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OUR UNIVERSE Week 8 The Sun & the Stars. Visible UV Sun’s Structure Sun’s Structure The ultra-hot core extends outward from the star's center to about 20% of its radius. The temperature at the centre of the core is around 15 million kelvin, and it gradually decreases further from the center. The core is the location within the Sun in which hydrogen fusion occurs. Above the core is the radiation zone. It extends from the top of the core outward to about 70% of the Sun's radius. Temperatures varies from 10 to 5 million kelvin. Energy is carried through the radiation zone via electromagnetic radiation (photons). Above the radiation zone, and extending all the way to the "surface" of the Sun, is the convection zone. This part of the Sun is relatively "cool", with temperatures ranging downward from a peak of around 2 million kelvin. Energy flows upward through this area in a different manner than in the underlying radiation zone. Gigantic blobs of matter, heated by the radiation zone below, rise to the Sun's surface, carrying heat with them. As these blobs of plasma emit their energy into space at the Sun's surface, they cool somewhat; enough so that their densities increase and they sink back down. This convective motion is akin to that seen in a lava lamp. Solar Granulation is due to convection cells The Sun’s internal Structure Convection cells At the topmost boundary of the convection zone lies the photosphere. The photosphere is often referred to as the "surface" of the Sun. The photosphere marks an abrupt transition in the optical properties of the material that makes up the Sun. Below the photosphere, the photons bounce around so much that they don't travel direct paths to viewers on Earth. Hence we cannot see deeper into the Sun than the photosphere. So the photosphere is the "visible surface" of the Sun. The temperature of the photosphere is about 5800 K. Above the photosphere the Sun's vast “atmosphere” extends outward into interplanetary space. In the case of the Sun, the density of material in the solar atmosphere is much less than is the case below the photosphere within the Sun's "interior". Also, the physical properties that control motions of material and the temperatures encountered are far different in the Sun's atmosphere than in the layers of the Sun beneath the photosphere. There are two major regions within the Sun's atmosphere: the lower and much smaller chromosphere, and the upper and much larger corona. The relatively thin chromosphere is just a few thousand kilometers deep, less than Earth's diameter. Although temperatures within the Sun gradually decrease as one moves outward, (from 15 million kelvin in the core to 5,800 kelvin at the photosphere) they begin to climb once again as we rise through the Sun's atmosphere. The temperature of the chromosphere increases from 4,300 kelvin (slightly above the photosphere) to around 50,000 kelvin (near the corona). Powerful magnetic fields in the Sun's atmosphere accelerate the plasma as they transfer energy to it, heating the material in ways that scientists still don't fully understand. Until relatively recent times, when special filters and space-based telescopes became available, the Sun's atmosphere was only visible during total solar eclipses. During an eclipse, the chromosphere could be seen as a colorful reddish zone around the edge of the occluded solar disk, thus earning the region its name (Greek "chromos" = "color"). Chromosphere The Sun's much larger upper atmosphere, the corona, extends unevenly for millions of kilometers into space. The temperature of the solar atmosphere climbs sharply in a narrow transition region between the chromosphere and the corona. The temperatures in the corona range from around 800,0000 K to 3 million K corona, Matter is continuously flung outward by the Sun. An electrically charged "soup" of protons, electrons, and lesser numbers of heavier atomic nuclei flows outward into space. This extremely tenuous plasma is called the solar wind. In a sense, the solar wind is a vast extension of the Sun's atmosphere. The solar wind flows past Earth and beyond. All of the planets are within the gigantic "bubble" of the solar wind. Eventually, on the far edge of our solar system, the solar wind merges with the outpourings of other stars, and the extended solar atmosphere ends. The gigantic region within this solar wind "bubble" is called the heliosphere. The boundary of the heliosphere, where the extended atmosphere of the Sun finally gives way to interstellar space, is called the heliopause. The location of the heliopause is something like 70AU from the Sun. Solar Prominence in UV Sunspots It appears that sunspots are the visible counterparts of magnetic flux tubes in the sun's convective zone that get "wound up" by differential rotation. Convection cells Differential rotation causes field lines to be wrapped around the Sun Sunspots migrate to equator where they cancel out and eventually reverse the overall field If the stress on the tubes reaches a certain limit, they curl up like a rubber band and puncture the sun's surface. Convection is inhibited at the puncture points; the energy flux from the sun's interior decreases; and with it surface temperature. Sunspots are depressions on the sun's surface. Sunspots come in pairs with opposite magnetic polarity. From cycle to cycle, the polarities of leading and trailing with respect to the solar rotation] sunspots change from north/south to south/north and back. Sunspots usually appear in groups. Sunspot Structure • The sunspot itself can be divided into two parts: • The central umbra, – which is the darkest part, where the magnetic field is approximately vertical. • The surrounding penumbra, – which is lighter, where the magnetic field lines are more inclined. • Sunspot activity cycles about every eleven years. • The point of highest sunspot activity during this cycle is known as Solar Maximum, and the point of lowest activity is Solar Minimum. • Early in the cycle, sunspots appear in the higher latitudes and then move towards the equator as the cycle approaches maximum. Sun spots have a 11 year cycle (magnetic field reversal) Differential rotation causes field lines to be wrapped around the Sun Sunspots migrate to equator where they cancel out and eventually reverse the overall field The Solar Corona Visible The Solar Corona A corona is a type of plasma "atmosphere" of the Sun. It extends millions of kilometers into space Most easily seen during a total solar eclipse, but also observable in a coronagraph. Visible The Solar Corona A corona is a type of plasma "atmosphere" of the Sun. It extends millions of kilometers into space Most easily seen during a total solar eclipse, but also observable in a coronagraph. Visible Light from the corona comes from three primary sources (called K, F and E), which are called by different names although all of them share the same volume of space. The K-corona (K for kontinuierlich, "continuous" in German). Created by sunlight scattering off free electrons; Doppler broadening of the reflected photospheric absorption lines completely obscures them, giving the spectrum the appearance of a continuum with no absorption lines. The F-corona (F for Fraunhofer). Created by sunlight bouncing off dust particles, and is observable because its light contains the Fraunhofer absorption lines that are seen in raw sunlight; the F-corona extends to very high elongation angles from the Sun, where it is called the Zodiacal light. The E-corona (E for emission). Result from spectral emission lines produced by ions that are present in the coronal plasma; it may be observed in broad or forbidden or hot spectral emission lines and is the main source of information about the corona's composition. The Solar Neighbourhood Transparency circles at 5, 10, 15 ly Transparency Measuring Stars Temperature T Distance d Luminosity L Radius R Mass M Element Abundances Measuring Stars Distances (d) needed for finding L • Stellar parallax (Hipparchos satellite yielded a revolutionary improvement) • Proper motion studies • Moving clusters - stars seem to get closer as the cluster recedes. • Comparison with standard stars of known distance Measuring the Stars. First a Reminder: Measuring Distances using the Earth’s orbit around the Sun as a baseline Orion We only observe a 2-dimensional projection of objects in the sky. Orion’s belt We need extra information to find their position in 3-dimensions their distance d Distance by Parallax d AB 2 (a) Planet observed from A & B against background of distant stars. (b) Photos taken from A & B show the planet’s image has moved against the background stars. Distance by Parallax d AB 2 (a) Planet observed from A & B against background of distant stars. (b) Photos taken from A & B show the planet’s image has moved against the background stars. AB d tan 2 Stellar Parallax Proper Motion The proper motion of a star is its angular change in position over time as seen from the Sun. It is measured in seconds of arc per year. Proper Motion This contrasts with radial velocity, which is the time-rate of change in distance toward or away from the viewer. (usually measured by the Doppler shift) Proper Motion Barnard’s Star (at 1.82 pc, 5.98 ly) moves over 22yrs by 230 = 3.8´ 1894 1916 Stellar motion Vt v Transverse velocity (measured by Vr Proper Radial velocity Motion) (measured by Doppler Shift) Stellar Motion. An example: Centauri Measuring Stars. Radius R Stellar Sizes Directly from imaging & Interferometry Indirectly from Luminosity The Sun’s Radius Directly from imaging RSun = 6.96108 m = 109 RE 1 AU = 1.4961011 m = 215 RE Mercury’s orbit = 83 Rsun Stellar Radii Directly from Imaging & Interferometry (but restricted to nearby large stars) Indirectly from L & T: Stellar Radii Directly from Imaging & Interferometry (but restricted to nearby large stars) Indirectly from L & T: Stefan’s Law Stellar Radii Directly from Imaging & Interferometry (but restricted to nearby large stars) Indirectly from L & T: Stefan’s Law Flux T 4 Stellar Radii Directly from Imaging & Interferometry (but restricted to nearby large stars) Indirectly from L & T: Flux = Power per unit area Stellar Radii Directly from Imaging & Interferometry (but restricted to nearby large stars) Indirectly from L & T: Luminosity = Power radiated Stellar Radii Directly from Imaging & Interferometry (but restricted to nearby large stars) Indirectly from L & T: Flux = Luminosity per unit area Stellar Radii Directly from Imaging & Interferometry (but restricted to nearby large stars) Indirectly from L & T: Lstar 4R T 2 star 4 HST Resolves a Star The Red Giant Betelgeuse in the constellation Orion Stellar Sizes vary greatly Betelgeuse 300 R THE END OF LECTURE 16 OUR UNIVERSE Lecture No. 17 Measuring Stars. Temperature T ( i.e. Surface T ) & Luminosity L The Black Body Spectrum Here plotted against wavelength The Sun’s continuous spectrum is well approximated by a Black Body or Planck Spectrum at 5800 K A Star’s Colour Depends on its Temperature Planck spectrum: & therefore the colours of stars only depend on T max T max T 1 Brightness through UBV Filters Depends on a Star’s Temperature Brightness through UBV Filters Depends on a Star’s Temperature Use of filters provides a quick and convenient method of estimating stellar properties Each filter samples a different part of the Planck spectrum. The ratio of brightness in B and V filters determines the COLOUR TEMPERATURE B-V log T UBV Filters are supplemented with 8 IR filters U 360 nm B 420 nm V 540 nm R 700 nm --------------------------------------I 900 nm = 0.90 m J 1250 nm = 1.25 m K 2200 nm = 2.20 m --------------------------------------L 3400 nm = 3.40 m M 4900 nm = 4.90 m --------------------------------------N 10200 nm = 10.20 m Q 20000 nm = 20.00 m The Black Body Laws Stefan-Boltzmann Law for the Flux Flux T 4 Watts -2 m The Total Power L* emitted by a star, of Radius R* and Area = 4 R*2 is L* 4R T 2 * 4 Watts Measuring Stars Luminosity L Watts needs distance d & brightness (Flux) at Earth -2 (b Watts m ) LUMINOSITY L* is the Total Power emitted by a star, 2 of Radius R* and Area = 4 R* L* 4R T 2 * 4 Watts The Measured apparent brightness b* is the Flux reaching the Earth at distance d* from the Star The Measured apparent brightness b* is the Flux reaching the Earth at distance d* from the Star L* b* 2 4d* Watts -2 m If we Measure both apparent brightness b* & distance d* we obtain Luminosity L* If we Measure both apparent brightness b* & distance d* we obtain Luminosity L* L* 4d b 2 * * Watts Measuring both d* and the apparent brightness b* as well as T (from spectrum) gives us the star’s Radius L* 4R T 2 * 4 Measuring both d* and the apparent brightness b* as well as T (from spectrum) gives us the star’s Radius L* R 4 4T 2 * L0 b0 2 4d 0 L* b* 2 4d* The Luminosity is often found in terms of a standard star such as the Sun. The Luminosity is usually expressed in terms of the Solar Luminosity Lsun For example, a Supergiant : 4 L *= 10 L s u n If distance d not known we must resort to other tricks e.g. to find the distance of a cluster of stars, compare its HR diagram with the standard Main Sequence of Stars with known distances Hertzprung Russell Diagram Recall Spectral information from stars • Peak or T = Temperature • Presence of Line Composition & T • Line intensity Composition & T • Line width T, density, rotation… •Doppler shift Line-of-sight velocity Principal Types of Stellar Spectra SUN Principal Types of Stellar Spectra 35,000 K 22,000 K 16,400 K 10,800 K 8,600 K 7,200 K 6,500 K 5,900 K 5,600 K 5,200 K 4,400 K 3,700 K 3,500 K Spectral Classes: O B A F G K M Stellar Classification • Astronomers classify of stars based on their spectral characteristics. • Based on which atomic excitations are most prominent in the light, • giving an objective measure of the temperature in this chromosphere. • Most stars are currently classified using the letters • O, B, A, F, G, K and M, • O stars are the hottest and the letter sequence indicates successively cooler stars up to the coolest M class. • According to an informal tradition: • O stars are "blue" • B "blue-white" • A stars "white" • F stars "yellow-white" • G stars "yellow" • K stars "orange" • M stars "red“ • Spectral letter is enhanced by a number from 0 to 9 indicating tenths of the range between two star classes. • E.g., A5 is five tenths between A0 and F0, but A2 is two tenths of the full range from A0 to F0. • Aditionally, the luminosity class expressed by the Roman numbers I, II, III, IV and V. • It expressed the width of certain absorption lines in the star's spectrum. • It has been shown that this feature is a general measure of the size of the star, and thus of the total luminosity output from the star. • Class I are generally called supergiants, • class III simply giants and class • V either dwarfs or more properly main sequence stars. • For example our Sun has the spectral type G2V, • (which might be interpreted as "a 'yellow' two tenths towards 'orange' main sequence star“) • The apparently brightest star Sirius has type A1V. Spectral Classes: O B A F G K M ++++ O Be A Fine Girl Kiss Me Girl Guy Strengths of Absorption lines in Stars across the HR Diagram (which lines dominate depends on temperature) SUN Principal Types of Stellar Spectra 35,000 K 22,000 K 16,400 K 10,800 K 8,600 K 7,200 K 6,500 K 5,900 K 5,600 K 5,200 K 4,400 K 3,700 K 3,500 K Spectral Class Hydrogen atom Spectral Series L L etc H H etc P P Populations in H levels vs Temperature 1 0.9 n=1 Ground State 0.8 Population 0.7 N( 1 T ) N( 2 T ) 0.6 N( 3 T ) 0.5 N( 4 T ) N( 5 T ) 0.4 Part shown expanded in the next slide 0.3 0.2 0.1 0 0 2 10 4 4 10 6 10 4 4 8 10 4 T Temperature 3000 to 100,000 K 1 10 5 n=5 n=4 n=3 n=2 Populations showing details for excited states. Note the expanded scale. 0 . 0 6 n=1 Population n=5 N ( 2 T ) N ( 3 T ) 0 . 0 4 n=4 N ( 4 T ) N ( 5 T ) n=3 N ( 1 T ) 0 . 0 2 0 n=2 4 2 1 0 4 4 1 0 4 6 1 0 T 4 8 1 0 Temperature 3000 to 100,000 K 5 1 1 0 Measuring Stars Temperature T, etc • U, B, V filters - colour Temperature • Fit the spectrum to Planck • Detailed modelling of the spectrum line shapes and strengths - this also gives the surface gravity & the elemental abundances. Measuring Stars Element Abundances SAD Solar Abundance Distribution Mass fractions: H X~ 0.73, He Y~ 0.25 Metallicity Z ~ 0.02 Heavier elements are “Metals” in astronomy gSun = 274 ms-2 28 gEarth The “Cosmic Abundance” of the Elements determined from the Sun, Stars and Meteorites NB: log Abundance CNO Fe “Cosmic Abundance” of the Elements NB: log Abundance Fe Stars are classified into 2 broad categories depending on Element Abundances Population I: H X~ 0.73, He Y~ 0.25 Metallicity Z ~ 0.02 Sun Stars in the disc of the Galaxy Population II: H X~ 0.75, He Y~ 0.25 Metallicity Z ~ 0.001 Globular Cluster Stars in the halo of the Galaxy The Herzsprung-Russell Diagram Luminosity Classes supergiants giants dwarfs Sun G2V Vega A0V Barnard’s Star (Dwarf) M4V Betelgeuse (Red Giant) M2Ia THE END OF Week 6