Lecture 11Stellar Evolution

How are stars born?

How do they die?

Exam #2

Avg: 34.9

Median: 34.9

Curve: 45- A 6

35-45 B 9

25-35 C 9

15-25 D 6

0-15 F 0

2 no-show, 1 excused absence

Quiz today.

One question

1. What is your name?

Temperature-Pressure relation

Pressure inside balances Pressure outside

Balloon cools, molecules inside slow down, pressure inside decreases

Balloon shrinks until inside and outside pressures again balance

Hydrostatic Equilibrium

Pressure-Temperature Thermostat

In a star, inward pull of gravity balanced by the internal pressure

As the star loses energy, the T and P would drop, except nuclear fusion is generating just enough energy to maintain the balance If reactions begin to produce too much energy, this extra

energy raises T, which raises P, so star expands, which cools it slightly. This slows the nuclear reactions.

If reactions slow, then inner T drops, lowering P. Gravity compresses the star slightly. Compression of gas raises T & P increasing nuclear fusion rate.

M-L Relation explained

Remember that most massive MS stars are also the most luminous?

Explained by GRAVITATIONAL EQUILIBRIUM

Interstellar Medium

Dark NebulaeDark Cloud

Dark Cloud / Cluster

Interstellar Medium - Gas

• Narrow absorption lines in stellar spectra– Line from the atmosphere of the star are broad due to

“doppler broadening.” (Remember temperature is motion of atoms).

– Cool interstellar gas (not much motion) results in narrow lines.

• Emission nebulae• Usually pink/red because of energies of electrons transitions

Emission Nebulae

Reflection Nebulae

• Look Blue!

Collapse of a Protostar• Stars form from the collapse of “dense” (~1000

atoms/cm3) molecular clouds– Cloud has few 100,000 or a million solar masses of

material– Temperature ~ 10K (COLD!)

• Why do they collapse? GRAVITY– Sitting in gravitational equilibrium, compressed slightly,

gravity takes over!– Converts gravitational potential energy to THERMAL

energy (infalling material heats up)– Cloud fragments as it collapses – each fragment becomes a

PROTOSTAR, emitting radiation because it is hot

Collapsing Interstellar Cloud

From Protostar to Star

• What slows and eventually stops the collapse? PRESSURE – Gas falls in, heats up – As the temperature rises, so does

the pressure!– Three kinds of pressure:

• Thermal pressure (Temp-Pressure related)• Radiation Pressure (due to photons)• Degeneracy Pressure (later)

• When the temperature rises high enough, FUSION (OH) begins, and A STAR IS BORN!– Surrounding gas/dust get blown away

Births of Stars

• Where on the HR diagram do new stars lie?

• THE MAIN SEQUENCE

• “Tracks” follow position of single star during its life (models)

Formation Timescales

• Most massive stars form the most quickly– Gravity collapses the cloud fragment more quickly

in these cases

Evidence of Star Formation Theory

• I should present EVIDENCE to support this theory– Not much time – see text for more details– See objects

that match our

expectations

Hot young stars evaporate surrounding material, revealing the cores where other stars are forming

Stellar Evolution

• “Evolution” means what happens to a star DURING its lifetime – (not over generations of stars)– How can we see this, since we don’t see any single

star evolve significantly during our lifetime?– Observe many different stars of different ages and

try to piece together the story – Like taking a snapshot of the human population

and figuring out how humans age

Use theory to model the evolution

And compare with the observed population

Rules of Stellar Evolution

• Births of stars governed by balance between gravity and pressure

• Structure of Main-Sequence stars governed by the same gravitational equilibrium (OH 81)

• EVERYTHING that happens to a star, from birth to death, is governed by a competition between gravity and pressure!

Modelling Stellar Evolution

• Apply the same rules that we did before to model MS stars– Gravitational Equilibrium– Energy Generation– Energy Transport– Energy conservation

Main Sequence Lifetimes

• Once a star is born, how long does it live on the Main Sequence? (OH: Table 9-2)– Stays on the main sequence while fusing H to He– Eventually, runs out of H in its center (core)– Energy generation changes => Leaves MS– Stars spend 90% of lifetime on MS – More massive stars use up fuel more quickly, so run out of

H FIRST! They spend LESS time on the MS! That’s one reason why there are more dim red MS stars than luminous blue MS stars

ConcepTest discussion

• Would you expect to find intelligent life on planets orbiting hot, blue, luminous main-sequence stars?

Post-Main-Sequence Stars

• What about the other stars on the HR diagram?– These stars have run out H in their cores– Core out of H => He “ash” in core => no energy

generated there => T,P drop– H is still fusing (“burning”) in a shell– Gravity collapses core => T,P rise at center => He

begins to fuse to Carbon

“Post-MS” – He fusion– Core collapses, heats and Helium

fusion begins due to higher Temperature

– Luminosity goes up (larger volume of material involved in fusion)

• Surface temperature increase results in pressure increase

• Since force of gravity (mass) hasn't changed, increased pressure causes outer layers to expand.

• Expansion causes outer layers to cool

• Star gets larger and cooler!

Red Giants

• We now have a bigger, cooler star, so where does it fall on the HR diagram?

• Sun-like stars will be red giants, very massive ones will be red supergiants

Red Giants – Now What?

• Eventually, will run out of He in core!– He burning won’t last as long as H “burning” because He

generates less Energy per unit mass (OH 60)

• MASS controls what happens next!

HST image of Betelgeuse

Fate of “Low-Mass” Stars• Stars like the Sun are considered “low-mass”

• When they run out of He in core:– Left with Carbon “ash”– Core contracts– Outer layers blown away by stellar winds/radiation

pressure– Collapsed core is called a “White Dwarf” (hot,

low-luminosity – more later)– Outer layers called a “Planetary Nebula”

• NOTHING to do with planets!

“Low-Mass” Star: Evolutionary Track

Stars run out of H at center, leave MS and become Red Giants

Run out of He at center, eject outer layers, leaving a hot, small, Carbon core

Planetary Nebulae:Testing Theory

• EVIDENCE: Do we see systems like we expect?

• YES – we see planetary nebulae with hot central stars! This SUPPORTS our theory.

White Dwarves

• Continuing the story of “low-mass” stars like the sun – what is this “white dwarf” that remains?– T,P never rise high enough for Carbon to fuse,

because there’s ANOTHER source of PRESSURE– Degeneracy Pressure - electrons refuse to pack

themselves into a higher volume

Degeneracy Pressure

• Quantum Mechanical Effects important for high density– Pauli Exclusion Principle – no two electrons can occupy

the same state

• Properties– Degenerate material RESISTS compression– Pressure NOT related to temperature (unlike normal gas) –

only depends on E levels– Add mass => increase gravity => material squeezed tighter

=> SHRINKS!– “Chandrasekhar Limit”: Mass = 1.4 solar masses would

imply a radius of 0, so is impossible!

White Dwarves (2)

• Supported against gravity by pressure of “degenerate electrons”

• Can never be more massive than 1.4 solar masses

• Shining because hot, but SMALL, so not very luminous

• Generating NO MORE ENERGY, just cooling

• DENSE: ~ mass of Sun in object ~ size of Earth!– ~1 cubic cm would weigh ~1 ton on Earth

Fate of “High-Mass” Stars

• Remember, evolution of stars depends on MASS – we’ve been discussing the fate of stars like the sun

• High-mass stars go through similar initial stages, but faster (remember M-L relation!)– Runs out of H in core, core collapses until T,P increase

enough for He to fuse, meanwhile outer layers expand and cool => red SUPERgiant

High-Mass vs Low-Mass

Fate of “High-Mass” Stars (2)

• Core becomes hot enough to fuse C and O into higher elements

• As the core runs out of each, it contracts due to gravity, heats up, and begins another round of fusion, resulting in an “onion-like” structure

High-Mass fates (3)

Finally, Si is fused to iron (Fe)– Fe is most tightly

bound nucleus (OH 60)– No reaction (either

fission or fusion) results in energy generation

– Can NO LONGER support itself against GRAVITY!

Death of a High-Mass star

• Core still supported by degenerate electrons• BUT matter is raining down from above• Eventually, core can’t hold itself up any more,

and COLLAPSES (forcing protons + electrons together p++e-n+ν, Energy is carried away by the neutrinos)

• Collapsing core becomes a NEUTRON STAR (held up by neutron degeneracy pressure) or a BLACK HOLE (held up by nothing)

• Envelope blasted apart in a SUPERNOVA

Death of a High-Mass star

• Core still supported by degenerate electrons• BUT matter is raining down from above• Eventually, core can’t hold itself up any more,

and COLLAPSES (forcing protons + electrons together p++e-n+ν, Energy is carried away by the neutrinos)

• Collapsing core becomes a NEUTRON STAR (held up by neutron degeneracy pressure) or a BLACK HOLE (held up by nothing)

• Envelope blasted apart in a SUPERNOVA

Neutron Star

• Core (1.4-3 solar masses) supported by degenerate neutrons

• Young ones have a rapid spin and strong magnetic fields– A beam of light comes out

of the magnetic poles if we're in the beam, we see pulsing. (Pulsar)

Black Hole

• So massive, nothing can hold it up.

• Star collapses to a single point? (Singularity)

• Event horizon, point at which not even light can escape.

• The only things we can find out about a BH are mass, charge and spin.

Life-Cycle of “High-Mass” Star

Comparison of Life Cycles:Low-Mass vs High-Mass

Star Stuff!

• We’ll see that universe began with mostly H, He

• Therefore all “heavy” elements were made in stars prior to (lighter than iron) and during (heavier than iron) explosions

• These were sent out to enrich the ISM by ejection of outer layers (planetary nebulae formation) or supernovae (exploding stars)

• New stars formed from these new IS clouds!

• We are made of dead stars!

Evidence on Origin of Elements

• Stars in older clusters (formed earlier) have fewer “heavy” elements

• Elements generated only in SN (like gold) are rare (as predicted)

• We are made of elements formed in stars! 100Other150

900O5

500N2

2000C3

2He2440

6500H7400

USIn every 10000 atoms

SUN

“Gas to Gas – Dust to Dust”

Massive-Star Supernovae: Theory

• When core mass reaches 1.4 M_sun, core collapses RAPIDLY

• No pressure to hold them up, so outer layers fall in

• They bounce off the dense core, and absorb some of the neutrinos

• BANG!

Massive-Star Supernovae: Theory

Why and How Bang?

• Energy is transferred to the surface layers, accelerating them away from the star

• VERY HOT during explosion, so fusion creates elements that are heavier than Fe

• Gas expands with velocities >~ 10,000 km/s

• When Betelgeuse explodes, it will be > 10x brighter than full moon (tonight to 30,000 years from now)

SN: Predictions of Theory

1) Previously “normal” star suddenly (~few days) becomes MUCH more luminous (~1010 L_sun), fades over months/years

1) Surrounding ISM should be disturbed by exploding matter, disturbance should grow

1) Neutron stars or black holes should be found in or near supernova remnants (SNR)

1) Neutrinos should be emitted during a SN as electrons and protons combine to form neutrons

1) Elements heavier than Fe (iron) should be present in the spectra of supernovae

SN: Observational Support

• 1054 AD: Crab SN seen by Chinese, Native American, African observers– Bright as full moon for several weeks, slowly faded

• Tycho saw one in 1572, Kepler in 1604

• We observe some in other galaxies!– Can rival entire galaxy in brightness for a few weeks

• SN 1993J in M31 – 11 million ly away– Not there one night, there the next (OH S.12)

Supernova Remnants

Disturbed ISM:

Left: Cygnus Loop SNR

Right: Vela SNR

Cygnus Loop SNR in X-rays

Filled with hot gas!

Crab Supernova – 1054 AD

Top: green = synchrotron

(radio) Red=hydrogen emission

lines (optical)

• Bottom:– Radio=red

– Optical=green

– X-ray=blue

Crab Supernova Remnant

• Theory predicts this remnant should be EXPANDING into the surrounding ISM

• Next week we’ll also see there’s a PULSAR in this SNR (a pulsar is a neutron star)

SN1006

SN 1987A – In a nearby galaxy

Originally a 20 M_sun supergiant

SN 1987A – Expanding materialAND neutrino detection!

First confirmation of FIFTY year-old theory!

SN 1987A – Light curve

Matches predictions!

SN 1987A More support for theory

• Saw gamma-rays with particular energies that could only come from short-lived radioactive Cobalt => fusion occurring during SN!

• At IR wavelengths, saw emission lines of freshly made cobalt, nickel, etc.

TESTING Stellar Evolution Models

• Evolution Models PREDICT:– Least massive stars take longest to form

– Most massive stars leave MS first

– Stars become red giants or supergiants after they leave the MS

– Some low and medium-mass stars become WDs

• How to test? – Watch sun for 10 billion yrs? IMPRACTICAL!

– Look at sets of stars that are ALL THE SAME AGE!

Star Clusters – Testing our Model

• Giant Molecular Cloud fragmented and collapsed into MANY stars!– All these formed at ~ same time in ~ same place

=> ~ same AGE and DISTANCE

• What do our models predict for a population of stars of varying masses, but all the same age?

Star Clusters – Predictions

• YOUNG CLUSTER PREDICTIONS:

– Lowest mass stars not yet on MS (not fusing H yet)

– MOST stars still on MS (not run out of H yet)

– Luminous stars are BLUE (not yet time to evolve red giants, WDs)

• OLD CLUSTER PREDICTIONS

– High-mass stars have left MS (run out of H)

– Only low-mass stars still on MS

– Lots of red giants, supergiants, WDs

Estimating Ages

• Theories predicted that some clusters should have HR diagrams like our prediction for “young” clusters, while others should match the “old” cluster predictions.

• Since this is the case, we can use the HR diagrams to estimate the AGES of stars in clusters!

Star Clusters - Observations

• Observations Match Predictions – HR diagram of observed

old cluster

Open Clusters Young, mostly MS

stars (no RG/WD)

Pleiades

Globular Clusters

• Old, no massive MS stars