© Sierra College Astronomy Department1 STAR STUFF: Stellar Birth, Stellar Evolution, and Stellar...

© Sierra College Astronomy Department 1

STAR STUFF:STAR STUFF:

Stellar Birth, Stellar Evolution, Stellar Birth, Stellar Evolution, and Stellar Deathand Stellar Death


Star Birth – Road to the Main SequenceStar Birth – Road to the Main Sequence

A Brief Woodland VisitA Brief Woodland Visit

• An Alien Visit• If you were alien from a treeless world and were sent to Earth for

one day to gather data from a forest, what do you think your chances are of developing the correct theory for the growth history of a tree?

• How would your chances change if you were given a basic knowledge base of Earth biology?

• Astronomers are in a Similar Position• The life cycle of stars takes a minimum of millions of years• Astronomers tackle the problem by observing tremendous

numbers of stars in various stages of development.• Combining this observational data with laboratory measurements

and theoretical models, the life cycle of a star is pieced together.



Stellar Nurseries – Where do Stars Form?Stellar Nurseries – Where do Stars Form?

• A First Look• The youngest star clusters, determined by their main

sequence turnoff points, are always associated with dark clouds of gas and dust

• The interstellar medium must be the birthplace of stars

• The interstellar clouds are of a special type: cold and dense - an indicator of the competing forces of gravity and pressure – often called molecular clouds

• A simple count shows that 2-3 new stars form each year in our part of the Milky Way




• Star-Forming Clouds• Stars are born in the coldest and highest density

interstellar clouds• 10-30 K, 300 molecules/cm3 (an average with high density

regions hundreds of times denser)

• Usually called molecular clouds since temperatures are low enough to form molecules

• H2 is the most abundant molecule – difficult to detect due to lack of emission lines at low temperatures

• More than 120 other molecules• CO is the most abundant and usually its emissions are used to

study a molecular cloud’s physical characteristics• Others: Water (H2O), ammonia (NH3), ethyl alcohol (C2H5OH)




• Molecular clouds and Interstellar Dust• Observing in the infrared allows observations of newly

forming stars (protostars) within and stars beyond the densest clouds

• Dust grains absorb the visible light and even some infrared light of the protostars

• The dust then emits the energy in the infrared and microwave bands

• Clouds that are dark in the visible “glow” in the longest wavelength infrared light and this glow characterizes the temperature distribution of the cloud



Stellar Nurseries – Why do Stars Form?Stellar Nurseries – Why do Stars Form?• Gravity vs Pressure

• Gravity can create stars from a gas cloud only if it can overcome the gas pressure, which depends on temperature and density through the ideal gas law

P = nkT

where P is the thermal pressure (to distinguish it from degeneracy pressure), T is the temperature, n is the number density, and k is Boltzmann’s constant

• Only when the density of a cloud is high enough and the temperature low enough will gravitational contraction start the stellar birthing process

• Gravitation equilibrium must be achieved as in the Sun.


Stages of Star Birth – The Onset of FusionStages of Star Birth – The Onset of Fusion

• Summary - Birth Stages on a Life Track

• Stage 1 – Assembly of a Protostar• Collapsing cloud fragment concealed in shroud of gas

and dust – creation of protostellar disc

• Protostellar winds and jets disrupt shroud and reveal protostar

• Photosphere temperature at 3000 K with surface much larger than Sun and a luminosity 10-100 times that of the Sun

• Stage 2 – Convective Contraction

• 3000 K surface maintained primarily by convective energy transport to surface

• Luminosity decreases as radius decreases


Stages of Star Birth – The Onset of FusionStages of Star Birth – The Onset of Fusion• Summary - Birth Stages on a Life Track (continued)

• Stage 3 – Radiative Contraction• Surface temperature rises as energy transport switches to

radiative diffusion within the protostar

• Even with radius decreasing, luminosity increases slightly

• Fusion in core commences

• Stage 4 – Self-Sustaining Fusion

• Core temperature continues to rise as does rate of fusion

• Hydrostatic (Gravitational) equilibrium is reached

• Star settles into main-sequence life



Stages of Star Birth – Binary Star SystemsStages of Star Birth – Binary Star Systems

• Single Star or Binary?• As the molecular cloud contracts it breaks into

fragments• These fragments can coalesce into one or they

can form into two stars which orbit each other – a binary star system

• Stars that end up within 0.1 AU of each other are called a close binary system.



Stages of Star Birth – The Onset of FusionStages of Star Birth – The Onset of Fusion

• From Protostar to Main Sequence• Only half the thermal energy released by gravitational

contraction is radiated away• This energy comes from the surface and induces further

contraction• The other half of the energy goes into heating the protostar’s

interior• When the core temperature hits 10 million K, fusion

occurs via the proton-proton chain and a main-sequence star is born

• The duration of a life track (evolutionary track) from protostar to main sequence varies with the eventual mass of the star (from less than one million years to well over 100 million years)



Masses of Newborn Stars – The SmallestMasses of Newborn Stars – The Smallest

• The Least Massive Stars• A star must be massive enough to initiate fusion• Degeneracy pressure, which only depends on density

and not temperature, prevents protostars less than 0.08 MSun (which is 80 times more massive than Jupiter), from reaching the fusion temperature

• Brown dwarfs are “failed stars” with masses between that of a planet and 0.08 MSun.

• The spectral classification of a brown dwarf is T and these objects are sometimes called T dwarfs with a surface temperature of less than 1400 K

• L dwarfs include brown dwarfs and hydrogen-burning stars with surface temperatures of 1400-2200 K



Masses of Newborn Stars - The LargestMasses of Newborn Stars - The Largest

• The Most Massive Stars• Radiation pressure will eventually dominate

gravity’s ability to accrete gas and dust onto a forming star, effectively blowing away any extra mass in their outer layers into space

• Theoretical maximum is about 150 Msun the observed mass of the Pistol Star is about this massive)

• Mass Distribution of Newborn Stars


Stellar EvolutionStellar Evolution

Life After the Main SequenceLife After the Main Sequence


Stellar Evolution - Life After the Main Sequence Stellar Evolution - Life After the Main Sequence

Why Do Stars Evolve?Why Do Stars Evolve?

The Changing of Chemical Composition • As a star uses fuel, it transforms light elements into

heavy elements.• There are three processes in a star which depend on

chemical composition:• The rate at which a star produces energy• The rate at which energy flows to the surface• The generation of pressure which resists gravitational collapse

• These processes are driven by:• Primarily mass• Internal structure a factor as well


Stellar Evolution - Life After the Main Sequence Stellar Evolution - Life After the Main Sequence

Why Do Stars Evolve?Why Do Stars Evolve?

Stellar Nuclear Fusion• The core temperature rises to about 10 million K after the

star’s contraction from a molecular core fragment.• Stars of low mass like the Sun (<1.5 M) use the proton-

proton chain to generate energy.• Stars of mass greater than 1.5 M have higher core

temperatures that allow the CNO cycle to fuse of hydrogen into helium (4H He).• The CNO cycle is more efficient at the higher core temperatures of

these stars• This series of reactions involves hydrogen with carbon, nitrogen, and

oxygen as catalysts.• Hydrostatic equilibrium (pressure balances gravity)

maintains fusion at a uniform rate.


Stellar Evolution - Life After the Main SequenceStellar Evolution - Life After the Main Sequence

Stellar MaturityStellar Maturity

Main Sequence Life (MSL)

• Recall: TMSL = 1010 Msun/Lsun years

• An aging main sequence star becomes more luminous and cooler as it attempts to maintain hydrostatic equilibrium.

• Stars start their main sequence lives on the left side of the main sequence “strip”, then move up and to the right as they age.

• The more massive stars do this the fastest.



Stellar MaturityStellar Maturity

Towards Star Death• Until their lives end on the main sequence, the

main difference between the evolution of stars of various masses is the amount of time they spend as protostars and main sequence stars.

• Stars can be grouped by mass as low-mass, intermediate-mass stars, or high-mass depending on how they reach their end state. [However, other groupings are possible.]



Low-Mass StarsLow-Mass Stars

Low-Mass Stars• In stars with a mass of less than about 0.4 M,

convection occurs throughout most or all of the volume of the star.

• Hydrogen from throughout the star is cycled through the core, and the entire star runs low on hydrogen at the same time.

• A low-mass star will take at least 100 billion years to completely burn its hydrogen.



Low-Mass StarsLow-Mass Stars

• Ultimately, low-mass stars will become white dwarfs through gravitational shrinkage.

• The hypothetical lifetime of a low-mass star is more than the assumed age of the universe.

• Consequently, white dwarfs currently observed must have originated in a different manner.

• Eventually, however, low-mass stars will follow the evolutionary sequence of intermediate-mass stars.



Leaving the Main SequenceLeaving the Main Sequence

Hydrogen Shell Burning• After the core is depleted of hydrogen, the star

has no source of nuclear energy and must turn to gravitation energy as a source

• Soon a region around the core, still rich in hydrogen begins to ignite and fuse into helium• For stars like the Sun (an intermediate-mass star) this

happens at almost the same time the core runs out of hydrogen

• For high-mass stars the core must contract for 100,000s to millions of years before shell burning can occur.



Leaving the Main SequenceLeaving the Main Sequence

Movement of the Hydrogen shell• As time goes on the shell steadily flows outward, leaving

behind helium which migrates towards the core.• While the star’s interior continues to contract the surface

layers expand and cool turning the star into a red giant.• For stars as massive as the Sun this process may go for 1 billion

years.• For a star which has 9 times the mass of the Sun, this process lasts

only a million years.

• This causes the star’s life track to steadily move to the right and upward on the H-R diagram.


Pressure Under Extreme Conditions• Under extremely high density conditions,

pressure no longer follows the ideal gas law and is produced by electron degeneracy.

• Electron degeneracy is a quantum state of a gas in which its electrons are packed as densely as nature permits.

• Pressure of such a high-density gas is not dependent on temperature as it is in a “normal” gas.


Degenerate MatterDegenerate Matter



Helium as a FuelHelium as a Fuel

Helium Fusion• For stars with a mass less than about 2 M the

ever growing core will become degenerate.• This prevents further contraction of the core,

though the temperature will continue to rise as more helium is dumped onto the core from the hydrogen shell burning.

• Eventually, the core reaches 100 million K and the triple alpha process begins to form carbon (and some oxygen) .




The Helium Flash• At this stage the star in nearly as big a 1 AU and 1000

times more luminous as the Sun• As the helium fuses into carbon the temperature

increases. Since the core is degenerate, it does not expand which would normally regulate the fusion rate

• Since the temperature continues to rise, the rate of the triple alpha process increases rapidly, and the helium in the core ignites in what is termed the helium flash, more energy released than needed to hold off core contraction.

• The core at this stage is 300 million K.




More Massive Red Giants• Stars with masses larger than 2 M are spared the

helium flash.• Instead, before the core becomes degenerate, it starts

the triple alpha process and steadily burns He into carbon.

• Regardless of a star’s mass, stars with helium fusion in the core and hydrogen fusion in an outer shell are referred to as helium-burning stars and they occupy the horizontal branch of the H-R diagram.



The Variable Star PhaseThe Variable Star Phase

Horizontal Branch Stars (Helium Burning Star)• After helium core burning begins, a stars core returns to an

ideal gas and its surface becomes hotter and smaller - settling into moving across the H-R diagram with roughly a constant luminosity passing into a yellow giant stage.

• Many of these yellow giants (whether an aging high-mass or low-mass star) swell and shrink rhythmically: they pulsate.• These pulsating yellow giants are located in the instability strip

of the H-R diagram.• High-mass pulsating giants are Cepheid variables (periods of

about 1-70 days).• Low-mass pulsating giants are RR Lyrae variables (periods of

about 12 hours).



Beyond Helium BurningBeyond Helium Burning

Exhausting the Helium Fuel in the Core• Eventually, the star will consume all its helium in the

the core, leaving behind carbon and oxygen.• Just like in a red giant, the core contracts and becomes

degenerate again, but now helium shell burning begins and the star is referred to as a double-shell burning star.

• The evolutionary track of a star at this stage is called the Asymptotic Giant Branch (AGB) and the star returns to being a red giant (or a red supergiant in the case of a high-mass star).



Beyond Helium BurningBeyond Helium Burning

More Exhaustion on the AGB• Hydrogen shell burning is present and provides most of the

energy of the star.• However, once enough helium has built between the shells, it

is consumed in a nearly explosive event called a thermal pulse.

• The thermal pulse is repetitive and increases the mass of the degenerate carbon/oxygen core.

• The pulses dredge up carbon from the core, enriching the surface with carbon and creating a red giant called a carbon star, which creates “carbon smog” that is easily blown into space by the star’s wind providing the largest source of interstellar dust



Approaching the EndApproaching the End

Mass Loss In AGB Stars• The solar wind carries away about 10–14 of the Sun’s

mass each year. Over the course of 10 billion years, the Sun will lose only 0.01% of its mass this way.

• In AGB stars, it is thought that core instabilities and pulsations are responsible for large mass losses.

• A typical AGB loses from 10–7 to 10–4 solar masses a year and hence can last at most 10 million to just 10,000 years.



Intermediate-Mass Stars After the Main SequenceIntermediate-Mass Stars After the Main Sequence

Final Stage of an Intermediate-Mass Star• A Planetary nebula is a spherical shell of gas that

is expelled by a low-mass red giant near the end of its life.

• The material in the shell glows because UV radiation from the central hot star causes it to fluoresce.

• Pulsations and/or stellar winds are thought to cause planetary nebulae.



More Quantum PhysicsMore Quantum Physics

The Chandrasekhar Limit• Degenerate electrons can withstand pressures

created by up to 1.4 solar masses.• Beyond that point - the Chandrasekhar limit

(or white dwarf limit) - white dwarf stars cannot exist.

• Main sequence stars with masses up to 6 solar masses (and perhaps higher) can end up as white dwarfs only if they lose enough mass during the red giant and planetary nebula phases.



Final Days for the High-Mass StarsFinal Days for the High-Mass Stars

High-Mass Stars on the AGB• Massive stars eventually become supergiants.• Typical supergiants have luminosities a million

times that of the Sun and absolute magnitudes of -10.

• The greater core temperatures and pressures, fuse elements beyond helium and up to iron on the periodic table.

• The creation of these elements is known as nucleosynthesis.



Supernova – Type IISupernova – Type II

Core Collapse• Type II supernovae (or massive star supernova) begin

with the conversion of silicon to iron. The fusing of silicon to iron in a supergiant star will take only a few days.

• Because the iron fusion reaction absorbs more energy than it releases, the core shrinks, heats up, but produces no new more massive elements.

• At the Chandrasekar limit, the core collapses violently. After reaching its minimum size, the core rebounds, colliding violently with infalling material.



Supernova RemnantsSupernova Remnants

On the Rebound• This collision between the infalling material and the

rebounding core produces two effects:• Enough energy is produced to fuse iron into heavier

elements.• Shock waves are sent outward that throw off the outer layers

of the supergiant. These shock waves may be further heated by the neutrinos escaping the collapsed core.

• The thrown-off outer layers create a supernova remnant – an expanding cloud of debris.

• Years of last five supernova observed without the need of a telescope: 1006, 1054, 1572, 1604, 1987.



Evolution in Stellar SystemsEvolution in Stellar Systems

The Lives of Close Binaries

• Most stars in binary systems proceed in their evolution as if they were alone.

• The Algol paradox • Close binary pair of which one star is a 3.7 M main-

sequence star and the other a 0.8 M subgiant.

• If they were born at about the same time, how can the more massive star be a main-sequence star?

• Answer: Mass Transfer

© Sierra College Astronomy Department1 STAR STUFF: Stellar Birth, Stellar Evolution, and Stellar...

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Transcript of © Sierra College Astronomy Department1 STAR STUFF: Stellar Birth, Stellar Evolution, and Stellar...