Stellar Evolution

Chapters 12 and 13

Topics

• Humble beginnings– cloud– core– pre-main-sequence star

• Fusion– main sequence star– brown dwarf

• Life on the main sequence• Retirement

– low mass stars (<10 solar masses)– high mass stars (>10 solar masses)

H-R Diagram

Mass related to luminosity

• For binary stars, that we can reliably measure their masses and luminosities, graph luminosity vs. mass

• HUGE changes in luminosity correspond to small changes in mass -- power relationship!

• L ~ M4 for main sequence stars

So how do stars grow?

How do we know?

• Develop computer models and theories based on physics

• Compare observations with predictions

• Although changes to stars generally occur over large time scales, there are enough stars that we occasionally see a change occur (like novae and supernovae)

“Oh, honey, let’s have a baby...”

• Cloud of dust and gas– mostly gas– lots of hydrogen

– diameter 10,000Ds.s.

– density <~ 1000 atoms/cm3

– in equilibrium

Milky Way

Milky Way in infrared (COBE)

Emission Nebulae

M20: Trifid Nebula (900 pc)

Barnard 68: Dark Nebula

Horsehead Nebula in Orion

Eagle Nebula in M16

“Pickles and Lamaze”• Gravitational Collapse

– A shock wave likely produced by a nearby nova or supernova disturbs the cloud.

– The cloud is no longer in equilibrium.

– Local regions of higher density.– Some of the dust and gas get close

enough to each other that the gravitational force is significant enough that they collide and begin to clump.

– dense cores form– these cores are protostars

• Internal temperature and pressure increases– loss of gravitational energy

results in a gain of kinetic energy and thermal energy

– temperature and pressure at the core increases

– rate of collapse slows down

– continues to contract although at a slower rate

– pre-main-sequence star

“The water breaks!A star is born!”

• fusion– as the star contracts, the

temperature and pressure at the core increase

– high temperature allows fusion to take place

– most common type of fusion at this stage is the proton-proton chain; six hydrogen atoms yield one helium and two hydrogen atoms

– mass is transformed into energy (E=mc2)

• equilibrium– temperature and pressure

increase in the core

– the outward pressure balances the inward gravitational force

– star is in hydrostatic equilibrium

– main-sequence star

“Fat stars die young”

• A greater mass star requires a greater pressure to achieve equilibrium.

• Greater mass stars are thus hotter.• M - L relationship!• The more massive stars “burn” energy (i.e.

convert hydrogen to helium) at a much higher rate.

• More massive stars die younger.

“When the birth goes wrong”

• What if the temperature of the star is not high enough for fusion to begin?– miscarriage: brown dwarf– brown dwarfs are different from planets in how they

form– they have approximately the same mass of large Jovian

planets (gas giants)– hard to detect; looking for lithium is one way– we define a brown dwarf as having mass 10-80 Jupiter

masses

Adolescence to adulthood

• The star is on the main sequence.

• It continues to convert mass to energy by the process of fusion.

• The more massive stars will “burn out” sooner.

• So which stars on the H-R diagram are younger?

H-R Diagram

Two retirement plans

• what happens next depends on the star’s mass• low mass stars (~<10 solar masses when on the main

sequence)– red giant– planetary nebula– white dwarf (<1.4 solar masses)

• high mass stars (~>10 solar masses when on the main sequence)– red giant– Type II supernova– neutron star or black hole

Low mass stars

• evolve from main sequence stars to red giants as it exhausts its hydrogen supply in its core for fusion and subsequently cools

• as it cools, its outer layers expand to form a planetary nebula

• its core contracts until reaching equilibrium• the core is so small and so dense that electrons cannot be

packed closer together• it is a white dwarf; a corpse• stable for M<1.4 solar massses (the Chandrasekhar

limit)

High mass stars

• If at the end of a star’s life, the mass of its core is greater than 1.4 solar masses, the pressure due to the “electron gas” is not great enough to balance gravitation.

• It undergoes further collapse until it reaches a new equilibrium where the pressure of a neutron gas is great enough to counteract gravitation.

• It is a neutron star.• For M > 2 or 3 solar masses, even a neutron gas cannot

withstand the gravitational forces.• For these masses, it becomes a black hole.

Novae

• A binary system of a white dwarf and red giant.• The high gravitational force of the white dwarf attracts

loosely held matter from the outer surface of the giant.• As the matter accretes onto the white dwarf, its

temperature increases.• When fusion begins, the outer layer of the dwarf

explodes.• Process can be repeated over and over.• Luminosity can be 10 or 100 times the luminosity of

the Sun.

Supernovae

• Type I– a white dwarf increases enough mass to exceed 1.4

solar masses– the entire star and core explode– nothing is left

• Type II– death of a massive star (blue or red giant)– core rapidly collapses, mass exceeds 1.4 solar masses– explosion– birth of a neutron star (or pulsar)

Crab Nebula - supernova remnant from 1054 A.D.

SN 1987A

Summary

• Gravitation births stars in clouds

• Gravitation kills massive stars through in supernovae explosions.

• Fusion generates heavier elements.

• Supernovae expel dust and gas back into the interstellar medium, only to form stars again.