Star Birth PDF

Star Birth Copyright © 2024 Pearson Education, Inc. All Rights Reserved 16.1 Stellar Nurseries Our goals for learning: – Where do stars form? – Why do stars form? Copyright © 2024 Pearson Education, Inc. All Rights Reserved Where Do Stars Form?...

Star Birth Copyright © 2024 Pearson Education, Inc. All Rights Reserved 16.1 Stellar Nurseries Our goals for learning: – Where do stars form? – Why do stars form? Copyright © 2024 Pearson Education, Inc. All Rights Reserved Where Do Stars Form? Copyright © 2024 Pearson Education, Inc. All Rights Reserved Star-Forming Clouds Stars form in dark clouds of dusty gas in interstellar space. The gas between the stars is called the interstellar medium. Copyright © 2024 Pearson Education, Inc. All Rights Reserved Composition of Clouds We can determine the composition of interstellar gas from its absorption lines in the spectra of stars. 70% H, 28% He, 2% heavier elements in our region of Milky Way Copyright © 2024 Pearson Education, Inc. All Rights Reserved Molecular Clouds (1 of 2) Most of the matter in star- forming clouds is in the form of molecules (H2 ,CO, etc.). These molecular clouds have a temperature of 10–30 K and a density of about 300 molecules per cubic centimeter. a A visible-light image of the nebula. The dark (horsehead-shaped) region is a molecular cloud. Copyright © 2024 Pearson Education, Inc. All Rights Reserved Interstellar Reddening (1 of 2) Stars viewed through the edges of the cloud look redder because dust blocks (shorter-wavelength) blue light more effectively than (longer-wavelength) red light. a A visible-light image of the dark molecular cloud Barnard 68. Copyright © 2024 Pearson Education, Inc. All Rights Reserved Interstellar Reddening (2 of 2) Long-wavelength infrared light passes through a cloud more easily than visible light. Observations of infrared light reveal stars on the other side of the cloud. b An infrared image of Barnard 68, showing the stars that lie behind the cloud. Copyright © 2024 Pearson Education, Inc. All Rights Reserved Observing Newborn Stars (1 of 2) Visible light from a newborn star is often trapped within the dark, dusty gas clouds where the star formed. Copyright © 2024 Pearson Education, Inc. All Rights Reserved Observing Newborn Stars (2 of 2) Observing the infrared light from a cloud can reveal the newborn star embedded inside it. Copyright © 2024 Pearson Education, Inc. All Rights Reserved Glowing Dust Grains (1 of 2) Dust grains that absorb visible light heat up and emit infrared light of even longer wavelength. Copyright © 2024 Pearson Education, Inc. All Rights Reserved Glowing Dust Grains (2 of 2) Long-wavelength infrared light is brightest from regions where many stars are currently forming. Copyright © 2024 Pearson Education, Inc. All Rights Reserved Why Do Stars Form? https://www.youtube.com/watch?v=YbdwTwB8jtc Copyright © 2024 Pearson Education, Inc. All Rights Reserved Gravity Versus Pressure Gravity can create stars only if it can overcome the force of thermal pressure in a cloud. Emission lines from molecules in a cloud can prevent a pressure buildup by converting thermal energy into infrared and radio photons. Copyright © 2024 Pearson Education, Inc. All Rights Reserved Mass of a Star-Forming Cloud 3 A typical molecular cloud (T ~ 30 K, n ~ 300 particles / cm ) must contain at least a few hundred solar masses for gravity to overcome pressure. Emission lines from molecules in a cloud can prevent a pressure buildup by converting thermal energy into infrared and radio photons that escape the cloud. Copyright © 2024 Pearson Education, Inc. All Rights Reserved Fragmentation of a Cloud (1 of 4) Gravity within a contracting gas cloud becomes stronger as the gas becomes denser. Gravity can therefore overcome pressure in smaller pieces of the cloud, causing it to break apart into multiple fragments, each of which may go on to form a star. Copyright © 2024 Pearson Education, Inc. All Rights Reserved Fragmentation of a Cloud (2 of 4) A turbulent cloud containing 500 solar masses of gas. a The simulation begins with a turbulent gas cloud 2.6 light-years across, containing 500MSun of gas. Copyright © 2024 Pearson Education, Inc. All Rights Reserved Fragmentation of a Cloud (3 of 4) The random motions of different sections of the cloud cause it to become lumpy. b Random motions in the cloud cause it to become lumpy, with some regions denser than others. If gravity can overcome pressure in these dense regions, they can collapse to form even denser lumps of matter. Copyright © 2024 Pearson Education, Inc. All Rights Reserved Fragmentation of a Cloud (4 of 4) Each lump of the cloud in which gravity can overcome pressure can go on to become a star. A large cloud can make a whole cluster of stars. c The large cloud therefore fragments into many smaller lumps of matter, and each lump can go on to form one or more new stars, which are represented by white dots. Copyright © 2024 Pearson Education, Inc. All Rights Reserved Isolated Star Formation Gravity can overcome pressure in a relatively small cloud if the cloud is unusually dense. Such a cloud may make only a single star. Copyright © 2024 Pearson Education, Inc. All Rights Reserved Thought Question 1 (1 of 2) What would happen to a contracting cloud fragment if it were not able to radiate away its thermal energy? A. It would continue contracting, but its temperature would not change. B. Its mass would increase. C. Its internal pressure would increase. Copyright © 2024 Pearson Education, Inc. All Rights Reserved Thought Question 1 (2 of 2) What would happen to a contracting cloud fragment if it were not able to radiate away its thermal energy? A. It would continue contracting, but its temperature would not change. B. Its mass would increase. Thecor ectan C. Its internal pressure would increase. sweri s Copyright © 2024 Pearson Education, Inc. All Rights Reserved What Have We Learned? (1 of 5) Where do stars form? – Stars form in dark, dusty clouds of molecular gas with temperatures of 10 − 30 K. – These clouds are made mostly of molecular hydrogen (H2 ) but stay cool because of emission by carbon monoxide (CO). Why do stars form? – Stars form in clouds that are massive enough for gravity to overcome thermal pressure (and any other forms of resistance). – Such a cloud contracts and breaks up into pieces that go on to form stars. Copyright © 2024 Pearson Education, Inc. All Rights Reserved 16.2 Stages of Star Birth Our goals for learning: – What slows the contraction of a star-forming cloud? – What is the role of rotation in star birth? – How does nuclear fusion begin in a newborn star? Copyright © 2024 Pearson Education, Inc. All Rights Reserved What Slows the Contraction of a Star-Forming Cloud? b Random motions in the cloud cause it to become lumpy, with some regions denser than others. If, gravity can overcome pressure in these dense regions, they can collapse to form even denser lumps of matter. Copyright © 2024 Pearson Education, Inc. All Rights Reserved Trapping of Thermal Energy As contraction packs the molecules and dust particles of a cloud fragment closer together, it becomes harder for infrared and radio photons to escape. Thermal energy then begins to build up inside, increasing the internal pressure. Contraction slows down, and the center of the cloud fragment becomes a protostar. Copyright © 2024 Pearson Education, Inc. All Rights Reserved Growth of a Protostar Matter from the cloud continues to fall onto the protostar until either the protostar or a neighboring star blows the surrounding gas away. Also called T-Tauri phase of stars Copyright © 2024 Pearson Education, Inc. All Rights Reserved What Is the Role of Rotation in Star Birth? b This image from the Atacama Large Millimeter/submillimeter Array (ALMA) shows the protostellar disk around the protostar HL Tau. (For more information about this image, see Figure 8.4b.) Copyright © 2024 Pearson Education, Inc. All Rights Reserved Evidence from the Solar System The nebular theory of solar system formation illustrates the importance of rotation. a Artist's conception of star birth. Copyright © 2024 Pearson Education, Inc. All Rights Reserved Conservation of Angular Momentum The rotation speed of the cloud from which a star forms increases as the cloud contracts. Rotation of a contracting cloud speeds up for the same reason skaters speed up as they pull in their arms. https://www.youtube.com/watch?v=FmnkQ2ytlO8 a Artist's conception of star birth. Copyright © 2024 Pearson Education, Inc. All Rights Reserved Flattening Collisions between particles in the cloud cause it to flatten into a disk. Collisions between gas particles in cloud gradually reduce random motions. Collisions between gas particles also reduce up and down motions. The spinning cloud flattens as it shrinks. a Artist's conception of star birth. Copyright © 2024 Pearson Education, Inc. All Rights Reserved Formation of Jets It is thought that the spinning disk twists the magnetic field of the star, channeling the jets. c This schematic drawing illustrates one hypothesis for how protostars create jets, which relies on the magnetic field lines thought to thread the protostellar disk. As the disk spins, it twists the magnetic field lines. Charged particles from the disk's surface can then fly outward along the twisted magnetic field lines. Copyright © 2024 Pearson Education, Inc. All Rights Reserved Formation of Jets The jets ram into interstellar gas, heating it and causing it to glow. a Two jets of material are shooting in opposite directions from a protostar within this star- forming cloud. The chapter opening image shows a larger region around the same protostar. Copyright © 2024 Pearson Education, Inc. All Rights Reserved How Does Nuclear Fusion Begin in a Newborn Star? Copyright © 2024 Pearson Education, Inc. All Rights Reserved From Protostar to Main Sequence A protostar looks starlike after the surrounding gas is blown away, but its thermal energy comes from gravitational contraction, not fusion. Contraction must continue until the core becomes hot enough for nuclear fusion. Contraction stops when the energy released by core fusion balances energy radiated from the surface—the star is now a main-sequence star. Copyright © 2024 Pearson Education, Inc. All Rights Reserved Thought Question 3 (1 of 2) With very low temperatures and luminosities, where would protostars be located on the H-R diagram? A. Upper left B. Upper right C. Lower left D. Lower right Copyright © 2024 Pearson Education, Inc. All Rights Reserved Thought Question 3 (2 of 2) With very low temperatures and luminosities, where would protostars be located on the H-R diagram? A. Upper left B. Upper right C. Lower left Thecor ectan D. Lower right sweri s Copyright © 2024 Pearson Education, Inc. All Rights Reserved Birth Stages on a Life Track A life track illustrates a star's surface temperature and luminosity at different moments in time. Copyright © 2024 Pearson Education, Inc. All Rights Reserved Assembly of a Protostar Luminosity and temperature grow as matter collects into a protostar. Copyright © 2024 Pearson Education, Inc. All Rights Reserved Convective Contraction Surface temperature remains near 3000 K while convection is main energy transport mechanism. Copyright © 2024 Pearson Education, Inc. All Rights Reserved Radiative Contraction Luminosity remains nearly constant during late stages of contraction, while radiation transports energy through star. Copyright © 2024 Pearson Education, Inc. All Rights Reserved Self-Sustaining Fusion Core temperature continues to rise until star begins fusion and arrives on the main sequence. Copyright © 2024 Pearson Education, Inc. All Rights Reserved Life Tracks for Different Masses Models show that the Sun required about 30 million years to go from protostar to main sequence. Higher-mass stars form faster. Lower-mass stars form more slowly. Copyright © 2024 Pearson Education, Inc. All Rights Reserved What Have We Learned? (2 of 5) What slows the contraction of a star-forming cloud? – The contraction of a cloud fragment slows when thermal pressure builds up because infrared and radio photons can no longer escape. What is the role of rotation in star birth? – Conservation of angular momentum leads to the formation of disks around protostars. Copyright © 2024 Pearson Education, Inc. All Rights Reserved What Have We Learned? (3 of 5) How does nuclear fusion begin in a newborn star? – Nuclear fusion begins when contraction causes the star's core to grow hot enough for fusion. Copyright © 2024 Pearson Education, Inc. All Rights Reserved What Is the Smallest Mass a Newborn Star Can Have? Copyright © 2024 Pearson Education, Inc. All Rights Reserved Fusion and Contraction Fusion will not begin in a contracting cloud if some sort of force stops contraction before the core temperature rises above 107 K. Thermal pressure cannot stop contraction because the star is constantly losing thermal energy from its surface through radiation. Is there another form of pressure that can stop contraction? Copyright © 2024 Pearson Education, Inc. All Rights Reserved Degeneracy Pressure (1 of 2) Thermal Pressure: Depends on heat content. Is the main form of pressure in most stars. Copyright © 2024 Pearson Education, Inc. All Rights Reserved Degeneracy Pressure (2 of 2) Degeneracy Pressure: Particles can't be in same state in same place. Doesn't depend on heat content. Copyright © 2024 Pearson Education, Inc. All Rights Reserved Brown Dwarfs (1 of 2) Degeneracy pressure halts the contraction of objects with  0.08MSun (about 80 times the mass of Jupiter) before core temperature becomes hot enough for fusion. Starlike objects not massive enough to start fusion are brown dwarfs. Copyright © 2024 Pearson Education, Inc. All Rights Reserved What Is the Greatest Mass a Newborn Star Can Have? Copyright © 2024 Pearson Education, Inc. All Rights Reserved Radiation Pressure Photons exert a slight amount of pressure when they strike matter. Very massive stars are so luminous that the collective pressure of photons drives their matter into space. Copyright © 2024 Pearson Education, Inc. All Rights Reserved Upper Limit on a Star's Mass Models of stars suggest that radiation pressure limits how massive a star can be without blowing itself apart. Maximum mass thought to be around 150MSun , but new observations indicate some may be even larger! Copyright © 2024 Pearson Education, Inc. All Rights Reserved