Stellar Evolution

Stellar EvolutionChapter 12

This chapter is the heart of any discussion of astronomy. Previous chapters showed how astronomers make observations with telescopes and how they analyze their observations to find the luminosity, diameter, and mass of stars. All of that aims at understanding what stars are.

This is the middle of three chapters that tell the story of stars. The preceding chapter told us how stars form, and the next chapter tells us how stars die. This chapter is the heart of the story—how stars live.

As always, we accept nothing at face value. We expect theory to be supported by evidence. We expect carefully constructed models to help us understand the structure inside stars. In short, we exercise our critical faculties

Guidepost

and analyze the story of stellar evolution rather than merely accepting it.

After this chapter, we will know how stars work, and we will be ready to study the rest of the universe, from galaxies that contain billions of stars to the planets that form around individual stars.

Guidepost (continued)

The structure and evolution of a star is determined by the laws of

Main Sequence Stars

• Hydrostatic equilibrium

• Energy transport

• Conservation of mass

• Conservation of energy

A star’s mass (and chemical composition) completely determines its properties.

That’s why stars initially all line up along the main sequence.

Maximum Masses of Main-Sequence Stars

h Carinae

(Eta Carinae)

a) More massive clouds fragment into smaller pieces during star formation.b) Very massive stars lose mass in strong stellar winds

Example: η Carinae: Binary system of a 60 Msun and 70 Msun star. Dramatic mass loss - major eruption in 1843 created double lobes.

Mmax = 100 solar masses

Minimum Mass of Main-Sequence Stars

Mmin = 0.08 Msun

At masses below 0.08 Msun, gas doesn’t get hot enough to ignite thermonuclear fusion.

These are called Brown Dwarfs

Gliese 229B

Brown DwarfsHard to find because they are very faint

and cool; emit mostly in the infrared.

Many have been detected in star forming regions like the Orion Nebula.

Evolution on the Main Sequence (1)

Zero-Age Main

Sequence (ZAMS)

Main-Sequence stars live by fusing hydrogen (H) into

helium (He).

A finite supply of hydrogen means a finite life time.

MS evolution

Evolution on the Main Sequence (2)

Luminosity L = M3.5

A star’s life time T = energy reservoir / luminosity

T = M/L = 1/M2.5

Energy reservoir = M

Massive stars have short

lives!

Evolution off the Main Sequence: Expansion into a Red Giant

When hydrogen (H) in the core is completely converted to helium (He), fusion stops.

H burning continues in a shell around the core.

He core and H burning shell produce more energy than needed for pressure support

Expansion and cooling of the outer layers of the star produces a Red Giant

Expansion onto the Giant Branch

Expansion and surface cooling during the phase of an inactive He core and a H burning shell

Sun will expand beyond Earth’s orbit!

Red Giant Evolution

4 H → He

He

Hydrogen burning shell keeps dumping helium onto the core.

He-core gets denser and hotter until the

next stage of nuclear burning can begin in

the core.

Helium Fusion

He nuclei can fuse to build heavier elements like carbon and oxygen

When pressure and temperature in the He core

become high enough,

Fusion Into Heavier Elements

Fusion into heavier elements (than carbon and oxygen) requires very high temperatures and occurs only in massive stars (more than 8 solar masses).

These stars fuse:He C and O then C Ne, Na, Mg, O then Ne O, Mg then O Si, S, P then Si Fe, Co, Ni….all in the final 0.00008 of it’s life!

Evidence for Stellar Evolution: Star Clusters

Stars in a star cluster all have approximately the same age!

More massive stars evolve more quickly than less massive ones.

If you put all the stars of a star cluster on a HR diagram, the most massive stars

(upper left) will be missing!

HR Diagram of a Star Cluster

Example: HR diagram of the star cluster M 55

High-mass stars evolved onto the

giant branch

Low-mass stars still on the main

sequence

Turn-off point

Estimating the Age of a Cluster

The lower on the MS

the turn-off point, the older the cluster.

Evidence for Stellar Evolution: Variable Stars

Some stars show brightness variations not caused by eclipsing

in binary systems.

Most important example:

d Cephei

Light curve of d Cephei

Cepheid Variables: The Period-Luminosity Relation

The variability period of a Cepheid variable is correlated with its luminosity.

=> Measuring a Cepheid’s period, we can determine its absolute magnitude!

The more luminous it is, the more slowly it pulsates.

Cepheid Distance MeasurementsComparing absolute and apparent magnitudes of Cepheids,

we can measure their distances (using the 1/d2 law)!

The Cepheid distance measurements were the first distance determinations that worked out to distances beyond our Milky Way!

Cepheids are up to ~ 40,000 times more luminous than our sun

=> can be identified in other galaxies.

Pulsating Variables: The Valve Mechanism

Partial He ionization zone is opaque and absorbs more energy than necessary to balance the weight from higher layers.

=> Expansion

Upon expansion, partial He ionization zone becomes more transparent, absorbs less energy => weight from higher layers pushes it back inward. => Contraction.

Upon compression, partial He ionization zone becomes more opaque again, absorbs more energy than needed for equilibrium => Expansion

Period Changes in Variable StarsPeriods of some Variables are not constant over time

because of stellar evolution. Another piece of evidence for stellar evolution.

conservation of mass lawconservation of energy law

stellar modelbrown dwarfzero-age main sequence (ZAMS)

degenerate mattertriple alpha processhelium flashopen clusterglobular clusterturnoff pointhorizontal branchvariable starintrinsic variableCepheid variable starRR Lyrae variable star

period–luminosity relationinstability strip 

New Terms