Chapter 20: The Birth of Stars and the Discovery of Planets Outside the Solar System

February 14, 2006February 14, 2006 Astronomy 2010Astronomy 2010 11

Chapter 20: Chapter 20: The Birth The Birth of Stars of Stars and the and the Discovery Discovery of Planets of Planets Outside the Outside the Solar Solar SystemSystem


Questions About Star FormationQuestions About Star FormationAre new stars still being created, or did creation Are new stars still being created, or did creation cease billions of years ago?cease billions of years ago?

Where are new stars being created?Where are new stars being created?

Are planets a natural result of star formation or Are planets a natural result of star formation or is our solar system unique in the universe?is our solar system unique in the universe?

How can we observe planets around distant How can we observe planets around distant stars?stars?


Basics About Stars (Table 20.1)Basics About Stars (Table 20.1)Stable (main-sequence) stars maintain equilibrium by Stable (main-sequence) stars maintain equilibrium by producing energy through nuclear fusion in their cores. producing energy through nuclear fusion in their cores. Generating energy by fusion defines a star. Generating energy by fusion defines a star.

Hydrogen is being converted to helium, but eventually Hydrogen is being converted to helium, but eventually the supply of hydrogen will run out.the supply of hydrogen will run out.

Stars range in mass from about 1/12 MStars range in mass from about 1/12 Msunsun to 200 to 200 Msun. Low mass stars are more common.

For main sequence stars, mass and luminosity are related such that high mass stars have high luminosity and low mass stars have low luminosity.

Galaxies, like the Milky Way, contain enough gas and Galaxies, like the Milky Way, contain enough gas and dust to create billions of new stars.dust to create billions of new stars.


Life Cycle of Stars: Life Cycle of Stars: from Birth to Maturityfrom Birth to Maturity

Stage 1Stage 1: Giant Molecular Cloud – : Giant Molecular Cloud – cold dust clouds in spacecold dust clouds in spaceClumps (dust bunnies) accrete matter from cloud to form Clumps (dust bunnies) accrete matter from cloud to form

protostarprotostar

Stage 2Stage 2: Protostar – : Protostar – energy generated by gravitational collapseenergy generated by gravitational collapse

Stage 3Stage 3: Wind formation – : Wind formation – protostarprotostar produces produces strong solar windsstrong solar winds winds eject much of the surrounding cocoon gas and dustwinds eject much of the surrounding cocoon gas and dustwinds blow mostly along the rotation axeswinds blow mostly along the rotation axes

Stage 4Stage 4: Main Sequence -- : Main Sequence -- the new star becomes stable the new star becomes stable Equilibrium: hydrogen fusion into helium Equilibrium: hydrogen fusion into helium in the core in the core balances balances

gravity. gravity. Stage continues until most of the hydrogen in the core is used Stage continues until most of the hydrogen in the core is used

up.up.


Stage 1: Giant Molecular Cloud Stage 1: Giant Molecular Cloud

giant molecular cloudgiant molecular cloud

large, dense gas cloud with dust large, dense gas cloud with dust

cold enough for molecules to form cold enough for molecules to form

thousands of giant molecular clouds in the thousands of giant molecular clouds in the galactic disk galactic disk

each giant molecular cloud contains vast each giant molecular cloud contains vast amount of material for amount of material for star formationstar formation

about one million solar massesabout one million solar masses


Star Formation (Reference Slide)Star Formation (Reference Slide)matter in part of giant molecular cloud begins to collapsematter in part of giant molecular cloud begins to collapse

tens to hundreds of solar massestens to hundreds of solar massescollapse can start by itself if matter is cool and massive collapse can start by itself if matter is cool and massive enough enough shock waves can trigger collapse by compressing the shock waves can trigger collapse by compressing the gas clouds into clump gas clouds into clump

the explosion of a nearby massive star – supernova the explosion of a nearby massive star – supernova gravity from nearby stars or groups of starsgravity from nearby stars or groups of stars

gravity pulls more matter to form sufficiently massive gravity pulls more matter to form sufficiently massive clumps clumps whatever the reason, the result is the same: gas clumps whatever the reason, the result is the same: gas clumps compress to become compress to become protostarsprotostars


Eagle Eagle Nebula Nebula M16M16

nearby star shinesnearby star shines

light on gas cloudlight on gas cloud

revealing protostarrevealing protostar

formationformation



Evaporating Gas GlobulesEvaporating Gas Globules


giant molecular cloudgiant molecular cloud

shock wave creates clumps in giant shock wave creates clumps in giant molecular cloudmolecular cloud

clusters: many stars forming clusters: many stars forming simultaneouslysimultaneously


Stage 2: ProtostarsStage 2: Protostarsgas clump collapses and heats gas clump collapses and heats up as gas particles collideup as gas particles collide

gravitational energy is converted to gravitational energy is converted to heat energyheat energy

heated clump produces infrared heated clump produces infrared and microwave radiation and microwave radiation at this stage the warm clump is at this stage the warm clump is called a called a protostarprotostarRotating gas clump forms a disk Rotating gas clump forms a disk with the protostar in the center with the protostar in the center other material in the disk may other material in the disk may coalesce to form another star or coalesce to form another star or planetsplanets


Behind the visible Behind the visible part of the Orion part of the Orion Nebula is a much Nebula is a much denser region of denser region of gas and dust that is gas and dust that is cool enough for cool enough for molecules to form.molecules to form.

Many stars are Many stars are now forming inside now forming inside it.it.


trapesizium cluster:

•stars that provide much of the energy which makes the brilliant Orion Nebula visible

•other stars obscured by nebula


Observation of protostarsObservation of protostarsInfrared detectors Infrared detectors enable observation of enable observation of protostars.protostars.

Many stars forming in Many stars forming in the Nebula above and the Nebula above and to the right of the to the right of the Trapezium stars. Trapezium stars.

They can only be seen They can only be seen in the infrared image. in the infrared image.

Images are from the Hubble Space Telescope

Visible Infrared


ProtostarProtostargravity pulls more gravity pulls more matter into clumpmatter into clump

energy from falling energy from falling matter creates heatmatter creates heat

protostar forms as protostar forms as hot matter begins to hot matter begins to glow in infraredglow in infrared

protostar surrounded protostar surrounded by "cocoon" of dust by "cocoon" of dust

matter falling into a matter falling into a rotating star tends to rotating star tends to pile up in a diskpile up in a disk




Social StarsSocial StarsYoung stars seem to be socialYoung stars seem to be social

Fragmentation of the giant molecular cloud Fragmentation of the giant molecular cloud produces protostars that form at about the same produces protostars that form at about the same time. time.

Stars are observed to be born in clusters.Stars are observed to be born in clusters.

Other corroborating evidence for this is that there Other corroborating evidence for this is that there are no isolated young stars. are no isolated young stars.

This observation is important because a valuable This observation is important because a valuable test of the stellar evolution models is the test of the stellar evolution models is the comparison of the models with star clusters. comparison of the models with star clusters.


Stage 3: Wind FormationStage 3: Wind Formationstrong stellar winds strong stellar winds

winds eject much of winds eject much of the surrounding gas the surrounding gas and dustand dust

Winds constrained to Winds constrained to flow preferentially flow preferentially along the rotation along the rotation axesaxes

With most of the With most of the cocoon gas blown cocoon gas blown away, the forming away, the forming star finally becomes star finally becomes visiblevisible

wind

wind

proto-planetarydisk

JetsJets


Jets from Stellar WindJets from Stellar Windgravitational gravitational contraction contraction continuescontinues

eventually enough eventually enough energy for stellar energy for stellar wind to form jetswind to form jets

jets blow away jets blow away cocooncocoon

fusion begins at end fusion begins at end of this stage of this stage

star reaches zero star reaches zero age main sequence age main sequence when fusion startswhen fusion starts




Stage 4: Main Sequence Stage 4: Main Sequence We define the star’s arrival on the main We define the star’s arrival on the main sequence as the time when fusion begins.sequence as the time when fusion begins.

Eventually becomes stable because Eventually becomes stable because hydrostatic equilibriumhydrostatic equilibrium is established. is established.

It settles down to spend about 90% of its life as It settles down to spend about 90% of its life as a a main sequencemain sequence star. star.

Fusing hydrogen to form helium Fusing hydrogen to form helium in the core.in the core.

http://www.astronomynotes.com/starsun/strsunc.htm#A5.3

•zero age main sequence •point at which star begins fusing hydrogen into helium.•moving to left – temperature is increasing

evolution to main sequenceevolution to main sequence

ages of forming stars in years as they grow towards main sequence

mass determines position on main sequence

evolution to main sequenceevolution to main sequence


Life Cycle of Stars: from Birth to Maturity (Recap)Life Cycle of Stars: from Birth to Maturity (Recap)

Stage 1Stage 1: Giant Molecular Cloud – : Giant Molecular Cloud – cold dust clouds in spacecold dust clouds in spaceClumps (dust bunnies) accrete matter from cloud to form Clumps (dust bunnies) accrete matter from cloud to form

protostarprotostar

Stage 2Stage 2: Protostar – : Protostar – energy generated by gravitational collapseenergy generated by gravitational collapse

Stage 3Stage 3: Wind formation – : Wind formation – protostarprotostar produces produces strong solar windsstrong solar winds winds eject much of the surrounding cocoon gas and dustwinds eject much of the surrounding cocoon gas and dustwinds blow mostly along the rotation axeswinds blow mostly along the rotation axes

Stage 4Stage 4: Main Sequence -- : Main Sequence -- the new star becomes stable the new star becomes stable Equilibrium: hydrogen fusion into helium Equilibrium: hydrogen fusion into helium in the core in the core balances balances

gravity. gravity. Fusion continues until most of the hydrogen in the core is used Fusion continues until most of the hydrogen in the core is used

up.up.


Summary of Birth Process Summary of Birth Process


•ages of forming stars in years as they grow towards main sequence•zero age main sequence – ZAMS •point at which star begins generating energy by fusion

Evolution to Main SequenceEvolution to Main Sequence

time to reach main sequence stage

short for big stars•as low as 10000 years

long for little stars•up to 100,000,000 years for low mass


0

100

200

300

400

500

600

0 1 2 3 4 5 6 7 8 9 10

height (feet)

weight (pounds)

HR Diagram: Analogy to Weight vs Height for People


0

50

100

150

200

250

300

350

0 1 2 3 4 5 6 7

height (feet)

weight (pounds)

Weight and Height changes as Age increases (Marlin Brando)


0

50

100

150

200

250

300

0 1 2 3 4 5 6 7

height (feet)

weight (pounds)

Brando

Allen

Different paths for different body types (Woody Allen)


20.3 Evidence that Planets Form 20.3 Evidence that Planets Form Around Other StarsAround Other Stars

It is hard to see a planet orbiting another star.It is hard to see a planet orbiting another star.Look for a disk of material before it clumps to form Look for a disk of material before it clumps to form planets -- big disk is more visible than small planet.planets -- big disk is more visible than small planet.

Look for evolution of disks -- evidence for clumping Look for evolution of disks -- evidence for clumping into planets.into planets.


proto-planetarydisk


Proto-planetary DisksProto-planetary Disks


Dust Ring Around a Young StarDust Ring Around a Young Star


Disk Around Epsilon ErdaniDisk Around Epsilon Erdani


20.4 Planets Beyond the Solar 20.4 Planets Beyond the Solar System: Search and DiscoverySystem: Search and Discovery

If we can’t directly observe planets, can we If we can’t directly observe planets, can we indirectly observe them?indirectly observe them?

3 methods have succeeded.3 methods have succeeded.

First method: doppler shiftFirst method: doppler shiftA planet must orbit its star to be stable.A planet must orbit its star to be stable.Search for the effect of the planet’s orbit on the star.Search for the effect of the planet’s orbit on the star.Both planet and star orbit around a common Both planet and star orbit around a common center center of massof mass..The star “wobbles” a bit as the planet orbits it.The star “wobbles” a bit as the planet orbits it.The wobble has the same period as the planet’s The wobble has the same period as the planet’s orbit.orbit.


Search for Doppler ShiftSearch for Doppler Shift


Velocity from measuredDoppler Shift vs. time --shows the star’s orbit about unseen partner


Second Method to Find PlanetsSecond Method to Find PlanetsLook for a small reduction of star light when an Look for a small reduction of star light when an orbiting planet moves between us and the star.orbiting planet moves between us and the star.

Works only when planet’s orbit is lined up properly.Works only when planet’s orbit is lined up properly.

Will block all visible wavelengths -- this is a cross check.Will block all visible wavelengths -- this is a cross check.

February 14, 2006

Astronomy 2010 46

Third Method to Find Planets

• Measure infrared (thermal) radiation of “hot” planet.


As of June As of June 2005, more than 2005, more than 155 extrasolar 155 extrasolar planets found.planets found.

Systems of 2, 3, Systems of 2, 3, and possibly and possibly more planets more planets are seen.are seen.

Masses are Masses are measured in measured in Jupiter-masses.Jupiter-masses.

DiscoveredDiscovered PlanetsPlanets


20.4.3 Explaining the Planets Seen20.4.3 Explaining the Planets SeenNow that we have a large sample of planetary Now that we have a large sample of planetary systems, astronomers can refine their models systems, astronomers can refine their models of planet formation.of planet formation.

Almost all the planets are Jupiter-sized, and Almost all the planets are Jupiter-sized, and many have highly eccentric orbits close to their many have highly eccentric orbits close to their star. This is a surprise and is hard for the early star. This is a surprise and is hard for the early models to explain.models to explain.

The formation of planetary systems is more The formation of planetary systems is more complex and chaotic than we thought.complex and chaotic than we thought.


Planet Mass DistributionPlanet Mass DistributionNot many brown-dwarf sized planets (M>10MJup).

Jupiter-sized planets are common.


Eccentric Orbits Are CommonEccentric Orbits Are Common


Planets and Star “Metallicity”Planets and Star “Metallicity”

Planets are more common around stars with more heavy elements (“metallicity”).

Documents

Chapter 20: The Birth of Stars and the Discovery of Planets Outside the Solar System