 25% He—stars make everythingelse.

Helium fusion can make carbon in low-mass stars.

The CNO cycle can change C into N and O.

Helium Capture

• High core temperatures allow helium tofuse with heavier elements.

Helium capture builds C into O, Ne, Mg …

Advanced Nuclear Burning

• Core temperatures in stars with >8MSunallow fusion of elements as heavy as iron.

Advanced reactions in stars make elements like Si, S, Ca,and Fe.

Multiple-Shell Burning• Advanced nuclear

burning proceeds ina series of nestedshells.

The Death Sequence of a High-Mass Star

Iron is a deadend for fusionbecause nuclearreactionsinvolving irondo not releaseenergy.

(Fe has lowestmass pernuclearparticle.)

Evidence forheliumcapture:

Higherabundances ofelements witheven numbersof protons

How does a high-mass star die?

Iron builds upin the core untildegeneracypressure can nolonger resistgravity.

The core thensuddenlycollapses,creating asupernovaexplosion.

The Death Sequence of a High-Mass Star

Supernova Explosion• Core degeneracy

pressure goes awaybecause electronscombine withprotons, makingneutrons andneutrinos.

• Neutrons collapse tothe center, forming aneutron star.

Energy and neutrons released in a supernova explosion enableelements heavier than iron to form, including Au and U.

Supernova Remnant• Energy released by

the collapse of thecore drives outerlayers into space.

• The Crab Nebula isthe remnant of thesupernova seen inA.D. 1054.

Multiwavelength Crab Nebula

Supernova 1987A

• The closest supernova in the last fourcenturies was seen in 1987.

How does a star’s massdetermine its life story?

Role of Mass

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

• High-mass stars have short lives, eventuallybecoming hot enough to make iron, and end insupernova explosions.

• Low-mass stars have long lives, never become hotenough to fuse carbon nuclei, and end as whitedwarfs.

Low-Mass Star Summary

1. Main Sequence: H fuses to Hein core

2. Red Giant: H fuses to He inshell around He core

3. Helium Core Burning:He fuses to C in core while Hfuses to He in shell

4. Double-Shell Burning:H and He both fuse in shells

5. Planetary Nebula: leaves whitedwarf behindNot to scale!

Reasons for Life Stages

• Core shrinks and heats until it’shot enough for fusion

• Nuclei with larger chargerequire higher temperature forfusion

• Core thermostat is brokenwhile core is not hot enoughfor fusion (shell burning)

• Core fusion can’t happen ifdegeneracy pressure keeps corefrom shrinking

Not to scale!

Life Stages of High-Mass Star

1. Main Sequence: H fuses to Hein core

2. Red Supergiant: H fuses to Hein shell around He core

3. Helium Core Burning:He fuses to C in core while Hfuses to He in shell

4. Multiple-Shell Burning:many elements fuse in shells

5. Supernova leaves neutron starbehindNot to scale!

How are the lives of stars withclose companions different?

Thought Question

The binary star Algol consists of a 3.7 MSun main-sequence star and a 0.8 MSun subgiant star.

What’s strange about this pairing?

How did it come about?

Stars in Algol are closeenough that matter canflow from the subgiantonto the main-sequencestar.

The star that is now asubgiant was originallymore massive.

As it reached the endof its life and started togrow, it began totransfer mass to itscompanion (massexchange).

Now the companionstar is more massive.