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  2. The Universe(A good video to watch)

  3. Solar system • Solar system has 8 planets (earlier 9 planets including Pluto) • Planets move around in elliptical orbits • The elliptical orbits are characterized by their eccentricities • Ellipse with ‘e’ close to 1 are more flatter • Near circular orbits have ‘e’ close to 0 • Inner planets are planets closest to Sun – Mercury, Venus, Earth and Mars • Outer planet are Jupiter, Saturn, Uranus, Neptune

  4. Eccentricity of an elliptical orbit • Eccentricity is the ratio between the distance between the two foci of the ellipse and the length of the major axis of the ellipse (e=0 is perfect circle and e=1 is straight line)

  5. Status of Pluto • Pluto first discovered in 1930 by Clyde W. Tombaugh • A full-fledged planet is an object that orbits the sun and is large enough to have become round due to the force of its own gravity. In addition, a planet has to dominate the neighborhood around its orbit. • Pluto has been demoted to be a “Dwarf planet” (2006) because it does not dominate its neighborhood. Charon, its large “moon,” is only about half the size of Pluto, while all the true planets are far larger than their moons.

  6. Solar system(Sidereal period is the Time required for a celestial body in the solar system to complete one revolution with respect to the fixed stars)

  7. Asteroid belt • Asteroid Belt is the region between the inner planets and outer plants where thousands of asteroids are found orbiting around the Sun • Asteroids are chunks of rock and metal that orbit around the Sun • The largest known asteroid is CERES

  8. Beyond solar system – Other stars • Other stars – There billions and billions of stars other than our sun in the universe - Nearest star system is Alpha Centauri which consists of 3 stars - Proxima Centauri at 4.22 light years and Alpha Centauri A, B (binary stars) at 4.35 light years • Stars are of different types – Giants, Super Giants, Red Giants, Neutron Star, White Dwarfs, Main Sequence Stars, Black Holes - all names based on their different stages of evolution

  9. Beyond solar system – Stellar clusters • Stellar clusters are groups of stars that are gravitationally bound • Two types of stellar clusters • Globular cluster – tight groups of hundreds of thousands of very old stars • Open cluster - contain less than a few hundred members, and are often very young - may eventually become disrupted over time and no longer gravitational bound – move in same direction in space – referred to as stellar association or moving group

  10. Beyond solar system - Galaxies • We belong to the Milky Way galaxy – spiral galaxy – 100,000 light years wide – 16,000 light years thick at the centre – has three distinct spiral arms - Sun is positioned in one of these arms about two-thirds of the way from the galactic center, at a distance of about 30,000 light-years • The Andromeda Galaxy, M31, is the nearest major galaxy to our own Milky Way. It is about 3 million light years away

  11. Clusters • Group of galaxies form a cluster • Milky Way belongs to “The Local Group” cluster that consists of over 30 galaxies • Local Group is held together by the gravitational attraction between its members, and does not expand with the expanding universe • Its two largest galaxies are the Milky Way and the Andromeda galaxy - most of the others are small and faint.

  12. Super-clusters • Groups of clusters and smaller galaxy groups • Not bound by gravity • Take part in expansion of universe • Largest known structure of cosmos • Our local cluster belongs to the local super cluster, also known as the virgo super-cluster

  13. Map of Super-clusters

  14. What is our address • If you mail something you need to let the post office know exactly where it needs to go. • So…. • What is our address in the universe?

  15. What is our address? Universe Local (virgo) super-cluster Local cluster Milky way Solar system Inner planets Earth North America Wisconsin Lincoln High School

  16. Beyond solar system - Nebula • Nebula is a huge, diffuse cloud of gas and dust in intergalactic space. The gas in nebulae (the plural of nebula) is mostly hydrogen gas (H2). • THEY ARE THE BIRTH PLACE OF STARS

  17. The Celestial Sphere

  18. The Celestial Sphere Zenith = Point on the celestial sphere directly overhead Nadir = Point on the c.s. directly underneath (not visible!) Celestial equator = projection of Earth’s equator onto the c. s. North celestial pole = projection of Earth’s north pole onto the c. s.

  19. Different sets of constellations are visible in northern and southern skies.

  20. Apparent Motion of The Celestial Sphere

  21. Apparent Motion of The Celestial Sphere (2)

  22. Constellation • A constellation is a group of stars that, when seen from Earth, form a pattern • The stars in the sky are divided into 88 constellations (12 based on zodiac signs) • The brightest constellation is Crux (the Southern Cross) • The constellation with the greatest number of visible stars in it is Centaurus (the Centaur - with 101 stars) • The largest constellation is Hydra (The Water Snake) which extends over 3.158% of the sky. • One of the most popular constellation is the Orion

  23. What we see… The stars of a constellation only appear to be close to one another Usually, this is only a projection effect. The stars of a constellation may be located at very different distances from us.

  24. Seasonal Changes in the Sky • The night-time constellations change with the seasons. • This is due to the Earth’s orbit around the Sun.

  25. The Sun and Its Motions Due to Earth’s revolution around the sun, the sun appears to move through the zodiacal constellations. (Imagine you look at the sun in the daytime. The constellation that would be in its background is the zodiac sign for that month)

  26. CONSTELLATIONS THAT WE MAY SEE IN THE NIGHT January  Caelum, Dorado, Mensa, Orion, Reticulum, Taurus February  Auriga, Camelopardalis, Canis Major, Columba, Gemini, Lepus, Monoceros, Pictor March  Cancer, Canis, Minor, Carina, Lynx, Puppis, Pyxis, Vela, Volans April  Antlia, Chamaeleon, Crater, Hydra, Leo, Leo Minor, Sextans, Ursa Major May  Canes Venatici, Centaurus, Coma Berenices, Corvus, Crux, Musca, Virgo June  Boötes, Circinus, Libra, Lupus, Ursa Minor July  Apus, Ara, Corona Borealis, Draco, Hercules, Norma, Ophiuchus, Scorpius, Serpens, Triangulum Australe August  Corona Austrina, Lyra, Sagittarius, Scutum, Telescopium September  Aquila, Capricornus, Cygnus, Delphinus, Equuleus, Indus, Microscopium, Pavo, Sagitta, Vulpecula October  Aquarius, Cepheus, Grus, Lacerta, Octans, Pegasus, Piscis Austrinus November  Andromeda, Cassiopeia, Phoenix, Pisces, Sculptor, Tucana December  Aries, Cetus, Eridanus, Fornax, Horologium, Hydrus, Perseus, Triangulum

  27. Source of stellar energy P-P Chain 109years H1 H1 He3 H1 1 sec He4 H1 106year H1 H1 H1 Gamma ray H1

  28. P-P Chain • The net result is 4H1 --> He4 + energy + 2 neutrinos where the released energy is in the form of gamma rays and visible light.

  29. Hydrostatic equilibrium

  30. Luminosity and Apparent Brightness * Luminosity is the total light energy emitted per second. (Power) * Apparent brightness is the light received per unit area per second at the earth’s surface. **The luminosity from our sun is 3.9 x 10^26W

  31. Black body • A black body is a good emitter of radiation as well as a good absorber of radiation

  32. Black body radiation • The intensity of light emitted by a black body is distributed over a range of wavelength. • The maximum intensity is radiated at a particular wavelength designated as lmax • The value of lmax decreases with increasing temperature as per the Wien’s Displacement given by • lmaxT = constant (2.9 x 10-3 mK) • The area under each curve gives the total energy radiated by the black body (luminosity) per second at that temperature and is governed by the Stefan-Boltzmann law, which is • L = sAT4 • where A is the surface area of the black body (for a sphere 4πr^2) and s (sigma) is the known as the Stefan constant(5.67 x 10-8 Wm-2K-4)

  33. Practice Problem • The sun has an approximate black-body spectrum with most of the energy radiated at a wavelength of 0.5 μm. Find the surface temperature of the sun.

  34. Practice Problem • The sun (radius R=7.0x10^8m) radiates a total power of 3.9x10^26W. Find its surface temperature.

  35. Practice Problem • The sun is 1.5 x 10^11m from Earth. Estimate how much energy falls on a surface area of 1m^2 in one year. • 3.9 x 10^26/(4pi(1.5 x 10^11)^2) • Ans x seconds in one year • = 4.4 x 10^10J

  36. Practice Problem 2 • The radius of star A is three times that of star B, and its temperature is double that of B. Find the ratio of the luminosity of A to that of B.

  37. Practice Problem 2 continued • The stars in the first part have the same apparent brightness when viewed from Earth. Calculate the ratio of their distances. • The radius of star A is three times that of star B, and its temperature is double that of B. Find the ratio of the luminosity of A to that of B.

  38. Practice Problem • The wavelength maximum in the spectrum of Betelgeuse is 9.6x10^-7m. The luminosity of Betelgeuse is 10^4 times the luminosity of the sun. Estimate the surface temperature of Betelgeuse and also its radius in terms of the radius of the sun.

  39. Practice Problem • The apparent brightness of a star is 6.4 x 10^-8 W/m^2. If its distance is 15ly, what is its luminosity? • 1ly = 9.46 x 10^15m

  40. Practice Problem • A star has half the sun’s surface temperature and 400 times its luminosity. How many times bigger is it?


  42. Stellar Spectra Absorption Lines and Classifications

  43. Spectral Classification of Stars Spectral Class Summary

  44. Spectral Classification of Stars Spectral Class Summary Mnemonics to remember the spectral sequence:

  45. Organizing the Family of Stars: The Hertzsprung-Russell Diagram We know: Stars have different temperatures, different luminosities, and different sizes. To bring some order into that zoo of different types of stars: organize them in a diagram of Luminosity Temperature (or spectral type) versus Absolute mag. Hertzsprung-Russell Diagram Luminosity or Temperature Spectral type: O B A F G K M

  46. Hertzsprung-Russell Diagram Betelgeuse Rigel Absolute magnitude Sirius B Color index, or spectral class