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TUESDAY 11/29/11 – “D” – Day. Aim: How do stars form? Do now: define the following words: 1. Shock 2. free-fall collapse 3. cooling lines 4. protostar 5. cocoons. CHAPTER 11 – FORMATION AND STRUCTURE OF STARS .
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TUESDAY 11/29/11 – “D” – Day Aim: How do stars form?Do now: define the following words:1. Shock2. free-fall collapse3. cooling lines4. protostar5. cocoons
CHAPTER 11 – FORMATION AND STRUCTURE OF STARS • How can we know what the life cycles and internal processes of stars are, given that you won’t live long enough to see them evolve, and you can’t see inside them?
11-1 Making Stars from the Interstellar Medium • Star Birth in Giant Molecular Clouds • Interstellar medium (ISM) –the gas and dust distributed between the stars - has same chemical composition of stars
11-1 Making Stars from the Interstellar Medium 2. Observing at far-infraredand radiowavelength, astronomers can look inside dense molecular clouds and find evidence that stars are forming 3. Giant molecular clouds have diameters much larger that a single star a. Can form into stars only if asmall region of the cloud can be compressed to high densityand temperature
11-1 Making Stars from the Interstellar Medium 4.4 factors resist compression (describe each using your text book pg. 211-212) • Thermal energy - • Magnetic field – • Rotation - • Turbulence –
11-1 Making Stars from the Interstellar Medium 5. Even with these factors there are some giant molecular clouds that form dense coresand many of these are destined to be stars a. Theory and observation suggest giant molecular clouds can be triggered to form stars by passing shock waves – shock (astronomical equivalence of a sonic boom)
11-1 Making Stars from the Interstellar Medium b. During a shock some regions of the molecular cloud can be compressed to high densities that the resisting factors no longer oppose gravity and stars form – first trigger c. Shock waves: • Supernova explosions • Ignition of very hot stars ionize nearby gas and causes it to flow rapidly away • New stars of all types seem to emit strong winds and jets while they are forming
11-1 Making Stars from the Interstellar Medium 6. Collision of molecular cloudscan compress parts of the clouds – second trigger 7. Spiral pattern of our Milky Way Galaxy –third trigger (spiral arms are shock waves as a molecular cloud passes through the arm it can be compressed)
SUMMARY QUESTIONS Answer the following questions using the chapter summary and information in the chapter: • How are stars born? • What makes giant molecular clouds contract? • What can produce shock waves that trigger further star formation?
11-2 The Orion Nebula: Evidence of a Star AIM: How do stars form? DO NOW: 1. Textbook pg. 230 #1 & 2 2. Define the following words: Bok globules Star-formation pillars Birth line Young Stellar Objects Protostellar disks
11-1 Making Stars from the Interstellar Medium AIM: HOW DO STARS FORM? DO NOW: 1. Define the following words: Protostellar disks Bipolar flows association T association OB association
11-1 Making Stars from the Interstellar Medium B. Heating by Contraction • The cloud of gas begins to contract and gravity draws the atoms toward the center a. Atoms are falling and are gaining speed – free-fall collapse
11-1 Making Stars from the Interstellar Medium b. Thermal Energy– agitation of particles in a gas, increase in speed of the atoms is a step towards heating the star c. Star is not heated until the motion of the atoms israndom- when atoms begin to collide with one another - temperature of the gas in increasing (gravitational energy is converted intothermal energy)
11-1 Making Stars from the Interstellar Medium 2. Thermal Energy was one of the factors that can resist gravityand keep a molecular cloud from contracting a. Does the contracting stop once the free-fall collapse heats the gas? – NO • so thermal energy generated by the collapse can escape the cloud and contraction can continue b. Astronomers observe cooling linesfrom dense cloud cores that are contracting
11-1 Making Stars from the Interstellar Medium C. Protostars 1. When the contracting core becomes hot enough it produces short-wavelengthradiation to which the core is not transparent – thermal energy can’t escape
11-1 Making Stars from the Interstellar Medium a. Now in a new stage – protostars shine with energy released gravitationally from a combination of matter fallinginward and slow contraction of the protostar
11-1 Making Stars from the Interstellar Medium 2. Develops higher-density region at the center and a low-density outer region (envelope) with material still flowing from the outer parts of the cloud – cloud contracts from the inside out, growing deep inside a surrounding cloud of cold, dusty gas
11-1 Making Stars from the Interstellar Medium 3. Star formation is a scientific theory that can never absolutely be proven but scientists have confidence in it because of repeated testingby observation
11-2 The Orion Nebula: Evidence of a Star II. The Orion Nebula: Evidence of Star Formation 1. Evidence means observations 2. Astronomers must therefore depend on observations at infrared and radio wavelengths to search for protostarsin their natural environment
11-2 The Orion Nebula: Evidence of a Star 3. On a clear night you can see the Great Nebula of Orionas a fuzzy blob in Orion’s sword. a. At the center of the nebula lie four brilliant blue-white stars known as the Trapezium, the brightest in a cluster of a few hundred
11-2 The Orion Nebula: Evidence of a Star A. Observing Star Formation 1. Observations give us evidence of star formation is a continuousprocess STAR FORMATION IN THE ORION NEBULA (Using your textbook pg. 216-217 describe the 4 important points)
SUMMARY QUESTIONS Answer Question # 3 & 4 in your textbook pg. 230
11-2 The Orion Nebula: Evidence of Star Formation AIM: What is the evidence that theories of star formation are correct? DO NOW: Answer the following questions: 1. Explain how stars are formed 2. How are astronomers able to find evidence of star formation?
11-2 The Orion Nebula: Evidence of a Star B. Contagious Star Formation 1. There is evidence that star formation can trigger more star formation One sign is the presence of star-formation pillars Exploding supernovas trigger star formation
11-2 The Orion Nebula: Evidence of a Star 2. Star formation can spread itself by creating massive stars Lower mass stars also form in the process but can trigger further star formation
11-2 The Orion Nebula: Evidence of a Star 3. Star formation has swept across Orion from NW to SE Starting near Orion's west shoulder Massive stars formed there triggered formation of stars in Orion's belt That star formation triggered new stars that we see in the Great Nebula of Orion 4. Great Nebula outline will change and new Nebula will form
11-3 Young Stellar Objects and Protostellar Disks 1. Protostar will not be hidden when the cocoon falls inward or is driven away a. Once a star crosses its birth line and become visible it continues to contract to become a main sequence star b. It is now called a Young Stellar Object
11-3 Young Stellar Objects and Protostellar Disks 2. The more massive the star the stronger its gravity and the more rapid it will contract a. Took the sun 30 million years from when its cloud began contracting to become a main- sequence star
11-3 Young Stellar Objects and Protostellar Disks 3. Majority of protostars are surrounded by protostellar disks a. important because planets form within these disks Earth formed in a disk around the protosun 4.6billion years ago b. Likely that most stars have planetary systems
11-3 Young Stellar Objects and Protostellar Disks Summary http://www.brainpop.com/science/space/lifecycleofstars/
11-3 Young Stellar Objects and Protostellar Disks AIM: How do stars maintain their stability? DO NOW: - Define the following words: Stellar winds Radiation pressure Hydrostatic equilibrium - Answer the following questions: • Why do astronomers believe the protostellar disks are important? • How is a Young Stellar Object formed? • How long did it take the sun to become a main sequence star?
11-3 Young Stellar Objects and Protostellar Disks 4. Describe the Observational Evidence of star formation on pg. 222-223 in your textbook 1. 2. 3. 4.
11-3 Young Stellar Objects and Protostellar Disks 5. Some evidence of stellar youth is not as obvious a. Some gas clouds give birth tocompact star clusters held together by their own gravity and others give birth to larger associationsnot bound together b. Associations must consist of young stars because the stars wanderapart quickly c. Associations of starts that are moving away from each other as we watch are clear evidence of recent star formation
11-3 Young Stellar Objects and Protostellar Disks 6. When a protostar becomes hot enough, it can drive away the gas and dust of the protostellar disks as well as the remaining traces of its cocoon a. Stars produce stellar wind – can be vigorous for hot young stars b. Stellar winds and radiation pressure combine to blow disks and remnant cocoons apart
11-4 STELLAR STRUCTURE A. What keeps a star stable? • Crucial idea to astronomy is balance • Stars are held together by their own gravity balanced by the support of their internal heat and pressure
11-4 STELLAR STRUCTURE 1. Think about the structure of a star as an onion a. Each layer has a variation in temperature, density, pressure, composition etc. (stars do not actually have separable layers)
11-4 STELLAR STRUCTURE 2. The deeper layers must support the weight of all the layers above a. Inside of a star is made up of gas so the weight pressing down on the layer must be balanced by gas pressure in that layer b. If the pressure is too low the weight from the above layer will compress and push down the layer c. If the pressure it too high the layer will expand and light the layers above
11-4 STELLAR STRUCTURE 3. This balance between weight and pressure is called hydrostatic equilibrium a. The pressure inside a star must grow larger with increasing depth to support the weight and keep the star stable
11-4 STELLAR STRUCTURE 4. Pressure in a gas depends on the temperature and density of the gas a. Principle of hydrostatic equilibrium – stars must have high temperature, pressure, and density inside to support their own weight and be stable Also need to learn about energy flows within the star to completely understand its structure
11-4 STELLAR STRUCTURE Summary Answer questions 5 & 6 on pg. 230 in your text book
11-4 STELLAR STRUCTURE AIM: How do stars maintain their stability? AIM: HOW DO STARS MAKE ENERGY DO NOW: - Define the following words: Energy Transport Opacity CNO cycle
11-4 STELLAR STRUCTURE B. Energy Transport • Inside of the star is hotter than the surface so energy must flow outward • Flow of energy through each shell determines it temperature – which determines how much weight the shell can balance
11-4 STELLAR STRUCTURE 1. Law of energy transport – energy flows from hot to cold by : • Conduction • Convection • Radiation
11-4 STELLAR STRUCTURE a. Conduction: most familiar form of heat flow – not a significant cause its not as efficient b. Radiation: principal means of energy transport in the interiors of stars Flow of energy by radiation is controlled by the opacity of the gas Opacity depends on temperature
11-4 STELLAR STRUCTURE c. Opacity high – radiation cant flow through the gas and it backs up if it backs up enough hot gas rises and cool gas sinks convection (important because it carries energy and mixes the gas)
11-5 Sources of Stellar Energy A. Proton – Proton Chain 1. the way the sun makes energy 2. operates at such high temperatures to overcome Coulomb barrier of electrical repulsion between positively charged atomic nuclei
11-5 Sources of Stellar Energy B. CNO Cycle 1. More efficient – requires higher temperature because of larger repulsion Both combine four hydrogen nuclei to make helium nucleus plus energy
11-5 Sources of Stellar Energy SUMMARY – pg. 230 # 7 & 8
11-5 Sources of Stellar Energy AIM: HOW DO STARS MAKE ENERGY DO NOW: Answer the following questions: • What is the law of energy transport? • What is the difference between conduction, convection, and radiation? • Why is convection important in stellar evolution?
