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Star Formation A Star is Born 8 Goals • • • • What is there between the stars? What are dust clouds? What are nebulae? How do these lead to the formation of star? – Where do baby stars come from? 8 The Stuff Between Stars • Space isn’t empty. • Interstellar Medium – The gas and dust between the stars. All the interstellar gas and dust in a volume the size of the Earth only yields enough matter to make a pair of dice. 8 The Distribution • Picture the dust under your bed. – Fairly uniform thin layer – Some small clumps – Occasional big complexes • Interstellar dust and gas is the same. 8 Dust • Space is dirty. • Dust blocks or scatters some light. • Result: black clouds and patterns against the background sky. • But what light gets through, and what light doesn’t? 8 Absorption and Scattering • Q: Why are sunsets red? • Light is absorbed or scattered by objects the same size or smaller than its wavelength. • Dust grains = wavelength of blue light • Dust clouds: – Opaque to blue light, UV, X-rays – Transparent to red light, IR, radio • A: Whenever there is a lot of dust between you and the Sun, the blue light is absorbed or scattered leaving the only the red light. 8 Interstellar Reddening • Same thing with dust clouds in space. • Since space is full of dust, the farther away stars are, the redder they look. • Enough dust and eventually all visible light is scattered or absorbed. 8 Dust and IR • In a dark dust cloud: – Even though all visible light may be gone, we can still use IR. – If dust is warm, IR will show its blackbody emission. 8 The IR Universe Orion - visible Orion – by IRAS And allows us to see dust where we wouldn’t otherwise expect it. 8 The Trifid Nebula – copyright Jason Ware 8 Interstellar Gas • In Lab 2 we talked about spectral lines and how they apply to hot and cool gases. • Let’s look at some hot and cool gases in space. Ha emission nebulae Copyright - Jason Ware 8 Horsehead Nebula – copyright Arne Henden Dust obscuring Ha emission nebula 8 Orion Nebula – copyright Robert Gendler • In order for the hydrogen to emit light, the atoms must be in the process of being excited. • The energy for the excitation comes from very hot stars (O and B stars) within the cloud. 8 Cold Dark Clouds • If dust clouds block light, then inside thick dust clouds there should be no light at all. • Without light, there is little energy. • With little energy, any gas inside is very, very cold. • Inside molecules can form. 8 Gravity vs. Pressure • Stars and other interstellar material are in a perpetual battle between forces pulling in (gravity) and forces pushing out (pressure). • Gravity comes from the mass of the cloud or star. • Pressure comes from the motion of the atoms or molecules. – Think of hot air balloons. – The hotter the air, the bigger the balloon. 8 Star Formation • Remember lecture 4: HOTTER COOLER • Cold interstellar clouds: No heat = no velocity = no outward pressure. Gravity wins. • Gas begins to contract. 8 How to Make a Star 1 2 3 8 1. The Interstellar Cloud • • • • Cold clouds can be tens of parsecs across. Thousands of times the mass of the Sun. Temperatures 10 – 100 K. In such a cloud: – Something makes a region denser than normal. – Force of gravity draws material to denser region. – Gravitational collapse begins. 8 Orion Nebula – copyright Robert Gendler 8 Contracting Fragments • Cloud about the size of solar system. • In the center: – Collapsing material continues to heat up. – Density causes heat to be retained. • Higher density makes center opaque. 8 Eagle Nebula – copyright J. Hester 8 2. Protostar • The central opaque part is called a protostar. • Mass increases as material rains down on it. 8 •Visible and IR image of the hot protostars in the Orion Nebula. 8 …and the Nebula? • Cloud around the protostar spins faster. • Flattens to a disk. – Pizza dough. 8 Planetesimals • Dust and gas condense onto dust grains. • Small clumps grow bigger. • Bigger clumps have more mass and attract more matter. • Planetesimals become the building blocks of the planets. Orion Nebula – Copyright O’Dell and Wong 8 3. T Tauri Phase • • • • Protostar still shrinks: 10x the Sun. Still heats up: surface = 4000 K Core temp = 5,000,000 K Violent surface activity creates strong winds that blow material away near the protostar’s surface. • Clear away the dust and gas between planets. 8 8 A Star is Born • Time: 40- 50 million years since the collapse started. • Radius: 1,000,000 km (Recall the Sun = 700,000 km) • Core temp: 10,000,000 K (Sun = 15,000,000 K) – Surface temp = 4500 K • Fusion begins in core. • Energy released creates the pressure needed to counter the contraction from gravity. • Contraction ends! 8 An H-R Life-Track 8 The Main Sequence • For the Sun: – While it took 40 – 50 million years to get here, the new star will spend the next 10 billion years as a main sequence star. • Bigger Stars: – Everything goes quicker. • Smaller Stars: – Everything longer. 8 Now what? • The mass of the star that is formed will determine the rest of its life! • Recall: the more massive the star, the more pressure in the core. • The more pressure, the more fusion. • More fusion: – More energy produced – Hotter – Shorter life span 8 Open Clusters • These are the new stars. • Small groups of young stars. • Slowly drifting apart. Jewel Box – copyright MichaelBessell 8 8 Homework #7 • For 10/16: • Read B16.3 – 16.5, B17.1 – 17.3 • Do: – Ch16 : Review Questions 9, 18, Problem 1 – Ch17: Review Question 1, Problem 4 8