Astro 201: Sept. 21, 2010 STARS

Transcription

1 Astro 201: Sept. 21, 2010 HW 3 and On- line Quiz 3 were due today Turn in IR Camera write- up on Thursday Don t forget to sign up for Telescope Lab First Midterm on Tuesday, Sept. 28. Will cover everything through Thursday s lecture See class web site and d2l for more info no on- line quiz this week Thursday: Dr. Bechtold will finish telescopes Today: Ken Wong on stars STARS What is a star? Why do they shine? How old are they? 1

2 Stars shine for millions to billions of years, much longer than a human lifevme. Yet, we've been able to piece together how stars are born, shine and eventually die. Why does the Sun shine? The Sun shines because it is hot. The Sun is hot because in its core, nuclear fusion is producing energy The nuclear fusion that is happening in the Sun's core is the fusion of hydrogen into helium. From models of stars and the Sun, we esvmate that this fusion process will keep the Sun shining as it does today for another 4-5 billion years. 2

3 The Sun is hot The surface of the Sun is 5800 degrees Kelvin. The temperature inside the Sun is much ho[er. Its convnuous spectrum is a blackbody with peak at a wavelength 500 nanometers (yellow) The total luminosity of the Sun is 390 trillion trillion wa[s. The density of the Sun is about 40% greater than water but the Sun is a gas. 3

4 The Sun is in HYDROSTATIC EQUILIBRIUM: The collapse of the Sun be gravity is countered by the pressure of the gas. The heat generated by nuclear fusion in the core of the Sun keeps the pressure high enough for the Sun to not collapse. FUSION: 4 hydrogen nuclei combine to yield one helium nucleus The helium nucleus weighs less than the 4 hydrogen nuclei, and the difference in mass is converted into energy, described by the famous equavon: E = m c 2 Note: A very small amount of m makes a lot of E The Sun is fusing 600 million tons of hydrogen per second into 596 million tons of helium. 4

5 E = m c 2 Einstein Speaks "It followed from the special theory of relavvity that mass and energy are both but different manifestavons of the same thing - - a somewhat unfamiliar concepvon for the average mind. Furthermore, the equavon E is equal to m c- squared, in which energy is put equal to mass, mulvplied by the square of the velocity of light, showed that very small amounts of mass may be converted into a very large amount of energy and vice versa. The mass and energy were in fact equivalent, according to the formula menvoned above. This was demonstrated by Cockcrog and Walton in 1932, experimentally." Fission vs. Fusion 5

6 Fusion only can occur if the gas temperature (ie the speed of the atoms) is very, very high greater than 10 million degrees Kelvin. In the Sun, the following chain of events makes the energy which keeps the Sun hot. 6

7 The Interior of the Sun Energy can be transported out of the Sun via: Conduc7on: atoms bump into other atoms ConducVon works best in solids like metals... not important in the Sun. Convec7on: Large scale circulavon of cells: hot blobs rise, cool blobs sink Radia7ve Diffusion: Photons flow from warm, bright regions to cool, dark regions (the surface) Important processes: ConvecVon and RadiaVve Diffusion 7

8 How do we know what the interior of the Sun looks like? 1. Computer Models How do we know what the interior of the Sun looks like? 2. Helioseismology Global OscillaVon Network Group, or GONG project, monitors the vibravons of the solar surface. The Sun vibrates at many frequencies, like a ringing bell, or the surface of a drum. The six sites comprising the GONG Network are: * The Big Bear Solar Observatory in California, USA. * The High AlVtude Observatory at Mauna Loa in Hawaii, USA. * The Learmonth Solar Observatory inwestern Australia. * The Udaipur Solar Observatory in India. * The Observatorio del Teide in the Canary Islands. * The Cerro Tololo Interamerican Observatory in Chile. 8

9 Helioseismology OperaVng since 1995, they make Doppler maps of the Sun: How do we know what the interior of the Sun looks like? 3. Neutrino Telescopes In addivon to photons, the fusion processes inside the Sun produce Vny parvcles called Neutrinos. Neutrinos move at or near the speed of light, and do not interact with ma[er very easily. Deep in mines, large vats of cleaning fluid can capture neutrinos, and produce li[le flashes of light. 9

10 The Homestake Mine in South Dakota: (Ray Davies won Nobel Prize in Physics in 2003). Super- Kamiokande Neutrino Telescope in Japan: 10

11 RadiaVve Diffusion: Photons Random Walk out of the Sun. Photons take about a million years to random walk out of the Sun. Neutrinos fly out at about the speed of light, in a few seconds. JAVA APPLET 11

12 DEFINITIONS: apparent brightness versus absolute brightness or luminosity ProperVes of Stars apparent m magnitude versus absolute magnitude Inverse Square Law Same Luminosity, Twice as far away - - > 4x dimmer Measure Apparent magnitude And Distance (parallax) To stars luminosity 12

13 Stellar Surface Temperatures Measure the surface temperature of stars by taking a spectrum of the star and using Wien's Law. 13

14 Originally classified as A,B,C,.. the classificavon of stellar spectra was recast into OBAFGKM by Cecilia Payne- Gaposchkin. OBAFGKM: Oh, Be a Fine Girl Kiss Me Stellar Masses Stellar masses are measured by observing binary stars, and using Kepler's 3rd Law to determine the mass of the stars from the period of their orbit. Types of Binary Stars: * Visual Binaries - - direct image shows two stars orbivng each other * Spectroscopic Binaries two stars are too close to see as separate stars, but spectrum shows absorpvon lines from two stars with variable Doppler shigs. * Eclipsing Binaries - - one star disappears when it passes behind the other 14

15 Visual Binaries: ArVst s concepvon of a visual binary 15

16 Eclipsing Binaries: 16

17 Spectroscopic Binaries: 17

18 Summary of Stellar Proper7es: Spectral Type MASS (solar masses) Luminosity (solar luminosi7es) Surface Temperature (degrees K) Radius (solar radii) O ,000 40, B 15 13,000 28, A , F , G , K , M ,