Chapter 30: Stars Sectin 1: The Sun Objectives 1. Explre the structure f the Sun. 2. Describe the slar activity cycle and hw the Sun affects Earth. 3. Cmpare the different types f spectra. A. Prperties f the Sun Is the largest bject in the slar system Gaseus interir because f very high temperature (~1 10 7 K in the center) Many gases exist as, but the uter layers are nt quite ht enugh t be plasma Cmpsitin Cnsists f H, He, and ther elements Is very similar t that f the gas giant planets Represents the cmpsitin f the galaxy as a whle B. The Slar Interir 1. Cre : the cmbining f lightweight nuclei int heavier nuclei In ur Sun s cre, where the pressure and temperature are extremely high, H nuclei fuse int He Hwever, the mass f the helium is less than the mass f the fused hydrgen atms, which means that mass is being lst. What happens t it? Accrding t Einstein s thery f special relativity:, where E is energy measured in jules (J) m is the mass cnverted t energy measured in kilgrams (kg) c is the speed f light measured in meters/secnd (m/s) Thus, mass and energy are equivalent, and the lst mass is cnverted t energy. At the Sun s rate f H fusing, it is abut halfway thrugh its lifetime Our Sun is and is expected t live. Fissin: 2. Znes (1) : abve the cre, radiatin transfers energy frm particle t particle, as atms cntinually absrb energy and then re-emit it (2) : abve the radiative zne, cnvectin carries the energy t the Sun s surface, r phtsphere 30-1
«It takes fr the energy prduced in the cre t travel thrugh bth znes t the phtsphere. C. The Sun s Atmsphere 1. Phtsphere Is ~400 km thick Has an average temperature = ~5800 K 2. Chrmsphere Is lcated abve the phtsphere Is ~2500 km thick 3. Crna Is the tp (r utermst) layer Extends several millin km utward frm the tp f the chrmsphere Emits, a fast-mving gas cmpsed f ins that flws thrugh the slar system the slar wind ins becme trapped by Earth s, which are tw huge rings in Earth s magnetic field, where they cllide with Earth s atmspheric gases and prduce aurras 4. Features : cler areas that frm n the surface (phtsphere) f the Sun and appear as dark spts; have a dark inner ring called the umbra and a light uter ring called the penumbra Crnal hles: areas f lw density in the crna that are ften lcated ver sunspt grups frm which the particles that cmprise the slar wind escape : vilent eruptins f particles and radiatin frm the surface (phtsphere) f the Sun that are assciated with sunspts; can interfere with cmmunicatins n Earth and damage satellites Prminence: D. Slar Activity Cycle Lasts (1) Starts with minimum sunspts and prgresses t maximum sunspts (2) : Sun s magnetic field reverses in plarity 30-2
(3) Restarts with minimum spts and prgresses t maximum spts (4) Sun s magnetic field reverts t the riginal plarity t cmplete the cycle Impact n Earth Evidence f subtle climate variatins within In the 1600s, the slar activity cycle stpped and there were n sunspts fr nearly 60 years knwn as the Little Ice Age because f very cld weather in Eurpe and N. America E. Energy frm the Sun Earth receives 1354 J f energy in 1 m 2 /sec (1354 W/m 2 ), but nt all f this energy reaches the grund because sme is absrbed and scattered by the atmsphere F. Spectra Spectrum: the arrangement f visible light accrding t wavelengths 3 types: (1) spectrum: is prduced by a ht slid r liquid r by a ht, highly-cmpressed gas; has n breaks in it (2) spectrum: is prduced when a cler gas (such as ur atmsphere) is in frnt f the ht slid r liquid r by a ht, highly-cmpressed gas; has dark (absrptin) lines that are due t different elements absrbing light at specific wavelengths (3) spectrum: is prduced when a clud f gas des nt have a ht slid r liquid r a ht, highly-cmpressed gas behind it; has bright (emissin) lines that depend n the element being bserved By cmparing labratry spectra f different gases with the dark absrptin lines in the slar spectrum, it is pssible t identify the elements that make up the Sun s uter layers. Sectin 2: Measuring the Stars Objectives 1. Describe star distributin and distance. 2. Classify the types f stars. 3. Summarize the interrelated prperties f stars. A. Grups f Stars 1. Cnstellatins Are the grups f stars named after animals, mythlgical characters, r everyday bjects : appear t mve arund the N r S ple; can be seen all year lng Summer, fall, winter, and spring: can be seen nly at certain times f the year because f Earth s changing psitin in its rbit arund the Sun 30-3
2. Star Clusters (1) : the stars are nt densely packed (2) : stars are densely packed int a spherical shape 3. Are tw stars that are gravitatinally bund tgether and that rbit a cmmn center f mass «>50% f stars are either binary stars r members f multiple-star systems B. Stellar Psitin and Distances Astrnmers measure lng distances using tw units: (1), r the distance that light travels in ne year = 9.461 10 12 km (2) = 3.26 ly r 3.086 10 13 km : the apparent shift in psitin f an bject caused by the mtin f the bserver The parallax shift f a star can be used t estimate the distance t the star (up t 500 pc using the latest technlgy). C. Basic Prperties f Stars 1. Size: diameters range frm 0.1X the Sun s diameter t 100s f times larger 2. Mass: varies frm <0.01X t >20X the Sun s mass 3. Magnitude: measure f a star s brightness, which depends n bth its distance frm Earth and its energy utput : based n the ancient Greek system f classificatin which rates hw bright a star appears t be 5 magnitudes 100X increase in brightness Negative numbers are used fr bjects brighter than +1 But nt all bjects are the same distance frm Earth, s astrnmers use : the brightness an bject wuld have if it were placed at a distance f 10 pc. Mst negative numbers = brightest bjects Mst psitive numbers = dimmest bjects Equals the surface energy utput/secnd Measured in watts (W; units f energy emitted per secnd) Sun s luminsity 3.85 10 26 W D. Spectral Classificatin f Stars All stars, including the Sun, Are classified accrding t the patterns f dark absrptin lines in their spectra 30-4
Have nearly identical cmpsitins, s the differences in their spectra are almst entirely a result f temperature effects Are assigned a spectral type, which cnsists f a letter (O, B, A, F, G, K, and M) and a number (0-9) Crrespnd t surface temperatures: Our Sun is a, which crrespnds t a surface (phtsphere) temperature f ~5800 K Mtin between the surce f light and the bserver causes spectral lines t shift in wavelength due t the Dppler Effect. If a star is mving tward the bserver, the spectral lines are shifted tward shrter wavelengths, r. If the star is mving away, the wavelengths becme lnger, r. = higher speeds, and thus, spectral line shifts can be used t determine the speed f a star s mtin E. Hertzsprung-Russell Diagrams AKA H-R diagrams Plt and n the hrizntal (x) axes Plt the n the vertical (y) axis Grups 1) Plt diagnally frm the upper-left crner t the lwer-right crner Includes ~90% f stars (including ur Sun) 2) : large, bright, cl, luminus stars, which plt tward the upper-right crner 3) : very large, bright, cl, luminus stars which plt in the upper-right crner 4) : small, dim, ht stars, which plt in the lwer-left crner Sectin 3: Stellar Evlutin Objectives 1. Explain hw astrnmers learn abut the internal structure f stars. 2. Describe hw the Sun will change during its lifetime and hw it will end up. 3. Cmpare the evlutins f stars f different masses. A. Basic Structure f Stars 30-5
Is the balance between gravity squeezing inward and pressure frm nuclear fusin and radiatin pushing utward Is gverned by the mass f a star Must hld fr any stable star; therwise, the star wuld expand r cntract (E = mc 2 ) Main sequence stars: prduce energy by fusing H int He Nn-main sequence stars: either fuse different elements r d nt underg fusin at all B. Stellar Evlutin As a star ages, its internal cmpsitin changes because fusin cnverts ne element int anther. As the cmpsitin changes, the star s density increases, its temperature rises, and its luminsity increases. When the nuclear fuel runs ut, the star s internal structure and mechanism fr prducing pressure must change t cunteract gravity. C. The Birth f a Star 1. A (pl. nebulae, a clud f interstellar gas and dust) cllapses n itself as a result f its wn gravity. 2. The clud cntracts and its rtatin frces it int a disk shape. 3. At the center f the disk is a ht cndensed bject a that will becme a new star. 4. Eventually, the prtstar becmes ht enugh fr fusin t begin. 5. Once fusin begins, the prtstar has sufficient internal heat t prduce the pressure needed t balance gravity. 6. The prtstar stabilizes and becmes a. «D. The Sun s Life Cycle 1. A star with the Sun s mass takes ~10 billin years t cnvert all f the cre s H int He. When the cre s H is used up, the cre is then made f He and the uter layers are made f H-dminated gas, sme f which cntinues t react. 2. The star becmes a red giant when the energy prduced in the thin H layer frces the star s uter layers t expand and cl. While the star is a red giant, it lses gas frm its uter layers as its cre becmes ht enugh (at 100 millin K) fr its He t react and frm C. When the cre s He is used up, the star is left with a cre made f C. 30-6
3. A star f the Sun s mass never becmes ht enugh fr C t react, s the star s energy prductin ends. The uter layers expand nce again and are driven ff, and the star becmes a shell f gas called a planetary nebula. 4. In the center f the planetary nebula, the star s C cre remains as a white dwarf. A white dwarf is stable because it is supprted by the resistance f e - being squeezed clse tgether and des nt require a surce f heat t be maintained. «A star less massive than the Sun has a similar life cycle, except that He may never frm C in the cre, and the star ends as a white dwarf made f He. E. Life Cycles f Massive Stars 1. A star mre massive than the Sun begins its life high n the main sequence, fusing H int He. It underges many reactins and prduces many elements in its interir. 2. As it expands at the end f each reactin phase, the massive star becmes a red giant. 3. As mre shells are frmed by the fusin f different elements, the star expands and becmes a supergiant. 4. Over time, the supergiant lses much f its mass and becmes a white dwarf. The white dwarf s cmpsitin is determined by hw many reactins the star went thrugh befre the reactins stpped. 5. A star that begins with a mass between 8 and 20X the Sun s mass will end up with a cre that is t massive t be supprted by e- pressure. Once n mre energy-prducing reactins can ccur, the cre vilently cllapses, and the p + and e - in the cre merge t frm n 0. The resistance f the n 0 t being squeezed creates pressure, which halts the cllapse f the cre, and ultimately results in a neutrn star. A neutrn star has a mass f 1.5-3X the Sun s mass but a radius f nly ~10 km. 6. When infalling gas strikes the hard surface f the neutrn star, it expldes utward. A supernva (pl. supernvae) is a massive explsin in which the entire uter prtin f the star is blwn ff and elements that are heavier than Fe are created. 7. 30-7
A star that begins with >~20X the Sun s mass will nt be able t frm a neutrn star. The resistance f n 0 t being squeezed is nt great enugh t stp the cllapse, s the cre f the star simply cntinues t cllapse frever, cmpacting matter int a smaller and smaller vlume. A black hle is a small, extremely dense remnant f a star whse gravity is s immense that light cannt escape its gravity field. 30-8