"Stellar Spectra - PowerPoint"
Stellar Spectra • Brightness of stars • Colors/spectra of stars • Classifying stars: H-R diagram • Measuring star masses • Mass-luminosity relation Brightness of stars • The brightness of a star is a measure of its flux. • Ptolemy (150 A.D.) grouped stars into 6 `magnitude’ groups according to how bright they looked to his eye. • Herschel (1800s) first measured the brightness of stars quantitatively and matched his measurements onto Ptolemy’s magnitude groups and assigned a number for the magnitude of each star. Brightness of stars • In Herschel’s system, if a star is 1/100 as bright as another then the dimmer star has a magnitude 5 higher than the brighter one. • Note that dimmer objects have higher magnitudes Apparent Magnitude Consider two stars, 1 and 2, with apparent magnitudes m1 and m2 and fluxes F1 and F2. The relation between apparent magnitude and flux is: F1 m1 m2 2.5 log10 F 2 F1 m2 m1 / 2.5 10 F2 For m2 - m1 = 5, F1/F2 = 100. Flux, luminosity, and magnitude L F 4D 2 F1 m2 m1 2.5 log 10 F 2 L1 4D2 2 4D 2 L m2 m1 2.5 log 10 1 2 L2 D2 m2 m1 2.5 log 10 5 log 10 L1 D1 Distance-Luminosity relation: Which star appears brighter to the observer? Star 1 10L L d Star 2 10d Flux and luminosity L2 D2 10 10 L1 D1 L2 D2 m2 m1 2.5 log 10 5 log 10 L1 D1 m2 m1 2.5 log 10 10 5 log10 10 m2 m1 2.5 5 2.5 Star 2 is dimmer and has a higher magnitude. Absolute magnitude • The magnitude of a star gives it brightness or flux when observed from Earth. • To talk about the properties of star, independent of how far they happen to be from Earth, we use “absolute magnitude”. • Absolute magnitude is the magnitude that a star would have viewed from a distance of 10 parsecs. • Absolute magnitude is directly related to the luminosity of the star. Absolute Magnitude Absolute magnitude, M, is defined as M m 5 log 10 D 5 where D is the distance to the star measured in parsecs. For a star at D = 10 parsecs, 5log10 = 5, so M = m. Absolute Magnitude and Luminosity The absolute magnitude of the Sun is M = 4.83. The luminosity of the Sun is L 4.83 M / 2.5 L L 10 L M 4.83 2.5 log 10 L Note the M includes only light in the visible band, so this is accurate only for stars with the same spectrum as the Sun. Absolute Bolometric Magnitude and Luminosity The bolometric magnitude includes radiation at all wavelengths. The absolute bolometric magnitude of the Sun is Mbol = +4.74. L M bol 4.74 2.5 log10 L Is Sirius brighter or fainter than Spica: (a) as observed from Earth [apparent magnitude] (b) Intrinsically [luminosity]? Sun Little Dipper (Ursa Minor) Guide to naked-eye magnitudes Which star would have the highest magnitude? 1. Star A - 10 pc away, 1 solar luminosity 2. Star B - 30 pc away, 3 solar luminosities 3. Star C - 5 pc away, 0.5 solar luminosities 4. Charlize Theron What can we learn from a star’s color? The color indicates the temperature of the surface of the star. Observationally, we measure colors by comparing the brightness of the star in two (or more) wavelength bands. U B V This is the same way your eye determines color, but the bands are different. Use UVRI filters to determine apparent magnitude at each color Stars are assigned a `spectral type’ based on their spectra • The spectral classification essentially sorts stars according to their surface temperature. • The spectral classification can also use spectral lines. Spectral type • Sequence is: O B A F G K M • O type is hottest (~25,000K), M type is coolest (~2500K) • Star Colors: O blue to M red • Sequence subdivided by attaching one numerical digit, for example: F0, F1, F2, F3 … F9 where F1 is hotter than F3 . Sequence is O … O9, B0, B1, …, B9, A0, A1, … A9, F0, … • Useful mnemonics to remember OBAFGKM: – Our Best Astronomers Feel Good Knowing More – Oh Boy, An F Grade Kills Me – (Traditional) Oh, Be a Fine Girl (or Guy), Kiss Me Very faint The spectrum of a star is primarily determined by 1. The temperature of the star’s surface 2. The star’s distance from Earth 3. The density of the star’s core 4. The luminosity of the star Classifying stars • We now have two properties of stars that we can measure: – Luminosity – Color/surface temperature • Using these two characteristics has proved extraordinarily effective in understanding the properties of stars – the Hertzsprung-Russell (HR) diagram HR diagram HR diagram • Originally, the HR diagram was made by plotting absolute magnitude versus spectral type • But, it’s better to think of the HR diagram in terms of physical quantities: luminosity and surface temperature If we plot lots of stars on the HR diagram, they fall into groups These groups indicate types of stars, or stages in the evolution of stars Stars come in a variety of sizes Recall the Stefan-Boltzmann law relates luminosity, temperature, and size: L = 4R2sT4 • Small stars will have low luminosities unless they are very hot. • Stars with low surface temperatures must be very large in order to have large luminosities. Sizes of Stars on an HR Diagram • We can calculate R from L and T. • Main sequence stars are found in a band from the upper left to the lower right. • Giant and supergiant stars are found in the upper right corner. • Tiny white dwarf stars are found in the lower left corner of the HR diagram. Hertzsprung-Russell (H-R) diagram • Main sequence stars – Stable stars found on a line from the upper left to the lower right. – Hotter is brighter – Cooler is dimmer • Red giant stars – Upper right hand corner (big, bright, and cool) • White dwarf stars – Lower left hand corner (small, dim, and hot) Luminosity classes • Class Ia,b : Supergiant • Class II: Bright giant • Class III: Giant • Class IV: Sub-giant • Class V: Dwarf The Sun is a G2 V star ‘Spectroscopic Parallax’ Measuring a star’s distance by inferring its absolute magnitude (M) from the HR diagram 1. If a star is on the main-sequence, there is a definite relationship between spectral type and absolute magnitude. Therefore, one can determine absolute magnitude by observing the spectral type M. 2. Observe the apparent magnitude m. 3. With m and M, calculate distance Take spectrum of star, find it is F2V, absolute magnitude is then M = +4.0. Observe star brightness, find apparent magnitude m = 9.5. Calculate distance: m M 5 log 10 D 5 D 10 (5.55) / 5 pc 126 pc Masses of stars • Spectral lines also allow us to measure the velocities of stars via the Doppler shift that we discussed in searching for extra-solar planets. Doppler shift measurements are usually done on spectral lines. • Essentially all of the mass measurements that we have for stars are for stars in binary systems – two stars orbiting each other. • The mass of the stars can be measured from their velocities and the distance between the stars. Binary star systems Classifications • Double star – a pair of stars located at nearly the same position in the night sky. – Optical double stars – stars that appear close together, but are not physically conected. – Binary stars, or binaries – stars that are gravitationally bound and orbit one another. • Visual binaries – true binaries that can be observed as 2 distinct stars • Spectroscopic binaries – binaries that can only be detected by seeing two sets of lines in their spectra – They appear as one star in telescopes (so close together) • Eclipsing binaries – binaries that cross one in front of the other. Visual Binary Star Krüger 60 (upper left hand corner) About half of the stars visible in the night sky are part of multiple-star systems. Kepler’s 3rd Law applied to Binary Stars 4 2 a P 3 2 G(m1 m2 ) Where: • G is gravitational constant • G = 6.67·10-11 m3/kg-s2 in SI units • m1, m2 are masses (kg) • P is binary period (sec) • A is semi-major axis (m) Simplified form of Kepler’s 3rd law using convenient units 3 a M1 M 2 2 P Where M in solar masses a in AU P in Earth years Example: a = 0.05 AU, P = 1 day = 1/365 yr, M1 + M2 = 16.6 Msun Mizer-Alcor : A double-double-double system! 10 arcmin Alcor Mizar A Mizar A+B Mizar B Note: Mizar B is also a binary with period of 6 months! Mizar A (Binary, P = 20.5 days) Historical Notes on Mizar-Alcor discoveries • Romans (c. 200BC): Used Mizar-Alcor (11 arcmin separation) as test of eyesight for soldiers • Benedi Castelli (c. 1613, student and friend of Galileo) Alcor discovers Mizar is a double star (separation 15") – "It's one of the beautiful things in the sky and I don't believe that in our pursuit one could desire better", Mizar remarked Castelli in letter to Galileo A+B Sidus Ludoviciana – Both Galileo and Castelli were interested in ‘optical doubles’ to prove the heliocentric view of solar system (nearer star would move w.r.t more distant star annually) • Johann Liebknecht (1722) announced that the 8thmag star SW of Mizar was a new planet! (Incorrect observation of motion) – He named it Sidus Ludoviciana (Ludwig’s Star) in honor of his local monarch King Ludwig. • 1887: Pickering at Harvard announces Mizar A is a spectroscopic binary, 20.5 day period • 1996: NPOI directly images the Mizar A binary (separation 0.008 arcsec) Mizar A: A Spectroscopic Binary • 1887 Spectroscopy of Mizar A shows periodic doubling of spectral lines, with 20.5 day period Note: Asymmetric light curves indicated ellipical orbits Mizar observations using the NPOI (Naval Prototype Optical Interferometer, near Flagstaff Arizona) Determining masses of Mizar-A binary stars from observations of period, angular separation, distance 1. Distance (from parallax) d = 24 pc (88ly) 2. Max. angular separation (NPOI meas.) Θ = 0.008" 3. Physical separation D = θ·d=0.19 AU 4. Sum of masses (Kepler’s 3rd law) a3 0.19 3 M1 M 2 2 2 2.2 P 0.056 5. Orbit shows a1 ~ a2 (NPOI meas.) so: M1a1 M 2 a2 M1 M 2 1.1M Spectroscopy makes it possible to study binary systems in which the two stars are very close together. Determining component masses of eclipsing binaries using velocity curves 1. Determine semi-major axis using observed velocity (V), period (P) 2a1 2a2 v1 v2 P P a a1 a2 2. Determine sum of masses using Kepler’s 3rd law 3 a M1 M 2 2 P 3. Determine mass ratio using a1, a2 M 1a1 M 2 a2 or M 1v1 M 2v2 a1 a2 4. Use sum, ratio to determine component masses a = a1 + a2 Tilt of Binary Orbits We have been assuming that we see the binary system face on when imaging the orbit and edge-on when measuring the velocity. In general, the orbit is tilted relative to our line of sight. The tilt, or inclination i, will affect the observed orbit trajectory and the observed velocities. In general, one needs both the trajectory and the velocity to completely determine the orbit or some independent means of determining the inclination. Light curves of eclipsing binaries provide detailed information about the two stars. Light curves of eclipsing binaries provide detailed information about the two stars. Eclipsing Binary EQ Tau Light curve from Astronomical Laboratory Course Fall 2003 Java-animation of binary stars Eclipse of an Exo-planet (HD209458) “Vogt-Russell” theorem for spheres of water • Spheres of water have several properties: mass, volume, radius, surface area … • We can make a “Vogt-Russell” theorem for balls of water that says that all of the other properties of a ball of water are determined by just the mass and even write down equations, i.e. volume = mass/(density of water). • The basic idea is that there is only one way to make a sphere of water with a given mass. “Vogt-Russell” theorem • The idea of the “Vogt-Russell” theorem for stars is that there is only one way to make a star with a given mass and chemical composition – if we start with a just formed protostar of a given mass and chemical composition, we can calculate how that star will evolve over its entire life. • This is extremely useful because it greatly simplifies the study of stars and is the basic reason why the HR diagram is useful. Mass in units of Sun’s mass Mass - Luminosity relation for main- sequence stars Mass-Luminosity relation on the main sequence 3.5 L M L M Mass-Lifetime relation • The lifetime of a star (on the main sequence) is longer if more fuel is available and shorter if that fuel is burned more rapidly • The available fuel is (roughly) proportional to the mass of the star • From the previous, we known that luminosity is much higher for higher masses • We conclude that higher mass star live shorter lives M M 1 t 3.5 2.5 L M M A ten solar mass star has about ten times the sun's supply of nuclear energy. Its luminosity is 3000 times that of the sun. How does the lifetime of the star compare with that of the sun? 1. 10 times as long 2. the same 3. 1/300 as long 4. 1/3000 as long M 10 1 t L 3000 300 Mass-Lifetime relation Mass/mass of Sun Lifetime (years) 60 400,000 10 30,000,000 3 600,000,000 1 10,000,000,000 0.3 200,000,000,000 0.1 3,000,000,000,000 Stellar properties on main sequence • Other properties of stars can be calculated such as radius (we already did this). • The mass of a star also affects its internal structure (solar masses) Evolution of stars • We have been focusing on the properties of stars on the main sequence, but the chemical composition of stars change with time as the star burns hydrogen into helium. • This causes the other properties to change with time and we can track these changes via motion of the star in the HR diagram. HW diagram for people • The Height-Weight diagram was for one person who we followed over their entire life. • How could we study the height-weight evolution of people if we had to acquire all of the data from people living right now (no questions about the past)? • We could fill in a single HW diagram using lots of different people. We should see a similar path. • We can also estimate how long people spend on particular parts of the path by how many people we find on each part of the path. HR diagram