* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Lecture 10 Spectra of Stars and Binaries
Rare Earth hypothesis wikipedia , lookup
Space Interferometry Mission wikipedia , lookup
Aries (constellation) wikipedia , lookup
Orion (constellation) wikipedia , lookup
Auriga (constellation) wikipedia , lookup
Corona Borealis wikipedia , lookup
Canis Minor wikipedia , lookup
Corona Australis wikipedia , lookup
International Ultraviolet Explorer wikipedia , lookup
Cassiopeia (constellation) wikipedia , lookup
Cygnus (constellation) wikipedia , lookup
Constellation wikipedia , lookup
Perseus (constellation) wikipedia , lookup
Aquarius (constellation) wikipedia , lookup
Canis Major wikipedia , lookup
Observational astronomy wikipedia , lookup
Future of an expanding universe wikipedia , lookup
Timeline of astronomy wikipedia , lookup
Star catalogue wikipedia , lookup
Corvus (constellation) wikipedia , lookup
H II region wikipedia , lookup
Stellar evolution wikipedia , lookup
Stellar classification wikipedia , lookup
The Spectra of Stars and Binary Stars (Masses and Radii) Colors of Stars • Stars are made of hot, dense gas – Con$nuous spectrum from the lowest visible layers (“photosphere”). – Approximates a blackbody spectrum. • From Wien’s Law, we expect: – hoJer stars appear BLUE (T=10,000‐50,000 K) – middle stars appear YELLOW (T~6000K) – cool stars appear RED (T~3000K) Spectra of Stars • Hot, dense lower photosphere of a star is surrounded by thinner (but sZll fairly hot) atmosphere. – Produces an Absorp$on Line spectrum. – Lines come from the elements in the stellar atmosphere. Spectral ClassificaZon of Stars • Astronomers noZced that stellar spectra showed many similariZes. • Can stars be classified by their spectra? • Draper Survey at Harvard (1886‐1897): – ObjecZve Prism Photography – obtained spectra of >100,000 stars – hired women as “computers” to analyze spectra ObjecZve Prism Spectra Harvard ClassificaZon • Edward Pickering’s first aJempt at a systemaZc spectral classificaZon: – Sort by Hydrogen absorpZon‐line strength – Spectral Type “A” = strongest Hydrogen lines – followed by types B, C, D, etc. (weaker) • Problem: Other lines followed no discernible paJerns. Edward Pickering Harvard “Computers” (c. 1900) Annie Jump Cannon • Leader of Pickering’s “computers”, she noZced subtle paJerns among metal lines. • Re‐arranged Pickering’s ABC spectral types, throwing out most as redundant. • Lef 7 primary and 3 secondary classes: • O B A F G K M (R N S) • Unifying factor: Temperature Annie Jump Cannon The Spectral Sequence O B A F G K M LT Hotter 50,000K Bluer Cooler 2000K Redder Spectral Sequence is a Temperature Sequence Spectral Types The Spectral Sequence is a Temperature Sequence • Gross differences among the spectral types are due to differences in Temperature. • ComposiZon differences are minor at best. – Demonstrated by Cecilia Payne‐Gaposhkin in 1920’s • Why? What lines you see depends on the state of excita$on and ioniza$on of the gas. Example: Hydrogen Lines • Visible Hydrogen absorpZon lines come from the second excited state. • B Stars (15‐30,000 K): Most of H is ionized, so only very weak H lines. • A Stars (10,000 K): Ideal excitaZon condiZons, strongest H lines. • G Stars (6000 K): Too cool, liJle excited H, so only weak H lines. O Stars • HoJest Stars: T>30,000 K • Strong lines of He+ • No lines of H B Stars • T=15,000 - 30,000 K • Strong lines of He • Very weak lines of H A Stars • T = 10,000 - 7500 K • Strong lines of H • Weak lines of Ca+ F Stars • T = 7500 - 6000 K • weaker lines of H • Ca+ lines growing stronger • first weak metal lines appear G Stars • T = 6000 - 5000 K • Strong lines of Ca+, Fe+, & other metals • much weaker H lines • The Sun is a G‐type Star K Stars • Cool Stars: T = 5000 - 3500 K • Strongest metal lines • H lines pracZcally gone • first weak CH & CN molecular bands M Stars • Very cool stars: T = 2000‐3500 K • Strong molecular bands (especially TiO) • No lines of H L & T Stars • Coolest stars: T < 2000 K • Discovered in 1999 • Strong molecular bands • Metal‐hydride (CrH & FeH) • Methane (CH4) in T stars • Probably not stars at all Modern Synthesis: The M‐K System • An understanding of atomic physics and beJer techniques permit finer disZncZons. • Morgan‐Keenan (M‐K) ClassificaZon System: Start with Harvard classes: • O B A F G K M L T Subdivide each class into numbered subclasses: • A0 A1 A2 A3 ... A9 Examples: • The Sun: G2 star • Other Bright Stars: Betelgeuse: M2 star (Orion) Rigel: B8 star (Orion) Sirius: A1 star (Canis Major) Aldebaran: K5 star (Taurus) Binary Stars • Apparent Binaries – Chance projecZon of two disZnct stars along the line of sight. – Ofen at very different distances. • True Binary Stars: – A pair of stars bound by gravity. – Orbit each other about their center of mass. – Between 20% and 80% of all stars are binaries. Types of Binaries • Visual Binary: Can see both stars & follow their orbits over Zme. • Spectroscopic Binary: Cannot separate the two stars, but see their orbit moZons as Doppler shifs in their spectra. • Eclipsing Binary: Cannot separate stars, but see the total brightness drop when they periodically eclipse each other. Visual Binary 1890 1940 1990 Center of Mass • Two stars orbit about their center of mass: a1 a2 M2 a M1 • Measure semi-major axis, a, from projected orbit and the distance. • Relative positions give: M1 / M2 = a2 / a1 Measuring Masses Newton’s Form of Kepler’s Third Law: • Measure Period, P, by following the orbit. • Measure semi‐major axis, a, and mass RaZo (M1/M2) from projected orbit. Problems • We need to follow the orbits long enough to trace them out in detail. – This can take decades. – Need to work out the projecZon on the sky. • Everything depends criZcally on the distance: – semi‐major axis depends on d – derived mass depends on d3 !! Spectroscopic Binaries • Most binaries are too far away to see both stars separately. • But, you can detect their orbital moZons by the periodic Doppler shiCs of their spectral lines. – Determine the orbit period & size from velociZes. B A B B A A A B Problems • Cannot see the two stars separately: – Semi‐major axis must be guessed from orbit – Can’t tell how the orbit is Zlted on the sky • Everything depends criZcally on knowing the distance. Eclipsing Binaries • Two stars orbiZng nearly edge‐on. – See a periodic drop in brightness as one star eclipses the other. – Combine with spectra which measure orbital speeds. • With the best data, one can find the masses without having to know the distance! Eclipsing Binary 4 Brightness 3 1 2 1 3 2 Time 4 Problems • Eclipsing Binaries are very rare – Orbital plane must line up just right • Measurement of the eclipse light curves complicated by details: – ParZal eclipses yield less accurate numbers. – Atmospheres of the stars sofen edges. – Close binaries can be Zdally distorted. Stellar Masses • Masses are known for only ~200 stars. – Range: ~0.1 to 50 Solar Masses • Stellar masses can only be measured for binary stars. Stellar Radii • Very difficult to measure because stars are so far away. • Methods: – Eclipsing binaries (need distance) – Interferometry (single stars) – Lunar OccultaZon (single stars) • Radii are only measured for about 500 stars Summary: • Color of a star depends on its Temperature – Red Stars are Cooler – Blue Stars are HoJer • Spectral ClassificaZon – Classify stars by their spectral lines – Spectral differences mostly due to Temperature • Spectral Sequence (Temperature Sequence) • O B A F G K M L T Summary: • Types of Binary Stars – Visual – Spectroscopic – Eclipsing • Only way to measure stellar masses: – Only ~150 stars • Radii are measured for very few stars. QuesZons: • What does the temperature of a star mean? • Are there stars with temperatures higher than 50000K? • Are hoJer stars brighter than cooler stars? Are they more luminous? • Why did it take so long to find L & T stars? QuesZons • What star do we know the mass of very precisely? • Why is it so unlikely that binaries are in eclipsing systems? • Most binaries are seen as spectroscopic. Why? • How can we know the sizes of more stars than masses?