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Astronomy 1020 Stellar Astronomy Spring_2017 Day-27 Course Announcements • Mar. 31 (Fri) – Last day to drop a class with W, F, FA • • • • • • • SW chapter 14 – Due Wednesday 3/29 SW chapter 15 – Due Monday 4/3 SW chapter 16 – Due Monday 4/10 SW chapter 17 – Due Friday 4/14 SW chapter 18 – Due Wednesday 4/19 SW chapter 19 – Due Wednesday 4/26 SW chapter 20 & 21 – Due Thursday 5/4 – 10:30 am – no late accepted • EXAM-3: Mon. 4/3 OR Wed. 4/5 • Chapters 13, 14, 15 (+??) Course Announcements • Next Dark Night observing: Tues. 3/28 • 8:30pm at the observatory • Weather dependent • There will 2 more after this – but don’t wait if the weather is good. • The LAST 1st Quarter night is Monday 4/3 at 7:30pm • On campus • Reports are due at Class Time on Monday April 24. • NO LATE REPORTS ACCEPTED! Lab This Week • • • • Hertzsprung-Russell Diagram NAAP lab (ClassAction Web site) What you need to know: Reviewing Chapter 13 will help. Where do Stars Form? Spirals Where don’t Stars Form? Ellipticals The interstellar medium: gas and dust between the stars. Near the Sun, chemical composition is similar. Most is gas; 1 percent is interstellar dust. The gas is very tenuous: about 1 atom per cubic centimeter. It emits various kinds of light, depending on its temperature. Dust is in the form of solid grains. “Interstellar soot” (iron, silicon, carbon, and more). Dust blocks visible light from stars, galaxies, etc.: interstellar extinction. Size of dust particles: large molecules up to 300 nanometers. Therefore, dust blocks short wavelengths more efficiently. More red light is let through: interstellar reddening. Long wavelengths (infrared and radio) penetrate dust. A star viewed through dust is fainter and redder due to interstellar extinction and reddening. The position of absorption lines are not affected. Dust also emits light when it blocks a star. Energy from absorbed light is transferred to the dust, heating it. Dust grains are typically cool (10–300 K). Therefore, they emit infrared radiation. iClicker Question ISM & Star Formation Wavelengths Most gas and dust is concentrated in relatively dense interstellar clouds. The material found between clouds is called intercloud gas. Some regions can be very hot (106 K). X-rays emitted, but extremely tenuous. The Sun resides in a local bubble of milliondegree gas. Most intercloud gas is 8000 K. H II regions (about 104 K): Hydrogen heated and ionized by ultraviolet light from hot, luminous stars (O and the hottest B). Ionized: stripped of one or more electrons. At lower temperatures, hydrogen is in single, neutral atoms. This gas emits radio waves with = 21 cm. Light of this wavelength penetrates the dust. Good for mapping the Milky Way. Many clouds are cold enough for hydrogen to be in the H2 molecule. These are called molecular clouds. Dense and cold, and appear dark. Temperatures are around 10 K, with densities as high as 1010 molecules/cm3. Emit radio waves. Many other molecules are in the mix. Some clouds can have masses as large as 10 million times that of the Sun: giant molecular clouds. On average, 120 light-years in size. Stars form in these molecular clouds. Molecular clouds are cold and dense. Some places in the cloud are denser than average. Self-gravity will make these regions collapse. Rate of collapse is slowed by magnetic fields, turbulence, and angular momentum (spin). Collapse and fragmentation lead to dense star-forming molecular-cloud cores. The Sun began in one of these cores. Molecular cores collapse under their own gravity. Center shrinks fastest; outer layers later. This produces a dense protostar. Spin of core produces a disk of material around the protostar. Material falls onto the growing protostar from the disk. Our Solar System began this way. iClicker Question ISM & Star Formation Molecular Clouds Star Formation Masses The interstellar material that becomes stars and planets must have the key elements for life if those planets are going to have it. Water and oxygen have been detected in some star-forming regions. Protostars Protostars are large, cool, and luminous. They will emit infrared light. Infrared studies of molecular regions reveal protostars and their disks. The protostar continues to accrete more material. It continues to shrink and radiate away energy, balancing pressure and gravity. The interior temperature and pressure rise. MATH TOOLS 15.1 The low temperature of dust means that it glows in the infrared. 100 K dust: 10 K dust: The protostar’s energy source is gravitational energy. As it shrinks, temperature rises in the core. Hydrogen fusion begins in the core: It becomes a main sequence star. The temperature in the core must be hot enough for fusion, 10 million K. Very low-mass stars (< 0.08 M) never start hydrogen fusion. These are called brown dwarfs. CONNECTIONS 15.1 A brown dwarf is not a star, nor a planet, but is in between. Classified as L, T, or Y (cooler than M stars). Glow in the infrared due to internal heat from gravitational contraction. Over 1,000 have been found since the mid1990s.