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Today: Surveying the Stars & Star Stuff

Lecture 18ISP205 Visions of the Universe

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Stellar properties depend on both mass and age Stars that have finished fusing H to He in their cores are no longer on

the main sequence.

All stars become larger and redder after exhausting their corehydrogen:giants and supergiants.

Most stars end up small and white after fusion has ceased: white dwarfs.

Off the Main Sequence

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Open cluster: A few

thousand looselypacked stars

Star clusters

Globular cluster: Up to

a million or more stars in adense ball bound together

by gravity

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How old are the Pleiades?

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A

B

C

DWhich of these

stars can be nomore than 10million years old?

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The Pleiadesstar clusternow has nostars with a lifeexpectancyless than

around 100million years.

Main-sequence

turnoff

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The main-sequence

turnoff pointof a clustertells us its

age.

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Stellar births, lives and deaths

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Star-Forming Clouds

Stars form in darkclouds of dustygas in interstellarspace

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Gravity Versus Pressure

Gravity within a contracting gas cloud becomesstronger as the gas becomes denser.

Gravity can create stars only if it can overcome theforce of thermal pressure in a cloud.

The cloud can prevent a buildup of thermal pressureby converting thermal energy into infrared and radiophotons that escape the cloud.

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Fragmentation of a Cloud

The simulation begins

with a turbulent gascloud containing 50 Msun

of gas

Random motions in the

cloud cause it to becomelumpy. If gravity can

overcome pressure inthese dense regions, theycan collapse to form evendenser lumps of matter

The large clouds

fragments into manysmaller lumps of matter.

Each lump can go on toform one or more new

stars

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Glowing Dust Grains

As stars begin to form, dust grains that absorb visible light

heat up and emit infrared light.

infrared

visible

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Thought Question

What would happen to a contracting cloud fragment if itwere not able to radiate away its thermal energy?

A. It would continue contracting, but its temperaturewould not change.

B. Its mass would increase.

C. Its internal pressure would increase.

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Cloud heats up as gravitycauses it to contract due to

conservation of energy.Contraction can continue ifthermal energy is radiatedaway.

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A protostar contracts and heatsuntil the core temperature issufficient for hydrogen fusion.

Contraction ends when energyreleased by hydrogen fusionbalances energy radiated from

the surface.

It takes 30 million years for astar like the Sun (less time for

more massive stars).

Protostar to MainSequence

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Protostar to Main Sequence

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A cluster of many stars can form out of a single cloud.

How massive are newborn stars?

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Very massivestars are rare.

Low-mass starsare common.

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Upper Limit on a Stars Mass

Very massive starsare so luminous thatthe collectivepressure of photons

drives their matterinto space.

Observations havenot found starsmore massive thanabout 300MSun.

Pistol Star; Credit: Don F. Figer (UCLA) and NASA

http://www.nasa.gov/http://www.nasa.gov/

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Lower Limit on a Stars Mass

If M > 0.08 Msun : Gravitational contraction heats thecore until fusion begins (Temperature above 107 K).

Energy generated by fusion provides thermalpressureto stop the collapse (star).

If M < 0.08 Msun : Core must collapse to higher

densities to reach temperature for H fusion.Degeneracy pressure stops gravitational collapsebefore the core temperature becomes hot enough forfusion.

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Laws of quantum mechanics prohibit two electrons from

occupying the same state in the same place.

Degeneracy Pressure:

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Thermal Pressure:

Depends on heat content

The main form of pressurein most stars

Degeneracy Pressure:

Particles cant be in same

state in same place

Doesnt depend on heatcontent

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Brown Dwarfs

Starlike objects (M < 0.08 Msun ca. 80 MJupiter) not massive

enough to start fusion are brown dwarfs.

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Brown Dwarfs in Orion

Brown dwarfs emitinfrared lightbecause of heat leftover fromcontraction.

Infraredobservations canreveal recently

formed browndwarfs becausethey are stillrelatively warm

and luminous.

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Stars more

massive than300MSun would

blow apart.

Stars lessmassive than

0.08MSun cantsustain fusion.

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Life stages of a low-mass star (M < 2 Msun)

A star remains on the main sequence as long as it

can fuse hydrogen into helium in its core.

Protostar,

30 millionyears

Yellow main

sequencestar, 10

billion years

Red giant

star, 1billion years

Double shell

fusion redgiant, 30

million years

Planetary

nebula,10,000 years

White

dwarf

Helium core-

fusion star,120 million

years

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Thought Question

What happens when a star can no longer fusehydrogen to helium in its core?

A. Its core cools off. B. Its core shrinks and heats up.C. Its core expands and heats up. D. Helium fusion immediately begins.

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Helium fusion does not begin right away because it

requires higher temperatures than hydrogen fusion( 100 million K vs. 10 million K ) - larger charge leadsto greater repulsion.

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Life Track After Main Sequence

Observations of starclusters show that astar becomes larger,redder, and moreluminous after itstime on the main

sequence is over.

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Broken Thermostat

As the core contracts, Hbegins fusing to He in a shellaround the core.

Energy from H fusion in shellcauses star to expand andcool (red giant phase)

H-burning shell makes moreHe, which is dumped intothe inert core

Core contracts further dueto added weight

H-shell burning accelerates

with time

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Helium Flash

Ignition of He core happens very quickly (helium flash)

The thermostat is broken in a low-mass red giant

because degeneracy pressure supports the core.

The core temperature rises rapidly when heliumfusion begins.

The helium fusion rate skyrockets until thermalpressure takes over and expands the core again.

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Helium core-fusion stars neither shrink nor grow becausethe core thermostat is temporarily fixed.

Helium Core-Fusion Star

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Models show that ared giant should

shrink and becomeless luminous afterhelium fusion beginsin the core.

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Life Track After Helium Flash

Observations of starclusters agree withthese models.

Helium core-fusionstars are found in a

horizontal branch onthe H-R diagram.

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Thought Question

What happens when a stars core runs out of helium?

A. The star explodes. B. Carbon fusion begins. C. The core cools off. D. Helium fuses in a shell around the core.

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Double Shell Fusion

After core helium fusion stops, He fuses into carbon in a shellaround the carbon core, and H fuses to He in a shell around thehelium layer.

Star becomes very cool & luminous, expansing in size (looks bigand red again)

Gravity is very weak at stellar surface, leading to mass lossthrough strong stellar wind

Continuing contraction of the core leads to greater and greaterluminosity. However, never gets hot enough to burn carbon core

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Double shellfusionends with a pulse

that ejects the H andHe into space as aplanetary nebula.

The core left behindbecomes a whitedwarf.

Planetary Nebulae(misleading name)

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White dwarf is verydense

- Suns mass in thesize of Earth

Very hot

- 20,000 K

But not internalenergy generation

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Radiation from whitedwarf ionizes ejected

gas which radiates asplanetary nebula

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End of Fusion

Fusion progresses no further in a low-mass starbecause the core temperature never grows hotenough for fusion of heavier elements (some He

fuses to C to make oxygen).

Degeneracy pressure supports the white dwarfagainst gravity.

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Life Track of a Sun-Like Star