Chapter 16 16.1 Stellar Nurseries Star Birth • Our goals for learning • Where do stars form? • Why do stars form? Star-Forming Clouds Where do stars form? • Stars form in dark clouds of dusty gas in interstellar space • The gas between the stars is called the interstellar medium Composition of Clouds Molecular Clouds • We can determine the composition of interstellar gas from its absorption lines in the spectra of stars • Most of the matter in star-forming clouds • 70% H, 28% He, is in the form of molecules (H 2 , CO,…) 2% heavier elements • These molecular clouds have a in our region of temperature of 10-30 K and a density of Milky Way about 300 molecules per cubic cm 1
Interstellar Dust Molecular Clouds • Tiny solid particles of interstellar dust block our view of stars on the other side of a cloud • Most of what we know about molecular • Particles are < 1 clouds comes from observing the emission micrometer in size and made of lines of carbon monoxide (CO) elements like C, O, Si, and Fe Interstellar Reddening Interstellar Reddening • Stars viewed • Long-wavelength through the edges of infrared light passes the cloud look through a cloud redder because dust more easily than blocks (shorter- visible light wavelength) blue light more • Observations of effectively than infrared light reveal (longer-wavelength) stars on the other red light side of the cloud Observing Newborn Stars Observing Newborn Stars • Visible light from a • Observing the newborn star is infrared light from a often trapped within cloud can reveal the the dark, dusty gas newborn star clouds where the embedded inside it star formed 2
Glowing Dust Grains Glowing Dust Grains • Dust grains that • Long-wavelength absorb visible light infrared light is heat up and emit brightest from infrared light of regions where many even longer stars are currently wavelength forming Why do stars form? 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 Resistance to Gravity Mass of a Star-Forming Cloud • A cloud must have • A typical molecular cloud (T~ 30 K, n ~ 300 even more mass to particles/cm 3 ) must contain at least a few hundred begin contracting if solar masses for gravity to overcome pressure there are additional forces opposing gravity • 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 • Both magnetic fields and turbulent gas motions increase resistance to gravity 3
Fragmentation of a Cloud Fragmentation of a Cloud • This simulation • Gravity within a contracting gas cloud begins with a becomes stronger as the gas becomes denser turbulent cloud containing 50 solar masses of gas • 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 Fragmentation of a Cloud Fragmentation of a Cloud • The random motions • Each lump of the of different sections cloud in which of the cloud cause it gravity can to become lumpy overcome pressure can go on to become a star • A large cloud can make a whole cluster of stars Isolated Star Formation The First Stars • Gravity can overcome pressure • Elements like carbon and oxygen had not yet been in a relatively small made when the first stars formed cloud if the cloud is unusually dense • Without CO molecules to provide cooling, the clouds that formed the first stars had to be considerably • Such a cloud may warmer than today’s molecular clouds make only a single star • The first stars must therefore have been more massive than most of today’s stars, for gravity to overcome pressure 4
What have we learned? Simulation of the First Star • 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 (H 2 ) but stay cool because of emission by carbon monoxide (CO) • Why do stars form? – Stars form in clouds that are massive enough • Simulations of early star formation suggest for gravity to overcome thermal pressure (and the first molecular clouds never cooled any other forms of resistance) below 100 K, making stars of ~100 M Sun – Such a cloud contracts and breaks up into pieces that go on to form stars What slows the contraction of a 16.2 Stages of Star Birth star-forming cloud? • Our goals for learning • What slows the contraction of a star- forming cloud? • How does a cloud’s rotation affect star birth? • How does nuclear fusion begin in a newborn star? Growth of a Protostar Trapping of Thermal Energy • Matter from the • As contraction packs the molecules and dust particles cloud continues to of a cloud fragment closer together, it becomes harder fall onto the for infrared and radio photons to escape protostar until either the protostar or a neighboring star • Thermal energy then begins to build up inside, blows the increasing the internal pressure surrounding gas away • Contraction slows down, and the center of the cloud fragment becomes a protostar 5
Evidence from the How does a cloud’s rotation Solar System affect star birth? • The nebular theory of solar system formation illustrates the importance of rotation Conservation of Rotation of a Angular Momentum contracting cloud speeds • The rotation speed up for the of the cloud from same reason a which a star forms skater speeds increases as the up as she pulls cloud contracts in her arms Flattening Collisions between gas particles in • Collisions between cloud particles in the gradually cloud cause it to reduce random flatten into a disk motions 6
Collisions Spinning between gas cloud particles also flattens as it reduce up shrinks and down motions Formation of Jets Jets are observed coming from • Rotation also the centers of causes jets of disks around matter to shoot out protostars along the rotation axis How does nuclear fusion begin in a newborn star? 7
Birth Stages on a Life Track From Protostar to Main Sequence • 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 • Life track illustrates star’s surface fusion balances energy radiated from the surface—the temperature and luminosity at different star is now a main-sequence star moments in time Assembly of a Protostar Convective Contraction • Luminosity and temperature grow as • Surface temperature remains near 3,000 K matter collects into a protostar while convection is main energy transport mechanism Radiative Contraction Self-Sustaining Fusion • Luminosity remains nearly constant during • Core temperature continues to rise until late stages of contraction, while radiation star arrives on the main sequence is transporting energy through star 8
What have we learned? Life Tracks for Different Masses • What slows the contraction of a star- • Models show that forming cloud? Sun required about – The contraction of a cloud fragment slows 30 million years to when thermal pressure builds up because go from protostar to infrared and radio photons can no longer main sequence escape • How does a cloud’s rotation affect star • Higher-mass stars birth? form faster – Conservation of angular momentum leads to the formation of disks around protostars • Lower-mass stars form more slowly What have we learned? 16.3 Masses of Newborn Stars • How does nuclear fusion begin in a newborn star? • Our goals for learning – Nuclear fusion begins when contraction • What is the smallest mass a newborn star causes the star’s core to grow hot enough for can have? fusion • What is the greatest mass a newborn star can have? • What are the typical masses of newborn stars? What is the smallest mass a Fusion and Contraction newborn star can have? • Fusion will not begin in a contracting cloud if some sort of force stops contraction before the core temperature rises above 10 7 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? 9
Thermal Pressure: Depends on heat content The main form of pressure in most stars Degeneracy Pressure: Particles can’t be in same state in same place Degeneracy Pressure: Laws of quantum mechanics prohibit two electrons Doesn’t depend on heat from occupying same state in same place content Brown Dwarfs Brown Dwarfs • Degeneracy pressure • A brown dwarf halts the contraction emits infrared light of objects with because of heat left <0.08 M Sun before over from core temperature contraction become hot enough for fusion • Its luminosity gradually declines • Starlike objects not with time as it loses massive enough to thermal energy start fusion are brown dwarfs What is the greatest mass a Brown Dwarfs in Orion newborn star can have? • Infrared observations can reveal recently formed brown dwarfs because they are still relatively warm and luminous 10
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