Observed Properties of Stars ASTR 2120 Sarazin
Extrinsic Properties Location Motion kinematics
Extrinsic Properties Location Use spherical coordinate system centered on Solar System Two angles (θ,φ) Right Ascension (α = RA) Declination (δ = Dec) (review in Chapt. 1) easy to measure accurately
Location Extrinsic Properties Use spherical coordinate system (r,θ,φ) centered on Solar System Two angles (θ,φ) Right Ascension (α = RA) Declination (δ = Dec) Radius r = distance d Very hard to measure
Distances Key Measurement Problem Celestial Sphere Astronomical objects are so far away, we have no ability to judge depth Stars which appear close on the sky can be very far apart
Distances - Parallax Earth travels around Sun è we view stars from different angles The stars will appear to shift back and forth every year (Review: Secs. 2.2, 2.6.2)
π Parallax
Parallax Earth travels around Sun è we view stars from different angles The stars will appear to shift back and forth every year Effect decreases with increasing distance
Parallax sinπ (rad) = AU d, π (rad)<<1 π (rad) = AU d 1 rad = 206,265!! (arcsec) π!! = 206265 AU d Define 1 pc 206265 AU = 3.09 10 18 cm = parsec π!! = 1, d pc = 1 d pc π!! AU d π
Parsec Basic unit of distance in astronomy parsec = 2.06 x 10 5 AU = 3.09 x 10 18 cm = 3.26 light years AU = 1.50 x 10 13 cm Memorize
Parallax Example: Nearest star Proxima Centauri d = 1.3 pc è π = 0.77 arcsec Spatial resolution of telescopes, seeing 1 arcsec So, very hard to center images < 0.01 Can only determine parallax, distance for relatively nearby stars, d << 100 pc
Extrinsic Properties Location Motion Separate into two components v r v r = radial velocity changes distance v t = tangential velocity changes angle v t v
Radial Velocity, Doppler Shift v r > 0 è distance increases Measured by Doppler Shift z λ λ obs em redshift or Doppler shift λ em z > 0 moving away for v << c, (true of all Milky Way stars) z v r / c v r = zc review?
Tangential Velocity Proper Motion v t è RA, Dec (angles) change Δθ d s = v t Δt tanδθ = s / d = v t Δt d
Tangential Velocity: Proper Motion tanδθ = s / d = v t Δt d Δθ = s / d = v Δt t d, Δθ <<1 radian, tanδθ Δθ Define "proper motion" µ dθ dt Δθ Δt µ = v t d measured in "/yr
Parallax + Proper Motion v orb (Earth) = 30 km/s v (nearby stars) ~ 20 km/s Proper motion (1 year) ~ parallax Parallax, periodic 1 year, east-west Proper motion continuous π μ
Distance Dependence Doppler shift: z = v r / c Independent of distance!! Can do across whole Universe Proper motion, parallax π = 1 / d pc, μ = v r / d, both ~ 1 / d Can only detect for nearby stars, < 100 pc
Parallax and Proper Motion Tough measurements from Earth due to seeing Adaptive Optics Observe from Space: Small field of view Hubble Space Telescope GAIA 1 billion stars observed 70x each in 5 years Positions accurate to 24 micro-arcsec Radial velocities for 150 million stars
GAIA
Intrinsic Properties of Stars
Luminosity and Flux L = luminosity = energy / time from star (erg/s) Brightness = flux F = energy / area / time at Earth (erg/cm 2 /s) F = L 4πd 2 inverse square law L = 4πd 2 F
Magnitudes Hipparchus 1) Classified stars by brightness, brighter = 1 st magnitude, 6 th magnitude 2) Used eyes, human senses logarithmic Magnitudes è go backwards, logarithmic
Magnitudes Write as 1 ṃ 3 or 1.3 mag 5 mag = factor of 100 fainter 2.5 mag = factor of 10 fainter (1 order of magnitude) Two stars, fluxes F 1, F 2 F 1 / F 2 =10 (m 1 m )/2.5 2 =10 0.4(m 1 m 2 ) m 1 m 2 = 2.5log(F 1 / F 2 ) (log log 10, ln log e )
Examples - 1 Two stars, a is twice as bright as b m =10 mag. What is m? b a F / F = 2 a b m m = 2.5log(F / F ) = 2.5log(2) a b a b = 2.5 0.3 = 0.75 m = m 0.75 =10 0.75 = 9.25 mag a b
Examples - 2 Sirius is -1.5 mag, Castor is 1.6 mag. Which is brighter, and by what factor? Brighter smaller mag Sirius Δm = 3.1 F / F =10 0.4 Δm =10 0.4 ( 3.1) =10 1.24 =17 S C
Magnitudes
Stellar Colors
Stellar Colors
Stellar Colors Star vary in color: Betalgeuse red, Sun yellow, Vega blue-white Use filters to get flux in one color, compare
Color Filters for Observing
Stellar Colors Star vary in color: Use filters to get flux in one color, compare Fluxes: F U, F B, F V, Magnitudes: m U = U, m B = B, m V = V,
Stellar Colors Color index, or just color CI = B V, Note: Given B V è fixed F B / F V Just measures shape of spectrum, not total flux Independent of distance Just gives color
Temperature & Color Color mainly determined by temperature of stellar surface Stellar spectra ~ black body λ max 0.3 cm / T (Wiens Law) (Review: Sec. 5.7) Higher T è shorter λ è bluer light Hot stars blue, B V negative Cool stars red, B V positive Solar spectrum vs. BB
Stellar Temperatures Range from T 3000 K to 50,000 K (brown dwarfs, planets cooler, some stellar corpses hotter)
Stellar Temperatures
Bolometric Magnitude Hard to measure all light from star, but Bolometric magnitude è magnitude based on total flux m bol
Luminosity & Absolute Magnitude F = L 4πd 2 L = 4πd 2 F Absolute magnitude M = magnitude if star moved to d = 10 pc F = L 4πd 2 F! = 10 # F d 10 " pc $ & % 2
Luminosity & Absolute Magnitude F = L 4πd 2 F! = 10 # F d 10 " pc (! m M = 2.5log* 10 *# " d ) pc $ & % 2 +, $ & % 2 - - = 5log ( 10 / d ) pc m M = 5logd pc 5 distance modulus
Luminosity & Absolute Magnitude m M = 5logd pc 5 distance modulus M = m 5logd pc + 5 = m + 5log ## π + 5
Luminosity & Absolute Magnitude From distance to Sun (AU) and flux M bol ( ) = +4.74 L = 3.845 x 10 33 erg/s = 3.845 x 10 26 J/s = W M bol = 4.74 2.5 log(l/l ) Memorize
Stellar Luminosities Very wide range 10-4 L L 10 6 L
Basic Numbers of Astronomy Memorize
For BB, Stellar Radii L = (area) σ T 4 = 4π R 2 σ T 4 σ = 5.67 x 10-5 erg/cm 2 /s/k 4 Stefan-Boltzmann constant Define effective temperature T eff such that L = 4π R 2 σ T eff 4 If BB, then T = T eff
Stellar Radii Measure flux F, distance d è L Measure color èt eff (estimate) Solve for radius R
Stellar Radii Find mainly three sets of radii Normal Stars: main sequence, dwarfs 0.1 R < R < 20 R sequence: small, cool, faint è big, hot, bright Giants: R > 100 R ~ AU cool, T ~ 3000 K White Dwarfs: R 0.01 R ~ R(Earth)