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Chapter 20: The Birth of Stars and the Discovery of Planets Outside the Solar System. Questions About Star Formation. Are new stars still being created, or did creation cease billions of years ago? Where are new stars being created?
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Chapter 20: The Birth of Stars and the Discovery of Planets Outside the Solar System Astronomy 2010
Questions About Star Formation • Are new stars still being created, or did creation cease billions of years ago? • Where are new stars being created? • Are planets a natural result of star formation or is our solar system unique in the universe? • How can we observe planets around distant stars? Astronomy 2010
Basics About Stars (Table 20.1) • Stable (main-sequence) stars maintain equilibrium by producing energy through nuclear fusion in their cores. Generating energy by fusion defines a star. • Hydrogen is being converted to helium, but eventually the supply of hydrogen will run out. • Stars range in mass from about 1/12 Msun 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 dust to create billions of new stars. Astronomy 2010
Life Cycle of Stars: from Birth to Maturity Stage 1: Giant Molecular Cloud – cold dust clouds in space Clumps (dust bunnies) accrete matter from cloud to form protostar Stage 2: Protostar – energy generated by gravitational collapse Stage 3: Wind formation – protostarproduces strong solar winds winds eject much of the surrounding cocoon gas and dust winds blow mostly along the rotation axes Stage 4: Main Sequence -- the new star becomes stable Equilibrium: hydrogen fusion into helium in the core balances gravity. Stage continues until most of the hydrogen in the core is used up. Astronomy 2010
Stage 1: Giant Molecular Cloud • giant molecular cloud • large, dense gas cloud with dust • cold enough for molecules to form • thousands of giant molecular clouds in the galactic disk • each giant molecular cloud contains vast amount of material for star formation • about one million solar masses Astronomy 2010
Star Formation (Reference Slide) • matter in part of giant molecular cloud begins to collapse • tens to hundreds of solar masses • collapse can start by itself if matter is cool and massive enough • shock waves can trigger collapse by compressing the gas clouds into clump • the explosion of a nearby massive star – supernova • gravity from nearby stars or groups of stars • gravity pulls more matter to form sufficiently massive clumps • whatever the reason, the result is the same: gas clumps compress to become protostars Astronomy 2010
Eagle Nebula M16 nearby star shines light on gas cloud revealing protostar formation Astronomy 2010
giant molecular cloud • shock wave creates clumps in giant molecular cloud • clusters: many stars forming simultaneously Astronomy 2010
Stage 2: Protostars • gas clump collapses and heats up as gas particles collide • gravitational energy is converted to heat energy • heated clump produces infrared and microwave radiation • at this stage the warm clump is called a protostar • Rotating gas clump forms a disk with the protostar in the center • other material in the disk may coalesce to form another star or planets Astronomy 2010
Behind the visible part of the Orion Nebula is a much denser region of gas and dust that is cool enough for molecules to form. Many stars are now forming inside it. Astronomy 2010
trapesizium cluster: • stars that provide much of the energy which makes the brilliant Orion Nebula visible • other stars obscured by nebula Astronomy 2010
Observation of protostars Visible Infrared Images are from the Hubble Space Telescope • Infrared detectors enable observation of protostars. • Many stars forming in the Nebula above and to the right of the Trapezium stars. • They can only be seen in the infrared image. Astronomy 2010
Protostar • gravity pulls more matter into clump • energy from falling matter creates heat • protostar forms as hot matter begins to glow in infrared • protostar surrounded by "cocoon" of dust • matter falling into a rotating star tends to pile up in a disk Astronomy 2010
Social Stars Young stars seem to be social • Fragmentation of the giant molecular cloud produces protostars that form at about the same time. • Stars are observed to be born in clusters. • Other corroborating evidence for this is that there are no isolated young stars. • This observation is important because a valuable test of the stellar evolution models is the comparison of the models with star clusters. Astronomy 2010
Stage 3: Wind Formation • strong stellar winds • winds eject much of the surrounding gas and dust • Winds constrained to flow preferentially along the rotation axes • With most of the cocoon gas blown away, the forming star finally becomes visible wind proto-planetary disk wind Astronomy 2010
Jets from Stellar Wind • gravitational contraction continues • eventually enough energy for stellar wind to form jets • jets blow away cocoon • fusion begins at end of this stage • star reaches zero age main sequence when fusion starts Astronomy 2010
Stage 4: Main Sequence • We define the star’s arrival on the main sequence as the time when fusion begins. • Eventually becomes stable because hydrostatic equilibrium is established. • It settles down to spend about 90% of its life as a main sequence star. • Fusing hydrogen to form helium in the core. Astronomy 2010
evolution to main sequence • zero age main sequence • point at which star begins fusing hydrogen into helium. • moving to left – temperature is increasing
evolution to main sequence • ages of forming stars in years as they grow towards main sequence • mass determines position on main sequence
Life Cycle of Stars: from Birth to Maturity (Recap) Stage 1: Giant Molecular Cloud – cold dust clouds in space Clumps (dust bunnies) accrete matter from cloud to form protostar Stage 2: Protostar – energy generated by gravitational collapse Stage 3: Wind formation – protostarproduces strong solar winds winds eject much of the surrounding cocoon gas and dust winds blow mostly along the rotation axes Stage 4: Main Sequence -- the new star becomes stable Equilibrium: hydrogen fusion into helium in the core balances gravity. Fusion continues until most of the hydrogen in the core is used up. Astronomy 2010
Summary of Birth Process Astronomy 2010
Evolution to Main Sequence • 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 Astronomy 2010
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
HR Diagram: Analogy to Weight vs Height for People Astronomy 2010
Weight and Height changes as Age increases (Marlin Brando) Astronomy 2010
Different paths for different body types (Woody Allen) Astronomy 2010
20.3 Evidence that Planets Form Around Other Stars • It is hard to see a planet orbiting another star. • Look for a disk of material before it clumps to form planets -- big disk is more visible than small planet. • Look for evolution of disks -- evidence for clumping into planets. Astronomy 2010
proto-planetary disk Astronomy 2010
Proto-planetary Disks Astronomy 2010
Dust Ring Around a Young Star Astronomy 2010
Disk Around Epsilon Erdani Astronomy 2010
20.4 Planets Beyond the Solar System: Search and Discovery • If we can’t directly observe planets, can we indirectly observe them? • 3 methods have succeeded. • First method: doppler shift • A planet must orbit its star to be stable. • Search for the effect of the planet’s orbit on the star. • Both planet and star orbit around a common center of mass. • The star “wobbles” a bit as the planet orbits it. • The wobble has the same period as the planet’s orbit. Astronomy 2010
Search for Doppler Shift Astronomy 2010
Velocity from measured Doppler Shift vs. time --shows the star’s orbit about unseen partner Astronomy 2010
Second Method to Find Planets • Look for a small reduction of star light when an orbiting planet moves between us and the star. • Works only when planet’s orbit is lined up properly. • Will block all visible wavelengths -- this is a cross check. Astronomy 2010
Third Method to Find Planets • Measure infrared (thermal) radiation of “hot” planet. Astronomy 2010
DiscoveredPlanets • As of June 2005, more than 155 extrasolar planets found. • Systems of 2, 3, and possibly more planets are seen. • Masses are measured in Jupiter-masses. Astronomy 2010
20.4.3 Explaining the Planets Seen • Now that we have a large sample of planetary systems, astronomers can refine their models of planet formation. • Almost all the planets are Jupiter-sized, and many have highly eccentric orbits close to their star. This is a surprise and is hard for the early models to explain. • The formation of planetary systems is more complex and chaotic than we thought. Astronomy 2010
Planet Mass Distribution Not many brown-dwarf sized planets (M>10MJup). Jupiter-sized planets are common. Astronomy 2010
Eccentric Orbits Are Common Astronomy 2010
