29 March 2005 AST 2010: Chapter 20 1 The Birth of Stars & the Discovery of Planets Outside the Solar System

29 March 2005AST 2010: Chapter 202 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?

29 March 2005AST 2010: Chapter 203 Basics about Stars (Table 20.1) Stable (main-sequence) stars maintain equilibrium by producing energy through nuclear fusion in their cores The ability to generate energy by fusion defines a star Each second in the Sun, about 600 million tons of hydrogen undergo fusion into helium, with about 4 million tons turning to energy in the process This rate of hydrogen use means that eventually the Sun (and all other stars) will run out of central fuel Stars’ masses range from 1/12 M Sun to 200 M Sun Low-mass stars are far more common For main-sequence stars, the most massive (spectral type O) are also the most luminous and have the highest surface-temperature, whereas the least massive (spectral type M or L) are the least luminous and the coolest A galaxy of stars, such as the Milky Way, contains enormous amounts of gas and dust, enough to make billions of stars like the Sun

29 March 2005AST 2010: Chapter 204 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 – protostar produces 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

29 March 2005AST 2010: Chapter 205 Molecular Clouds Vast clouds of gas dot the Milky Way A giant molecular cloud is a large, dense cloud of gas cold enough (10-20 K) for molecules to form Each giant molecular cloud contains a vast amount of material from which a star may be assembled The cloud may have a mass as large as 3 million times the Sun’s mass Within the clouds are lumps, regions of high density and low temperature These conditions are believed to be just what is required to make new star The combination of high density and low temperature makes it possible for gravity to overcome pressure

29 March 2005AST 2010: Chapter 206 Star Formation (Reference Slide) matter in part of giant molecular cloud begins to collapse tens to hundreds of solar masses 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

29 March 2005AST 2010: Chapter 207 Pillars of high-density dust in the central regions of the M16 nebula

29 March 2005AST 2010: Chapter 208 Dense Globules in M16 Nebula One of the pillars in M16 appears to have very dense globules These structures have been termed evaporating gas globules (e.g.g.s) They may harbor embryonic stars

Evaporating Gas Globules

29 March 2005AST 2010: Chapter 2011 Giant Molecular Cloud A shock wave creates clumps in a giant molecular cloud clusters: many stars forming simultaneously

29 March 2005AST 2010: Chapter 2012 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

29 March 2005AST 2010: Chapter 2013 trapesizium cluster: stars that provide much of the energy which makes the brilliant Orion Nebula visible other stars obscured by nebula

29 March 2005AST 2010: Chapter 2014 Observation of protostars Infrared detectors enable observation of protostars Many stars appear to be forming in the Nebula above and to the right of the Trapezium stars They can only be seen in the infrared image Images are from the Hubble Space Telescope VisibleInfrared

29 March 2005AST 2010: Chapter 2015 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

29 March 2005AST 2010: Chapter 2016

29 March 2005AST 2010: Chapter 2017 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

29 March 2005AST 2010: Chapter 2018 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

Jets

29 March 2005AST 2010: Chapter 2020 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 fusion starts when star reaches zero age main sequence

29 March 2005AST 2010: Chapter 2021

29 March 2005AST 2010: Chapter 2022 Stage 4: Main Sequence We define the stars arrival on the main sequence as the time when fusion begins Eventually become stable because hydrostatic equilibrium has been established hydrostatic equilibrium hydrostatic equilibrium They settle down to spend about 90% of their lives as main sequence stars Fusing hydrogen to form helium in the core

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 evolution to main sequence

29 March 2005AST 2010: Chapter 2025 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 – protostar produces 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.

29 March 2005AST 2010: Chapter 2026 Summary of Birth Process

29 March 2005AST 2010: Chapter 2027 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 Sequence

29 March 2005AST 2010: Chapter 2028 time to reach main-sequence stage short for big stars as low as 10,000 years long for little stars up to 100 million years for low mass

29 March 2005AST 2010: Chapter 2029 HR Diagram: Analogy to Weight versus Height for People

29 March 2005AST 2010: Chapter 2030 Weight and Height Change as Age Increases (Marlon Brando)

29 March 2005AST 2010: Chapter 2031 Different Paths for Different Body Types (Woody Allen)

29 March 2005AST 2010: Chapter 2032 Evidence that Planets Form around Other Stars It is very hard to see a planet orbiting another star Planets around other stars may be detected indirectly One way is to look for disks of material from which planets might be condensing A big disk is more visible than a small planet Look for evolution of disks -- evidence for clumping into planets.

29 March 2005AST 2010: Chapter 2033 Disks around Protostars Four disks around stars in the Orion Nebula The red glow at the center of each disk is believed to be a young star, no more than a million years old

29 March 2005AST 2010: Chapter 2034 Dust Ring around a Young Star A debris disk has been found around a star called HR 4796A The star is estimated to be young, about 10 million years old If there are newly formed planets around the star, they will concentrate the dust particles in the disk into clumps and arcs

29 March 2005AST 2010: Chapter 2035 Disk around Epsilon Erdani Evidence for a clumpy disk has been found around a nearby star named Epsilon Eridani The star is surrounded by a donut-shaped ring of dust that contains some bright patches The bright spots might be warmer dust trapped around a planet that formed inside the donut Alternatively, the spots could be a concentration of dust brought together by the gravitational influence of planet orbiting just inside the ring

29 March 2005AST 2010: Chapter 2036 Planets Beyond the Solar System: Search and Discovery If we can’t directly observe planets, can we indirectly observe them? Kepler’s and Newton’s laws apply A planet usually orbits a star Both the planet and the star orbit around a common center of mass The planet’s motion has an effect on the star’s motion As a result, the star wobbles a bit From the observed motion and period of the wobble, the mass of the unseen planet can be deduced using Kepler’s laws It is a planet if its mass is less than 1/100 the Sun’s mass (or about 10 times Jupiter’s mass)

29 March 2005AST 2010: Chapter 2037 Doppler Method for Detecting Planets The star slightly wobbles due to the motion of the unseen companion planet

29 March 2005AST 2010: Chapter 2038 The plot of the velocity from measured Doppler shift versus time shows the star’s orbit about unseen partner Second technique: The light curve ( luminosity versus time) shows shows planet’s transit

29 March 2005AST 2010: Chapter 2039 To date, more than 100 star systems with “planets” have been found Systems of 2, 3, and possibly more “planets” have been seen The masses of the “planets” are measured in Jupiter-masses. Discovered Planets

29 March 2005AST 2010: Chapter 2040 Some Properties of the First 100 Extrasolar Planets Found

29 March 2005AST 2010: Chapter 2041 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 difficult for the early models to explain The formation of planetary systems is more complex and chaotic than we thought