CH10 - Astronomy 1F03 2010/11 Fall Term 2010/11 Chaisson...

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Unformatted text preview: Astronomy 1F03 2010/11 Fall Term 2010/11 Chaisson & McMillan, Astronomy Chapter 10 Measuring the Stars From the Sun to the Stars From It is said that the Sun is a typical star – how was this determined? Where does the Sun fit in among other stars? stars? How do we put the stars on equal footing for comparison? footing The Sun versus the stars The Fair comparison: Fair Sun and other stars at the same Sun distance distance At the distance of a typical close star the Sun is ordinary, just is ordinary, another faint yellow star another We need to be able to measure distance to stars distance Distances to the stars Distances Parallax iis a precise Parallax s way to measure distance to nearby stars stars Stellar Parallax Stellar The apparent position of objects changes as the viewer moves viewer The angle change gets smaller with larger distances smaller angle = 1/distance The Parsec The 1 D (parsecs) = parallax (arcsec) When the Sun-Star–Earth Sun Earth angle iis one arc second angle s = 1/3600 of a degree 1/3600 The distance iis one parsec distance s one = 206,000 AU = 3.3 Light years = 31,000,000,000,000 km Parsecs and nearby stars Parsecs The nearest star is around a parsec away The Parsecs are a convenient unit to measure star distances star The best we can measure on Earth is about 0.03 arc seconds => 30 parsecs 0.03 In space: Hipparcos satellite Hipparcos satellite => 100 parsecs => The Sun’s Neighbourhood The Neighbourhood Stars within 4 parsecs parsecs (13 light years) Mostly small red stars red Stars move… Stars A nearby star drifting at several km/s can cover nearby star arc seconds in a few years arc Barnard’s star: 1.3 parsecs away Barnard 10.3 arcseconds per year (proper motion) 10.3 arcseconds per (proper 88 km/s transverse velocity 88 transverse Doppler Effect gives radial velocity radial Proper Motion of Stars Proper Transverse and radial motion combined give the total velocity of the star give Appearances… Appearances Distance objects appear to be fainter be Based on apparent brightness alone it is not possible to tell how far away a star is star Luminosity and distance and Distance objects appear fainter appear fainter because the light is spread out over a wider area wider The Luminosity (total Watts) is conserved – the same light energy is the spread out over larger and larger spheres as it moves outward spheres Brightness and distance and What we think of the brightness of a star is the Flux of light hitting our detector: W m-2 (e.g. our eyes) detector: (e.g. L F= 2 4π d Double the distance d gives ¼ the the apparent brightness F apparent Magnitudes Magnitudes Hipparcus (Greek 2nd Century BC) ranked Hipparcus Century stars from 1st magnitude (brightest) to magnitude 6th magnitude (faintest visible) Modern astronomers adopted this scale Modern +2.5 magnitudes is 10 times fainter -2.5 magnitudes is 10 times brighter Magnitudes Magnitudes Absolute Magnitude iis s Absolute brightness at 10 parsecs away (Sun +4.8) away Absolute Magnitude is a measure of total luminosity measure Apparent Magnitude is brightness at the true distance (Sun –27) Stellar Temperatures Stellar Brightess and distance tell us only Brightess and so much (and distances are only easy for nearby stars) easy There is one other fairly easy measurement: measurement: The Colour of a star The Colour of From Colour we get temperature… Colour Red Blue Orion Colour and Colour and Blackbody Radiation Blackbody Stars are pretty good blackbody radiators blackbody The peak light wavelength (colour) depends on depends temperature BUT what if we can’t see the peak? (No can see Wien’s Law) Wien Relative brightness at two wavelengths is enough to indicate the general shape of the curve the The Colour of Stars Colour Using filters, starlight of two colours (e.g. Blue colours (e.g. and Yellow) is measured and The ratio of the light in each colour picks out colour picks one particular blackbody curve and thus the temperature temperature Stellar Temperatures and Colours Colours Stellar Spectra Stellar Absorption lines in the spectra of Absorption stars are due to a combination of of Elements present Elements Surface Temperature Surface Surface Gravity (size) Surface Spectra can give more accurate temperatures than the colour alone colour Spectral Classification Spectral Stars are given spectral classes ranked by hot to cold: cold: O,B,A,F,G,K,M O,B,A,F,G,K,M The classes are determined by The which lines are prominent: which Hot Stars: Strong Hydrogen, Helium Lines Sun-like stars: Metal lines Sun Cold Stars: Molecular lines Cold Stellar Sizes Stellar A few stars are nearby and large enough to measure size directly directly For most stars we infer the size from the amount of light emitted emitted Betelguese: A red giant star Size-Temperature-Luminosity Size Relation A Blackbody surface generates light in proportion to Temperature4 proportion The size of a star’s surface gets bigger as the The surface star’s radius squared star L = 4π R 2 STAR σT 4 STAR To get 16 times the luminosity out: Double the star’s temperature OR Double Make the star 4 times bigger Make Giants and Dwarfs Giants Consider two stars with the same colour Consider colour (i.e. the same temperature) A giant star can put out millions of giant star times more light than a dwarf star just by being larger by Giants and Dwarfs Giants 1-10x Sun’s radius: radius: Dwarf Dwarf 10-100x Sun’s 10 radius: Giant Giant 1000x Sun’s radius: 1000x radius: Super-Giant Super The Sun is a G dwarf Luminosity Classes Luminosity Same surface temperature – same same colour colour Larger stars have weaker surface gravity – less surface pressure Spectral lines are narrower Classify: (I) Supergiant (V) Dwarf (V) Spectral Classes: Same Temperature, Different Radius Same The spectrum tells you Which luminosity class of star it is L = 4π R 2 STAR σT 4 STAR Colour versus Brightness: Colour Star’s cover a broad range: Star cover small cold ones (red dwarfs) small hot ones (white dwarfs) large cold ones (red giants) large large hot ones (blue giants) large What does the distribution look like? look Hertzsprung-Russell Diagram Hertzsprung The idea of getting samples of stars and plotting where they fall in colour and colour and luminosity was championed by Ejnar Hertzsprung (Danish) Ejnar (Danish) and and Henry Russell (American) Henry The Hertzsprung-Russell Hertzsprung Russell (H-R) Diagram (H Increasing Increasing Radius Radius Giants L~ R2T4 Dwarfs Spectral Classes: Same Temperature, Different Radius Same Super-giants are at the top giants of the HR diagram of Full Sample Full If you are able to detect all stars they fill out the whole diagram – favouring favouring a diagonal region called the main sequence sequence In practice there are In selection effects: selection Some types are rare Some type are faint Some A Fair Sample: Nearby Stars Fair Nearby stars are mostly small and red small They tend to fall along the main sequence main Note the lines of constant Radius from SizeRadius Temperature-Luminosity Luminosity relation relation A Biased Sample: Bright Stars Biased Bright stars can be far away away They are not representative but rare representative Most stars in the galaxy are small and red and thus faint thus The Main sequence The The Main sequence stretches from small red stars to large blue stars blue We expect some explanation for: We 1. Why do stars prefer the Main 1. sequence? sequence? 2. What determines where stars 2. sit on the sequence? sit Off the Main Sequence Off There are also a few obvious There groups not on the Main Sequence: Sequence: White Dwarfs: White Small hot stars Red Giants: Red Large cool stars What’s missing? What Via spectral type we can know the intrinsic Luminosity and Temperature and infer radius and But what makes it have those properties? properties? The answer is a combination of mass and age – but how can you tell mass? Stellar Masses: Stellar Binary Stars Many (perhaps most) stars are in binary systems (the Sun is unusual) systems Newton’s Law of Gravity allows us to Newton Law measure masses from the orbits measure Binary Stars Binary Visual Binary: Distinct images Visual Spectroscopic Binary: Spectrum is Spectroscopic Spectrum Doppler shifted by the orbital motion Doppler Binary Orbits to Masses Binary Eclipsing Binary: Eclipsing Stars pass in front of each other of For an eclipsing binary we know the orbit is edge on: Accurate masses can be obtained be The method is the same as for extraThe solar planets The Main Sequence The Determined by the mass of the star mass More massive stars are: stars Larger, brighter Larger, and bluer and The Candle that burns twice as bright … Massive stars produce light out of proportion to their mass proportion They are burning their fuel very fast They L∝M 3 .5 (approx) If we assume that the mass of the star is fuel we can estimate the maximum lifetime of Main Sequence stars… lifetime Stellar Lifetimes Lifetimes t Main Sequence M − 2.5 ∝ ∝M L Massive stars burn out in a million years Massive Sun ~ 10 billion years Very low mass stars burn for trillions Very The Main Sequence Understood Understood Why are stars there? Why They spend a long time burning fuel on They the main sequence the Why do the stars’ positions vary? Why More massive stars are larger, bluer and More more luminous on the main sequence more Main Sequence Main Higher up the main sequence is just a matter of more mass matter Off the main sequence is something else – before or after the before main sequence phase main Before and After the Main Sequence? Sequence? Red giants, White dwarfs, Supernovae and Black Holes… Black Forming Stars and Stellar Evolution – up up next next Distances to Stars: Distances “Spectroscopic Parallax” Spectral Type including Luminosity class accurately predicts star brightness accurately A star of known brightness is a standard candle candle L L F= ⇒d = 2 4π d 4π F “Spectroscopic Parallax” iis a term used to s describe this way of finding distance describe Hipparcos Hipparcos F= Sample L L ⇒d = 2 4π d 4π F Hipparcos Satellite gave very accurate Hipparcos Satellite distances for stars in 1000 pc distances This calibrates the “spectroscopic parallax” distance method: for known d, F we can get distance precise L precise Then for other stars of the same spectral type, we can be confident in the intrinsic luminosity L and use the apparent brightness F to get distance d based on how b distance Distance Ladder Distance Overview Overview ...
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