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ASTR 1020 * Spring 2017 MW 5:30-6:45 pm Stellar and Galactic Astronomy • Your instructor: Ryan Norris – Switched from Dr. Doug Gies • Email: [email protected] • Remember labs start week of Jan 23 • Please get a syllabus from me if you don’t have one • For now slides will be on www.astro.gsu.edu/~norris/ASTR1020 – Up by tomorrow at 12PM Chapter 13 Taking the Measure of Stars Why do stars look different? Bryce Canyon National Park Properties of Stars • • • • • Distances Luminosities Temperatures Radii Masses Astrometry •Measurement of position and movement of stars and other celestial bodies •Uses •Shape and formation of galaxies •Exoplanet detection Gaia, the newest European Space Agency astrometry mission •Dark matter distribution •Detection of near Earth objects Distance We use the effect known as (ASTR1010) parallax: the apparent shift of a nearby object against a background of more distant objects As Earth orbits the Sun, nearby stars change their positions slightly against the background. Comparing the position six months apart yields the distance. The greater the parallax (or parallactic angle), the smaller the distance. The only direct way to measure the distance to a star is from the parallax By definition, a star with a parallax of 1 arcsecond (arcsec) is at a distance of 1 parsec (pc). 1 arcsec = 1/3,600 degree. 1 parsec = 206,265 AU =3.26 light-years Parallax and Distance • Parallax angle and distance are inversely proportional: a small parallax angle means the star is far away • Distances measured from parallax angles are expressed in parsecs • Nearest star: Alpha Centauri system at 1.3 parsecs, which includes three stars (closest = Proxima Centauri). • Using the motion of stars to determine their parallax and to measure distance works well out to 200 parsecs (pc) • The local 10 pc neighborhood (Adric Riedel, GSU): https://www.youtube.com/watch?v=up_MqNBv0FE Thought Question The parallax of Proxima Centauri (the closest star to the Sun) is roughly twice that of Sirius (the brightest star in the Earth’s night sky). This means that Sirius is: A. Twice as close as Proxima Centauri B. Almost the same distance but moving faster C. Twice as far as Proxima Centauri D. None of the above Thought Question The parallax of Proxima Centauri (the closest star to the Sun) is roughly twice that of Sirius (the brightest star in the Earth’s night sky). This means that Sirius is: A. Twice as close as Proxima Centauri B. Almost the same distance but moving faster C. Twice as far as Proxima Centauri D. None of the above Luminosity (L) Total energy radiated by star each second L = surface area x rate/unit area L = 4πR 2 σ T 4 where R = stellar radius T = stellar effective temperature (sun~5800K) σ = Stefan-Boltzmann constant Apparent brightness: Rate at which we receive that energy With increasing distance, luminosity is spread over a larger area Area of sphere = 4π (distance)2 Divide luminosity by area to get brightness Inverse-Square Law: brightness proportional to 1/distance2 Thought Question How would the apparent brightness of Alpha Centauri change if it were three times farther away? A. B. C. D. It would be only 1/3 as bright It would be only 1/6 as bright It would be only 1/9 as bright It would be three times brighter Thought Question How would the apparent brightness of Alpha Centauri change if it were three times farther away? A. B. C. D. It would be only 1/3 as bright It would be only 1/6 as bright It would be only 1/9 as bright It would be three times brighter Stellar luminosities can be hugely different, ranging from 1/10,000th to 1,000,000 times that of the Sun The Sun is in the middle of the luminosity range, but there are way more faint stars than bright ones in the Universe The Magnitude system (Hipparcus, ~150BC): - express this range with friendlier numbers (-30 to ~30 currently) - low magnitudes = bright stars - high magnitudes = faint stars +5 magnitudes → 100 x fainter +1 magnitude → ~2.5 x fainter -1 magnitude → ~2.5 x brighter -5 magnitudes → 100 x brighter The apparent magnitude = what we measure on Earth → measures the brightness of a star Sun apparent magnitude = -26.7 The absolute magnitude = apparent magnitude if it were 10 pc away from the Sun → measure luminosity of a star Sun absolute magnitude = 4.8 Thought Question How would the absolute magnitude of Alpha Centauri change if it were three times farther away? A. It would be +3 magnitudes fainter B. It would be -3 magnitudes brighter C. It would stay the same Thought Question How would the absolute magnitude of Alpha Centauri change if it were three times farther away? A. It would be +3 magnitudes fainter B. It would be -3 magnitudes brighter C. It would stay the same (its absolute magnitude is an intrinsic property of a star) Thought Question How would the absolute magnitude of Alpha Centauri change if it suddenly got 100x fainter ? A. It would gain +1 magnitude B. It would gain +100 magnitudes C. It would stay the same D. It would gain +5 magnitudes Thought Question How would the absolute magnitude of Alpha Centauri change if it suddenly got 100x fainter ? A. It would gain +1 magnitude B. It would gain +100 magnitudes C. It would stay the same D. It would gain +5 magnitudes Thought Question Star A and star B appear equally bright but star A is twice as far away from us as star B. Which is true? A. Star A is twice as luminous as star B B. Star B is twice as luminous as star A C. They are the same luminosity D. Star A is four times as luminous as star B Thought Question Star A and star B appear equally bright but star A is twice as far away from us as star B. Which is true? A. Star A is twice as luminous as star B B. Star B is twice as luminous as star A C. They are the same luminosity D. Star A is four times as luminous as star B Listen to the history of astrometry! Sound file and explanation of sounds available at: http://sci.esa.int/gaia/58311-fromhipparchus-to-hipparcos-a-sonification-of-stellar-catalogues/ Temperature: Thermal Radiation 1. Hotter objects emit more light per unit area at all frequencies. 2. Hotter objects emit photons with a higher average energy. The color of a star is indicative of its temperature. To estimate it we can e.g. compare blue B and visual (yellow-green) V magnitudes Red stars are cooler than the Sun, while blue stars are hotter. Photometry ● Measurement of flux (amount of light) ● Generally measured through different filters (or “passbands”) ● Example B (Blue) and V (“visual”, yellow-green) ● B-V→ color index ● Measurements are in magnitudes ● Thus a smaller number means more blue and hotter ● Example ● Sun: B-V: 0.656, Teff: 5772 K ● Vega: B-V: 0.0, Teff: 9602 K Spectroscopy Study of the components of an object’s electromagnetic radiation split into components ● Emission and absorption lines provide information about composition, density, temperature, even motion (via Doppler effect) ● Kirchoff’s Laws 1: A continuous ● The spectrum of an incandescent light is an example of 1st law spectrum is produced by a hot, glowing solid Emission lines in heated hydrogen are examples of the 2nd law 2: Bright line or emission spectrum is produced by a hot, low density gas The lines observed by Fraunhofer in the Sun are examples of the 3rd law 3: If light from a continous spectrum passes through a cool, low density gas an absorption spectrum is produced ● ●