Concepts from ASTR1010 & Names, Catalogs, URLs

1. Concepts from ASTR1010&Names, Catalogs, URLs ASTR 3010 Lecture 2 Chapter 1 & 4

2. Luminosity, flux density, and surface brightness • Luminosity = Total energy emitted by the source per unit time (ergs/sec) • independent on the distance to the source • Lsun = 3.825×1026 W = 3.825×1033 ergs/sec • Apparent Brightness = luminosity/area • Flux density (aka irradiance) • Flux density of the Sun at the Earth position 1,370 W/m2 = Solar constant • Surface brightness = flux density at the surface of the source over 1 steradian

3. Blackbody radiation • For a given blackbody of temperature T, the surface brightness is Stefan-Boltzmann Law, and the wavelength with the peak flux changes as effective temperature Rayleigh-Jean Approx.

4. Astronomical magnitude system • Greek astronomer, Hipparchus (BC 2C) cataloged about 600 stars into 6 brightness bins. Later it was found that stars in the 6th bin (mag=6) are about 100 times fainter than stars in the first bin. • this means that 1 mag difference is roughly 2.52 times of difference in brightness. • From this fact, following formulae can be derived. m: apparent magnitude, M: absolute magnitude (when a stars is at 10pc), F: flux, d: distance in parcsec, F0: zero magnitude flux

5. Absolute Magnitudes • Absolute magnitude measured in a band-pass (MV, MB, etc.) • Absolute bolometric magnitude (Mbol = MV + BCV) • Sun, MV=4.83, Mbol=4.75

6. Names, Catalogs, URLs

7. Naming Stars • about 5000 stars can be seen by naked eyes in the whole sky • ~2500 stars at any given time (down to 6thmag) • about 50 stars that are very bright (m <~ 2ndmag) • We could name individual stars (e.g., Betelgeuse, Sirius, Antares, etc.) • effective up to few hundred stars • Ptolemy (2nd century astronomer) “the brightest reddish star on the right shoulder” • Al-Sufi (10th century, Persian) • “Armpit of the Central One” in Arabic • “Bed Elgueze” in Latin • “Betelgeuse in English • Bayer Scheme: order stars in order of brightness in a given constellation • prefix: 24 Greek alphabets • suffix: 3 first letters from the Latin genitive constellation name • e.g., Betelgeuse = αOri = brightest star in Orion

8. Continue… • αCMa = the brightest star in Canis Major (big dog) = Sirius • since there are 88 constellations in the sky, at most we can name ~2000 stars this way (88 x 24 alphabets = 2112) • Flamsteed scheme (18th century) • Why? with the advent of telescopes, there are too many stars. • within each constellation, number stars in order of increasing Right Ascension. • e.g., 58 Ori = Betelgeuse, Vega = 3 Lyr • this scheme is still in use How about variable stars?

9. Naming Variable Stars • in a given constellation, variable stars are named in order of discovery. • 1st discovered variable star in Ori R Ori • R, S … Z, RR, RS … RZ, SS … SZ, TT … TZ, ZZ, AA … AZ, BB…BZ, … QZ (334 cases) • then, any more discoveries will be named by “V” + number + constellation. • e.g., RR Lyr, V 353 Ori, etc. • Super Nova • with a prefix “SN” followed by the discovered year, and discovered sequence. • A…Z, aa … az, ba … bz … • SN 1987A = the first discovered super nova in 1987

10. Stellar Catalogs • Durchmesterung numbers (≈300,000 stars, down to 9-10thmag) • based on photographic measurements of stars with telescopes • Bonner Durchmusterung = BD, northern hemishpere • Cordoba Durchmusterung = CD = CoD, southern hemisphere • within a given declination strip, number stars in order of increasing RA • e.g., Vega = BD +38 3238  3238th star in the declination strip +38 degrees. • Henry Draper (HD) Catalog • not only name, brightness, and positions. • it also contains spectral type (e.g., temperature) info for 225,000 stars • one of the most important catalog

11. Continue… • Subsequent major catalogs • Hubble Guide Star Catalogs (GSC) : the need for good positions of many stars down to m≈16th mag. • about 15 million stars • divided the whole sky into 9537 regions where each region contains roughly the same number of stars  efficient entry look up. • Hipparcos and Tycho catalogs  European space satellite “Hipparcos” • Hipparcos : measured distance, proper motions, brightness, and positions (≈120,000 stars, down to m≈9thmag) • Tycho : positions, proper motions, and brightness for (≈2.4 million stars down to m≈11thmag). • USNO catalogs: extended version of Tycho • USNO-B : ≈500 million stars • UCAC-3 : ≈100 million stars, UCAC = USNO CCD Astrographic Catalog • Other stellar catalogs: HR, SAO, FK5, Giclas, Gliese, CCDM, etc.

12. Catalogs of non-stellar sources • Catalogs of non-stellar sources • Messier : 18th century, a catalog of 103 nebulae (galaxies, nearby interstellar clouds, etc.) • late 19th century: New General Catalog (NGC) of 7840 nebulae • e.g., Andromeda galaxy = M 31 = NGC 224 • Lots of other catalogs (at different wavelength regimes)  currently there are ≈10,000 catalogs ?? How can we navigate through these many catalogs??

13. Important astronomical websites • SIMBAD : http://simbad.u-strasbg.fr/simbad • using a name of a star, get basic info (position, brightness, distance, etc.) • Vizier : http://vizier.u-strasbg.fr/viz-bin/VizieR • can access ~10,000 astronomical catalogs • Aladin : http://aladin.u-strasbg.fr/aladin.gml • display image of a source • over-plotting catalog data on the image • ADS : http://adsabs.harvard.edu/abstract_service.html • bibliographical database

14. In summary… Important Concepts Important Terms Luminosity Surface brightness Stefan-Boltzmann Law Wien’s displacement Law Planck function Rayleigh-Jeans approximation Effective Temperature • Nomenclature of Stars • Stellar catalogs • Non-stellar catalogs • Important websites • Astronomical magnitude system • apparent magnitude • absolute magnitude • bolometric magnitude • Bolometric correction • Chapter/sections covered in this lecture : Chapters 1 & 4