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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

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
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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