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Astronomy 115 – Section 4 Week 13

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Astronomy 115 – Section 4 Week 13 Astronomy 115 – Section 4 Week 13 Adam Fries • SF State • [email protected] Important Notes • Start Galaxies, Ch. 11 • HW# 4 is posted on the webpage, due at the Final. • Final is in 2 weeks: May 19th same time/place • Extra credit is due a week before the final. Recall: • Massive stars (Mstar > 8 M ) core fusion stops at iron. • Once the iron core grows to 2 M , Type II Supernova (neutron star) • If the neutron star exceeds 3 M , you get a black hole. • Event Horizon is a sphere devoid of light – no reflection or emission • If light crosses the Event Horizon, it cannot escape the black hole – point of no return • The radius of the sphere is expressed as the Schwarzchild radius: 2G R = M Sch c2 Detecting Black Holes • need to look at binary systems (companion star orbiting a black hole) • black hole strips off the atmosphere of its companion • the gas and dust fall into the black hole, but bottle-necks – heats up and emits X-rays Ch. 11 Galaxies • Determine a galaxy’s type by its appearance • how spiral arms form • evidence for dark matter • evidence for supermassive black holes Difficulty in identifying shapes. There are 3 main galaxy types, and galaxies come in a wide range of sizes • Elliptical • Spiral • Everything else – Irregular Hubble’s Tuning Fork – not an evolutionary track Stellar Motions Give Galaxies Their Shapes Elliptical Galaxy (Type E), Messier 87 – Virgo • In ellipticals (Eggs). • . stars are almost randomly orbiting the center from a variety of angles • some stars are falling in while others are climbing out • stars are moving in all possible directions • gas poor, stars have stopped forming • most abundant type of galaxy Spiral Galaxy (Type S), Messier 81 – Ursa Major • In spirals... • . stars mostly move together as a flat, rotating disk spiral arms • there is a central bulge, where stars move more randomly, like in ellipticals • contain large amounts of dust and cold, dense molecular clouds, stars are still forming Barred Spiral Galaxy (Type SB), NGC 1300 – Eridanus Irregular Galaxy, NGC 55 – Sculptor • In irregulars... • . stars move in assymetric orbits • shape is not well-defined • result of galaxy collision • make up nearly 25% of all galaxies How arms form in spiral galaxies • Gas (H and He and other metals), • dust (small particles of C and silicon), • hot, young (blue) stars. • . are concentrated in the arms • (Old, red stars are concentrated in the central bulge) • The disk rotates! this naturally produces the arm structure • But to maintain the arms, there needs to be a sustained gravitational disturbance • Most bulges are elongated (Eggs), this is enough to sustain the structure • Different shaped bulges generate different shaped arms So where are we in the Milky Way (our galaxy)? By measuring the local density of stars, William Herschel believed that the Sun was at the center (1780s). But this was wrong. Story begins with Henrietta Swann Leavitt • in 1912, one of the Harvard Computers, Henrietta Leavitt, completed a study on a special type of star in a neighboring dwarf galaxy (SMC) • these stars ‘pulsated’ with a very predictable period • . brightening and then dimming • she deduced a period-luminosity relationship to find distances with these stars Small Magellanic Cloud (SMC) Omega Centauri Globular Clusters • Some of the oldest objects in the Universe • Contain 100,000s - 1,000,000s of stars • Some GC are visible to the naked eye • By 1920, Harlow Shapley used the P-L relationship of Cepheid Variable Stars to map the distribution of 93 globular clusters. • Found that the GC are located in a spherical distribution not centered on the Earth! • He suggested that the GC orbit the center of the Milky Way and we are not there. To find the center of the Milky Way, look for the Teapot! • In 1970s, it was hypothesized that a supermassive blackhole lived in the center of the MW • From 1995 to 2012, Keck/UCLA Galactic Center Group mapped the motion of stars about the center. • These are orbits, and orbits follow Kepler’s Laws! • Gross version of Kepler’s 3rd law: 4π2 P2 ' A3 GM • By measuring the period, P, and semimajor axis, A, the mass can be found! • SMBH is estimated to be about 3–4 million solar masses! Measuring the Mass of a Galaxy Any Ideas? We could count up all the light we see from the stars? But this method leaves out all of the really faint stars, black holes, brown dwarfs (failed stars) We can use the same trick of estimating the SMBH mass! Measure the motion (period) of the stars on the outer edge of the Milky Way. The orbits of these stars should follow Kepler’s Laws, thus we can estimate all the mass interior to these orbits. Because the MW is not spherical, astronomers need to know how the mass is distributed interior to these outer stars. It was hypothesized that mass and light were distributed in the same way throughout the galaxy. Wherever there was light, there must be mass. In the 1970s, Vera Rubin set out to measure the orbital velocities of stars. What’s going on? Any ideas? The idea that ’mass and light are distributed in the same way’ must be wrong! Astronomers hypothesized that there must be another component of undetected matter which is not stars, gas, or dust– this is called Dark Matter Astronomers believe that the Milky Way’s dark 12 matter halo mass is ∼ 10 M ! So what is dark matter? So what is dark matter? Maybe it’s all the stuff we forgot to count like BH, Brown Dwarfs, Neutron stars, White Dwarfs (MACHOS – massive compact halo objects)? Turns out not to be a good candidate. Not enough lensing events to account for the mass. How about WIMPS – weakly interacting massive particles? This turns out to be the leading candidate so far. Heavy elementary particles that don’t interact with normal matter. Experiments in the LHC are under way to try to detect this exotic matter. Adjust Newton’s Law of Gravitation at large scales (MOND)?.
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