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

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Concepts from ASTR1010 & Names, Catalogs, URLs

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Concepts from astr1010 names catalogs urls

Concepts from ASTR1010&Names, Catalogs, URLs

ASTR 3010

Lecture 2

Chapter 1 & 4


Luminosity flux density and surface brightness

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


Blackbody radiation

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.


Astronomical magnitude system

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


Absolute magnitudes

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


Names catalogs urls

Names, Catalogs, URLs


Naming stars

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


Continue

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?


  • Naming variable stars

    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


    Stellar catalogs

    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


    Continue1

    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.


    Catalogs of non stellar sources

    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??


    Important astronomical websites

    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


    In summary

    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


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