Gravity and Heat Stellar Evolution -...

StellarEvolution

The Formationof Stars Likethe SunGravity and Heat

Stars of OtherMasses

Star Clusters

Leaving theMainSequence

Evolution of aSun-like Star

The Death ofa Low-MassStar

Evolution ofStars MoreMassive thanthe Sun

SupernovaExplosions

Stellar Evolution

The Lives And Deaths of Stars

10/29/2009

My Office Hours:Tuesday 3:30 PM - 4:30 PM206 Keen Building

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SupernovaExplosions

Test 2: 11/05/2009

Now is a good time to startreviewing.

• Use the study guides!• Look again at homeworks

and miniquizzes!

1 Test is mandatory2 Covers chapters 7-123 Consists of 35 questions

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Outline

1 The Formation of Stars Like the SunGravity and HeatStars of Other MassesStar Clusters

2 Leaving the Main Sequence

3 Evolution of a Sun-like Star

4 The Death of a Low-Mass Star

5 Evolution of Stars More Massive than the Sun

6 Supernova Explosions

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Stage 1: An Interstellar Cloud

Interstellar cloud starts to contract, probably triggered byshock or pressure wave from nearby star. As it contracts,the cloud fragments into smaller pieces.

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Stages 2 and 3: A ContractingCloud Fragment

Stage 2

Individual cloud fragments begin to collapse. Once thedensity is high enough, there is no further fragmentation.

Stage 3

The interior of the fragment has begun heating, and isabout 10,000 K.

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The Orion Nebula is thoughtto contain interstellar cloudsin the process of condensing,as well as protostars.

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The Orion Nebula is thoughtto contain interstellar cloudsin the process of condensing,as well as protostars.

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Stage 4: A Protostar

Some 100,000 years afterfragment formed, it reachesstage 4:

• Nuclear reactions havenot yet begun.

The core of the cloud is nowa protostar, and makes itsfirst appearance on the H-Rdiagram.

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Planetary formation has begun, but theprotostar is still not in equilibrium All heating comes from gravitational

collapse.

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Stage 5: Protostellar Evolution

Last stages can be followedon the H-R diagram:

The protostar’s luminosityfinally decreases even as itstemperature rises because itis becoming more compact.

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Stage 6: A Newborn Star

At stage 6, the core reaches10 million K: nuclear fusionbegins. The protostar hasbecome a star.

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Stage 7: A Newborn Star

Star continues to contractand increase in temperature,until it is in equilibrium: thestar has reached the MainSequence and will remainthere as long as it has H tofuse in its core.

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Prestellar Evolutionary Tracks

This H-R diagram shows theevolution of stars somewhatmore and somewhat lessmassive than the Sun. Theshape of the paths is verysimilar, but they wind up indifferent places on the MainSequence.

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Binary Systems: Gliese 623

If the mass of the original nebular fragment is too small, nuclearfusion will never begin. These “failed stars” are called browndwarfs.

Below: Hubble image of Gliese 623. This binary system maycontain a brown dwarf.

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Newborn Cluster: NGC 3603

Because a single interstellar cloud can produce many stars ofthe same age and composition, star clusters are an excellentway to study the effect of mass on stellar evolution.

The cluster contains≈ 2000 bright stars.

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Open Cluster: Pleiades or M45

This is a young star cluster – The Pleiades. The H-R diagramof its stars is on the right. This is an example of an open cluster.

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Globular Cluster: Ω Centauri

This is a globular cluster. Note the absence of massive MainSequence stars, and the heavily populated Red Giant region.

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Young Stars in Orion

These images are believedto show a star cluster in theprocess of formation withinthe Orion nebula.

Visible image dominated by emissionnebula. However, IF image shows anextensive star cluster.

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Star Cluster NGC 346

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Protostellar Collisions

The presence of massive,short-lived O (T ≈ 30, 000 K)and B (T ≈ 20, 000 K) starscan profoundly affect their starcluster, as they can blow awaydust and gas before it has timeto collapse.

This is a simulation of such acluster.

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Equilibrium

During its stay on the MainSequence (steadily burning),any fluctuations in a star’scondition are quickly restored Hydrostatic Equilibrium.

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Equilibrium

Eventually, as hydrogen inthe core is consumed, thestar begins to leave theMain Sequence.

Its evolution from then ondepends very much on themass of the star:

1 Low-mass starsleave quietly.

2 High-mass stars goout with a bang!

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Solar Composition Change

Even while it is on the MainSequence, the compositionof a star’s core is changing.

The change speeds up as thenuclear burning rate increaseswith time.

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Hydrogen Shell Burning

As the fuel in the core is used up, the core contracts;when it’s used up, the core begins to collapse.

The hydrogen begins tofuse outside the core:

1 Core has shrunk to afew tens of thousandsof kilometers

2 Star’s surface is tentimes the original size

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Evolution of a Sun-like Star

Stages of a star leaving the Main Sequence:

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Stage 9: The Red-Giant Branch

As the core continues toshrink, the outer layers ofthe star expand and cool.

The star is a red giant now,extending out as far as theorbit of Mercury.

Despite its slightly coolertemperature, its luminosityincreases enormously dueto its large size.

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Stage 10: Helium Fusion

Once the core temperaturehas risen to 100,000,000 K,the helium in the core startsto fuse.

Helium Flash:

The helium begins to fuseextremely rapidly; withinhours the enormous energyoutput is over, and the staragain reaches equilibrium.

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Back to the Giant Branch

As the helium in the core fuses to carbon, the core becomeshotter and hotter, and the helium burns faster and faster.

Stage 11:

The star is now similar toits condition just as it leftthe Main Sequence, exceptnow there are two shells.

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Stage 11: Reascending theGiant Branch

The star has become a redgiant for the second time:

Lack of nuclear burning atthe center causes the coreto contract and overlyinglayers to expand.

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G-type Star Evolution

This graphic shows the entire evolution of a Sun-like star:

Such stars never become hot enough for fusion pastcarbon to take place.

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The Death of a Low-Mass Star

There is no more outward fusion pressure being generatedin the core, which continues to contract.

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Planetary Nebulae

Meanwhile, the outer layers of the star expand to form aplanetary nebula:

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The Death of a Low-Mass Star

The star has now two parts:

1 A small, extremely dense carbon core2 An envelope about the size of our solar system

The envelope is called a planetary nebula, even thoughit has nothing to do with planets – early astronomersviewing the fuzzy envelope thought that it resembled aplanetary system.

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White Dwarf on H-R Diagram

Stages 13 and 14:

Once the nebula hasgone, the remaining coreis extremely dense andextremely hot, but quitesmall (size of Earth).

It’s luminous only due toits high temperature.

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Sirius Binary System

The small star Sirius B isa white-dwarf companionof the much larger andbrighter Sirius A.

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Sirius Binary System

The small star Sirius B isa white-dwarf companionof the much larger andbrighter Sirius A.

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Distant White Dwarfs

The Hubble Space Telescope has detected white dwarfstars (circled) in globular clusters:

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White Dwarfs

As the white dwarf cools, its size does notchange significantly; it simply gets dimmerand dimmer, and finally ceases to glow.

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Nova

A nova is a star that flares up very suddenly and then returnsslowly to its former luminosity. These novae are the result ofexplosions on the surface of faint white dwarf stars, caused bymatter falling onto their surfaces from the atmosphere of largerbinary companions.

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Close Binary System

A white dwarf that is part of a semidetached binary system canundergo repeated novas:

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Nova Matter Ejection

Material falls onto the whitedwarf from its main-sequencecompanion.

When enough material hasaccreted, fusion can reignitevery suddenly, burning off thenew material.

Material keeps being trans-ferred to the white dwarf, andthe process repeats.

Nova Persei (a) brightened by afactor of 40,000 in 1901.

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Nova Matter Ejection

Material falls onto the whitedwarf from its main-sequencecompanion.

When enough material hasaccreted, fusion can reignitevery suddenly, burning off thenew material.

Material keeps being trans-ferred to the white dwarf, andthe process repeats.

Nova Cygni (b) is more than10,000 light-years away (1992).

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High-Mass Evolutionary Tracks

This H-R diagram showsthat stars more massivethan the Sun follow verydifferent paths when theyleave the Main Sequence:

• Mstar > 2.5 M⊙

No helium flash,helium burningstarts gradually

• Mstar ≥ 4 M⊙

No sharp moveson H-R diagram; itmoves smoothlyback and forth

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Heavy-Element Fusion

High-mass stars, like all stars, leave the Main Sequencewhen there is no more hydrogen fuel in the cores.

The first few events arevery similar to those inlower-mass stars:

1 First a hydrogen shell

2 Then a core burninghelium to carbon,surrounded by helium-and hydrogen-burningshells.

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Mass Loss from Supergiants

(a) Eta Carinae is one of the most massive and most luminousstars known. An HST image shows blobs of ejected materialracing away from the star at hundreds of kilometers per sec.(b) Star V838 Monocerotis (poorly understood red supergiant)illuminates shells of gas and dust expelled long ago.

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The End of the Road

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Supernova

A star of more than 8 solar masses can fuse elements farbeyond carbon in its core, leading to a very different fate.

Its path across the H-R diagram is essentially a straightline – it stays at just about the same luminosity as it coolsoff.

Eventually the star dies in a violent explosion called asupernova.

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Supernova Light Curves

A supernova is incredibly luminous, as can be seen from thesecurves – more than a million times as bright a nova.

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Supernova 1987A

A supernova is a one-time event – once it happens, thereis little or nothing left of the progenitor star.

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Two Types of Supernova

1 Carbon-detonation supernova

2 Death of a high-mass star: core collapses, then rebounds

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Carbon-Detonation Supernova

A white dwarf can accumulate too much mass from itsbinary companion. If the white dwarf’s mass exceeds1.4 solar masses, electron degeneracy can no longerkeep the core from collapsing:

Then carbon fusion begins throughout the star almostsimultaneously, resulting in a carbon explosion.

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Carbon-Detonation Supernova

Supernovae leave remnants – the expanding cloudsof material from the explosion.

The Crab nebula isa remnant from asupernova explosionthat occurred in theyear 1054.

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Stellar Recycling

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