Today:

• How a star changes while on the main sequence

• What happens when stars run out of hydrogen fuel

• Second stage of thermonuclear fusion

• Star clusters and stellar evolution

• Two kinds of stellar populations

Imagine…

Life on the main-sequence

• At their cores, all main-sequence stars are alike:– Convert hydrogen to helium (hydrogen core fusion)

• The total time a star will spend fusing hydrogen into helium is called its main-sequence lifetime.– About 12 billion years for our Sun, which is now about

4.56 billion years old.

• What happens to a star after all its core hydrogen has been used up? That depends strongly on the mass of the star.– 0.4 M is the important cutoff

Changes in the Sun’s composition

• Protostar becomes main-sequence star when core fusion and hydrostatic equilibrium are established.

– Star at this point is called “zero-age main-sequence” star

• Zero-age main-sequence stars are different from main-sequence stars since a star’s luminosity, surface temperature and radius change while on the main-sequence.

– Result of hydrogen burning which changes core composition.

Changes in the Sun’s composition

• Number of nuclei in core decreases with time.

– 4H->He– Fewer particles to provide pressure so

core compresses.– Compression leads to higher density,

temperature and pressure.– He accumulates in core

• Outer layers expand and shine more brightly.

– Why? More energy from core.

• Sun has become 40% more luminous, radius 6% larger and T 300K higher.

Stars with <0.4M

• These stars are small and cool.– Red dwarfs– 85% of all stars in our galaxy

• No He accumulation in core– Convection replaces He with

“fresh” H– After 100s of billions of

years the entire star will be He.

• Compare to 13.7 billion year age of universe…

Mass determines lifetime

What happens after the main-sequence?

• Mass < 0.4 M:– Red dwarf ends life as slowly cooling ball of pure

helium

• Mass > 0.4 M:– All hydrogen in core used up and H fusion in core

ceases.– Hydrogen fusion continues only in hydrogen rich

material just outside of core.• Called shell hydrogen fusion

• Shell hydrogen fusion leads to two things:

1) The core gets smaller

• End of fusion in core means no energy is generated there.– Temperature and pressure decrease due to lack of energy

source– Core compresses under weight of outer layers

• Core compression leads to conversion of gravitational energy to thermal energy which flows outward and accelerates shell H fusion.– He falls back into core and continues core shrinking and

heating– 1 M star: Temperature increases from 15 billion K to 100

billion K.

2) Outer radius gets larger

Stars join the main sequence when core hydrogen fusion begins. They leave the main sequence and become giants when core hydrogen is depleted.

Mass loss in giant stars

• Giant stars are giant– Gravity is weak at outer

reaches of star

– Gas can easily escape into space

– Typical mass loss is 10-7 M per year

• Compare to 10-14 M per year from solar wind

Helium fusion in giant stars

• At end of main sequence lifetime a star (>0.4 M) has a pure He core

• Helium is fuel for further fusion reactions:– Requires higher

temperatures and pressures– When core reaches 108 K

helium fusion begins

• Star again has central energy source

Onset of helium core fusion

• In larger stars (>2-3 M) onset of He fusion is gradual as core temperature approaches 108 K.

• In smaller stars (<2-3 M) onset of He fusion is sudden.– Called helium flash

Helium flash

• At He fusion temperatures all core gas is ionized– Further compression of free

electrons prohibited by Pauli exclusion principle

– Temperature independent core pressure allows for “runaway” He fusion for short time

– Called helium flash

• Core He burning lasts only ~108 years.

Star evolution and H-R diagrams

• Star evolution after main-sequence lifetime can be tracked on H-R diagram

• Stars move off zero-age main sequence as they age and move towards life as a giant star.– Explains why H-R diagram

is broad band rather than narrow line.

H-R diagram from Hipparcos satellite

Star clusters

• Star clusters contain stars of many masses– All stars in cluster have

essentially same age

• Star clusters allow for study of stellar evolution– Let’s look at computer

simulation of cluster of 100 stars of varying mass but same age.

Simulated star cluster

Simulated star cluster

Simulated star cluster

Age of star clusters

• M35 must be relatively young due to presence of luminous, blue, high-mass stars.

• NGC 2158 must be older since there are no high-mass blue main sequence stars.

Measuring ages of star clusters

• High-mass, high luminosity stars are first to consume core hydrogen and move off main sequence.

• Age of cluster can be found from “turnoff point.”– Top of surviving portion of

the main sequence on the cluster’s H-R diagram.

Distinct star populations

• Stars in youngest clusters are metal rich– Population I stars

• Stars in oldest clusters are metal poor– Population II stars

• Metals produced through thermonuclear reactions in most massive stars

Pulsating stars

• The Sun pulsates on a small scale.

• Other stars pulsate on a much larger scale.– Called pulsating variable

stars.

– These are evolved, post-main sequence stars.

• First discovered in 1595 with Mira.

Pulsating stars

Cepheid variables

Cepheid variables

• Cepheid variables allow for determination of intergalactic distances.– 1) Very luminous.

– 2) Direct relationship between period and luminosity

• Luminosity of Cepheid variables depends on the composition of the star.

Mass transfer in close binary systems