* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Document
Constellation wikipedia , lookup
International Ultraviolet Explorer wikipedia , lookup
Dyson sphere wikipedia , lookup
Canis Minor wikipedia , lookup
Aries (constellation) wikipedia , lookup
Star of Bethlehem wikipedia , lookup
Corona Borealis wikipedia , lookup
Auriga (constellation) wikipedia , lookup
Astronomical unit wikipedia , lookup
Theoretical astronomy wikipedia , lookup
H II region wikipedia , lookup
Cassiopeia (constellation) wikipedia , lookup
Corona Australis wikipedia , lookup
Stellar classification wikipedia , lookup
Canis Major wikipedia , lookup
Cygnus (constellation) wikipedia , lookup
Star catalogue wikipedia , lookup
Perseus (constellation) wikipedia , lookup
Observational astronomy wikipedia , lookup
Timeline of astronomy wikipedia , lookup
Aquarius (constellation) wikipedia , lookup
Stellar evolution wikipedia , lookup
Stellar kinematics wikipedia , lookup
Corvus (constellation) wikipedia , lookup
Introduction to Astronomy ! AST0111-3 (Astronomía) ! ! ! ! ! ! ! ! ! ! ! ! Semester 2014B Prof. Thomas H. Puzia Temperatures and Colors WIEN’s Law λ = b/T The colors of the stars reveal their surface temperature. For example, a Sun-like star with a surface temperature of 6000K is yellow. Taking images of stars in a few wide-spectrum bands and taking the ratio of intensities thus can be a very CHEAP way of characterizing them. Propagation of Light f = L / d2 (ergs/s/cm2) f=1 f = 1/4 f = 1/9 Apparent brightness or flux, f, depends both on true light output (luminosity) and distance, and decreases as the square of the source distance. Very important concept in astronomy Apparent Magnitude Hipparchus, a century BC classified the stars according to their brightness in six categories: ! 1-6 = 1-100 in brightness/flux •scales m=6 , brightest m=1 •weakest The scale is logarithmic (which •reflects the response of the rods and cones in the human eye). Absolute Magnitude Intrinsic Luminosity ! What magnitude will a star have if we put it at a distance of 10 parsec? " " Example, for the Sun with apparent magnitude m=-26.5 Using the 1/d2 law, the Sun would provide only 1/2,000,0002 (1 pc ≈ 200,000 AU) of the light we receive now. ! ! ! ! ! In general i.e., Photometry Measuring the flux of light in a given band. Shown here is the Johnson UBV system. ! The color Index measures the color of a star using two filters, e.g. B-V. It can be calibrated to the temperature of the star. ★ V=Vo-2.5log(lV), B=Bo-2.5log(lB) ★ B-V=(Bo-Vo)-2.5log(lB/lV) ✓ Sun: V=-26.78, B=-26.16, U=-26.06, B-V=0.62, U-B=0.10 ✓ Sirius: V= - 1.46, B= -1.46, U= -1.52, B-V=0.00, U-B=-0.06 The star YY has a temperature of 10000 K with a peak thermal emission at 290nm. The star ZZ has a temperature of 5000 K. Therefore, the peak thermal emission from ZZ is at A. B. C. D. E. 145nm 580nm 1160nm 4680nm nowhere close to any of these. Recall that WIEN’s Law states that the wavelength of the blackbody peak is inversely proportional to temperature as λ = b/T where b = 2897768.5 nm·K What is the difference between spectroscopy and photometry? What are the advantages/disadvantages of each? What is the difference between apparent and absolute magnitudes/fluxes? What are the differences between an O star and a G star? What do we mean when we say an object is a standard candle? Independent Distance Determination are very far away. How do we measure their distances so we can • Stars determine their intrinsic luminosities? • The most direct and “cleanest” measures are geometric • Parallax (only practically achievable for very nearby stars) • Expansion Parallax (watch something expand with known speed) the Greeks rejected the heliocentric theory because they did not • Example: detect stellar parallax. Tycho later realized that this is because stars are too far away to measure with current precision. • Indirect measures are made using comparisons and calibrations. • Comparing apparent vs. known intrinsic brightness of similar stars • Variability tied to known luminosity (Cepheids, RR Lyrae, SNe) • Can be applied out to great distances, although less accurate ! These are the first steps in the distance scale ladder (more later) Stellar Parallax Earth's orbit around the Sun provides the base of a triangle with the star at the apex. We can compare the shift of the star with respect to background (“fixed”) stars over the course of 6 months and use that to measure its geometrical distance. Parallax angle p is defined using the triangle. This angle p is very small. When p = 1", the star is 1 parsec (Parallax Second). Note: 1 pc = 206,265 AU = 3.26 ly Parallaxes of Nearby Stars Parallax angles are very small because the stars are far apart. ! EXAMPLE: Proxima Centauri, the nearest star to us is measured to have a parallax of p = 0.75 arcsec. This yields a distance of d = 1 / p = 1.333 pc = 275000 AU = 4.3 ly. The limit of ground-based telescopes is p > 0.01 arcsec, which means the method is limited to stars with d < 100 pc. The space mission Hipparcos measured parallaxes accurate to d < 500 pc for 5000 stars. A new mission, GAIA, is doing this for 109 stars to d<50,000 pc from 2013-2018! For scale, our galaxy is ~30,000 pc (100,000 lyr) wide. If Alpha Centauri were placed at a distance ten times farther from the Earth than it is now, its parallax would A. increase B. decrease C. stay the same D. change color A star’s luminosity is the A. B. C. D. E. surface temperature of the star total amount of light that the star will radiate over its entire lifetime apparent brightness of the star in our sky total amount of power that the star emits into space lifetime of the star Cosmic Dust reddening absorption scattering/polarization Causes several observational effects: ! Creates a problem for determining distances and temperatures! What is Cosmic Dust? Smooth chondrite interplanetary dust particle. Complex particles between ~0.1-10 µm in size and consisting of a few molecules (“cores”) up to 100s of molecules are generically called “dust grains”. ! There are many varieties, depending on where it was formed: intergalactic, interstellar, interplanetary and circumplanetary dust. ! Dust cores of Carbon, Silicon, or Iron-Sulfur-Nickel need a dense and relatively warm environment to initially form. But the cores can then accrete further material at later stages in sparser, colder environments. ! Its interaction with light depends on size and type. ! Grains can be destroyed by absorbing too much UV radiation (leading to dust explosions), as well as evaporation, sputtering, and grain-grain collisions. Porous chondrite interplanetary dust V B U UV Extinction by dust is a major nuisance for optical astronomers, because it means that an intrinsically red star with a considerable quantity of interstellar dust between it and us will appear: A. B. C. D. E. redder with the same flux/magnitude same color with fainter flux/magnitude no change because it is already red same temperature but redder and fainter none of the above Hertzsprung-Russell Diagram Theoretical plane is H-R diagram Observational plane is colormagnitude diagram. Intrinsically brightest stars are highest. Intrinsically hottest stars are to the left. ! Sequences: • • • • Main (Hydrogen burning stage) Red Giant Horizontal Branch White Dwarfs ! The position of a star in a curve or branch turns out to be related to their state of evolution (AGE). ! The presence of clearly defined tracks means that stars are simple, predictable objects governed by Determining other Stellar Parameters Aside from temperature (color) and luminosity (absolute magnitude), what other physical parameters are needed to characterize a star? • Chemical composition • Mass • Radius • Age These parameters can be measured directly using: • nearby/bright stars, (spectrum = composition) • binary stars, (binary orbit/separation = mass) • variable stars, (variation time + velocity = size) • clusters of stars. (i.e., at the same distance = coeval age) In general, the distance introduces the largest uncertainty. A large error in this means the stellar parameters will be poorly known/constrained. Stars are relatively “simple”: theoretical understanding + known constraints can fill in gaps: + w/ a ~ 3.5 for much of main sequence (although ~1 - 4 at extremes)