Understanding Clicker Q. What light gets through? HST Sharpness of Images. ASTR 1040 Accel Astro: Stars & Galaxies

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1 ASTR 1040 Accel Astro: Stars & Galaxies Prof. Juri Toomre TA: Kyle Augustson Lecture 5 Tues 29 Jan 08 zeus.colorado.edu/astr1040-toomre toomre Topics for Today Further remarks about telescopes Overview of how Sun is put together Why is a star spherical,, and does not collapse? (Gravitational equilibration) Why does it shine? What is the energy source? (Fusion of H to He) New HW #3 Set out today,, HW #2 due Discussion question for Thur class: Pros and cons of a lunar observatory,, where to position it on the Moon (respond CULearn) Understanding Clicker Q B. Which wavelength regions CAN be studied with ground-based telescopes? A. All light with wavelengths longer than ultraviolet B. Radio, visible, and very limited portions of infrared and ultraviolet C. All light with wavelengths shorter than infrared D. Infrared, visible, and ultraviolet What light gets through? B. Atmosphere protects us from most high energy photons But many very hot objects shine brightest in such UV, X-ray X and gamma-ray photons And cool star-forming regions are brightest in IR HST Sharpness of Images HST Resolution: 0.05 arcseconds (D) Compare with best seeing ground-based observations at 0.5 arcseconds (B), and typical 2 arcsecond seeing (A). Pre-fix of HST image is (C). HUBBLE TROUBLE repaired by astronauts inserting corrective optics from Boulder A B C D 1

2 X-Ray Telescopes do it their own way! X-ray photons can pass right through a mirror Such photons can only be reflected at shallow angles, like skimming stones off water surface Chandra X-RayX Observatory Bigger view of Chandra s s X-ray X Imaging grazing reflections Chandra X-ray X Observatory (at L 2 Lagrangian) Nonvisible Light X-ray, UV, IR, Radio Most light is invisible to human eye Special detectors can record such light Digital images built using false-color coding Chandra X-ray X image of center of our Milky Way Galaxy Radio Telescopes Biggest Single Dish Green Bank Telescope (100+m) 305 m Arecibo PR 2

3 Radio Interferometry many small look big! Two (or more) radio dishes observe the same object Signals from each interfere with each other Can construct image whose angular resolution is like that from a huge dish! Very Large Array (VLA) NM 27 25m antennas 36 km baseline VLA Large Aperture Synthesis VLBA in Owens Valley CA ALMA: : Atacama Large mm-submm (M) Array ( mm) Very high desert Chile (altiplano 5000 m!) continental baselines 10 25m dishes One of 64 (80) 12m diam antennas 15km max separation (0.005 best resolution) Reading Clicker Q What really is a light year? A. Year that has less calories B. B. Distance travelled by light in a year C. C. Time before some politicians tell the truth (as in It will be many light years before the truth is known.. ) D. Travel time for light to get to us from the Sun D. B Light year? B. DISTANCE travelled if moving at the speed of light (300,000 km/sec) 1 light-year = 9.46 trillion km = km 12 km Light travel time from surface of Sun to us is about 8 minutes 3

4 Next to Our Nearest Star Chap 14 Big Qs about the Sun (and any star) Why is a star ROUND? What keeps a star from collapsing inward? What keeps it shining? Why does it rotate and have varying magnetic fields? OVERVIEW of the Sun Round, rotates, burns H to He Big System View of Sun EUV from SOHO core radiative zone convection zone photosphere chromosphere corona solar wind 4

5 SUN IN PROFILE Vast range in temperature and density SURFACE photosphere CORE Convection Zone and Radiative Interior Just below the photosphere: Deep shell (30%) of very turbulent convection chromosphere corona ATMOSPHERE Drives strong differential rotation Pull of gravity = Push of pressure gradient SPHERICAL nature of gravity makes it ROUND Gravitational equilibrium PRESSURE vs GRAVITY High PRESSURE needed at CENTER HOT CENTER Sun is a big ball of plasma Hydrogen and helium are ionized by the high temperature throughout most of star Such electrically-conducting conducting GAS is called a PLASMA Movement of plasma has currents flowing, builds magnetic fields and electric fields PLASMA DISK How to get high central pressure? In gases, plasmas, equation of state is roughly PRESSURE = DENSITY x TEMPERATURE 1. Making the CENTER HOT yields high pressure that keeps star from collapsing 2. If really hot, NUCLEAR BURNING can supply the energy that always leaks away from hot places 5

6 Fusion or fission as star s s energy source? Need high temperatures to make fusion happen High temperature gives high speeds Plenty of H for fusion, almost no `heavy fuel for fission: H converted to He SUN as a SPHERE Proton-Proton (P-P) P) Chain Thermonuclear FUSION NUCLEAR BURNING near center P-P chain Hans Bethe (1937) 6